大气成分与环境气象

大气成分与环境气象研究进展

Advances in Research on Atmospheric Composition and Environmental Meteorology

1 大气成分及相关特性变化观测研究

1 Observational studies of atmospheric composition and related properties

1.1 Advances in sunphotometer-measured aerosol optical properties and related topics in China:Impetus and perspectives

Aerosol is a critical trace component of the atmosphere. Many processes in the Earth’s climate system are intimately related to aerosols via their direct and indirect radiative effects. Aerosol effects are not limited to these climatic aspects. They are also closely related to human health, photosynthesis, new energy, etc.,which makes aerosol a central focus in many research fields. A fundamental requirement for improving our understanding of the diverse aerosol effects is to accumulate high-quality aerosol data by various measurement techniques. Sunphotometer remote sensing is one of the techniques that has been playing an increasingly important role in characterizing aerosols across the world. Much progress has been made on this aspect in China during the past decade, which is the work reviewed in this paper. Three sunphotometer networks have been established to provide high-quality observations of long-term aerosol optical properties across the country. Using this valuable dataset, our understanding of spatiotemporal variability and long-term trends of aerosol optical properties has been much improved. The radiative effects of aerosols both at the bottom and at the top of the atmosphere are comprehensively assessed. Substantial warming of the atmosphere by aerosol absorption is revealed. The long-range transport of dust from the Taklimakan Desert in Northwest China and anthropogenic aerosols from South Asia to the Tibetan Plateau is characterized based on ground-based and satellite remote sensing as well as model simulations. Effective methods to estimate chemical compositions from sunphotometer aerosol products are developed. Dozens of satellite and model aerosol products are validated, shedding new light on how to improve these products. These advances improve our understanding of the critical role played by aerosols in both the climate and environment. Finally, a perspective on future research is presented. (Che Huizheng)

1.2 The dominant mechanism of the explosive rise of PM2.5 after significant pollution emissions reduction in Beijing from 2017 to the COVID-19 pandemic in 2020

The Chinese government implemented strict emission reduction measures of air pollution between 2013 and 2017. However, from the winter of 2017 until February 2020, during the COVID-19 pandemic,the twenty explosive rise (ER) events of PM2.5mass in twelve heavy aerosol pollution episodes (HPEs)still appeared in Beijing and its vicinity (BIV). To explore the controlling mechanism for the ER under the condition of drastically reduced emissions, the vertical structure of meteorological elements by L-band second-level sounding and aerosol properties by Lidar were investigated associating with the analysis of surface concentration in PM2.5mass, its main precursor gases, as well as black carbon (BC) by sevenwavelength Aethalometer. The planetary boundary layer height (BLH) was also estimated together with an analysis of the unfavorable meteorological index (PLAM) that can quantify the impact of unfavorable meteorological conditions to cause the change of PM2.5concentration. The results suggested that the ER reoccurrence’s fundamental cause is that the emissions have not yet fallen sufficiently to a level to decouple HPEs from unfavorable meteorological conditions. During the ER period, the BLH dropped significantly.The fact that PM2.5, its precursor gases, and black carbon increased almost in a similar proportion, indicating that the boundary layer structure change caused by aerosol accumulation is the dominant reason for the ER phenomenon compared to the chemical conversion factor. The two-way feedback effect between the further worsened meteorological conditions and the accumulation of PM2.5typically interpreted 54%‒93% of the ER. An HPE starting 8 Feb. 2020 during the COVID-19 period underwent one of the worst meteorological conditions, quantified by PLAM, in BIV since 2013. However, with a similar level of unfavorable meteorological conditions, the average PM2.5concentration during the HPE in 2020 was only about 66% of that of a similar HPE in 2016. It shows that the substantial reduction of emissions reduces the PM2.5pollution level primarily as before when facing an equivalent level of unfavorable meteorological conditions. These results combined suggest that China’s continuous efforts to reduce emissions proceed in the right direction and have achieved the desired results. (Che Huizheng)

1.3 Robust prediction of hourly PM2.5 from meteorological data using LightGBM

Retrieving historical PM2.5data is a key for evaluating the long-term impacts of PM2.5on the environment,human health, and climate change. Satellite-based aerosol optical depth has been used to estimate PM2.5,but estimations have largely been undermined by massive missing values, low sampling frequency, and weak predictive capability. Here, using a novel feature engineering approach to incorporate spatial effects from meteorological data, we developed a robust LightGBM model that predicts PM2.5at an unprecedented predictive capacity on hourly (R2= 0.75), daily (R2= 0.84), monthly (R2= 0.88), and annual (R2= 0.87)timescales. By taking advantage of spatial features, our model can also construct hourly gridded networks of PM2.5. This capability would be further enhanced if meteorological observations from regional stations were incorporated. Our results show that this model has great potential in reconstructing historical PM2.5datasets and real-time gridded networks at high spatial-temporal resolutions. The resulting datasets can be assimilated into models to produce long-term reanalysis that incorporates interactions between aerosols and physical processes.(Zhong Junting, Zhang Xiaoye, Gui Ke)

1.4 Attribution of the worse aerosol pollution in March 2018 in Beijing to meteorological variability

Fine particle matter (PM2.5) pollution frequently occurs in winter with increased consumption for heating and decreased radiation and boundary-layer height. Under strict emission controls since 2013, the mass concentrations of PM2.5in Beijing decreased substantially in Winter 2017/2018. However, in March 2018,the mean PM2.5concentration doubled from about 40 to 87 μg m−3, the reason for which is still unclear in the context of significant emission reductions. Here, using PM2.5measurements, vertical observations, and reanalysis data, we found that this worsening was attributed to increasingly stable stratification characterized by temperature differences between 850 hPa and 1000 hPa, which reached the maximum value (−4 ) for March from 1951 to 2018. Not limited to Beijing, positive anomalies in temperature differences occurred widely as a result of a westward transition in the center of the polar vortex. Away from the cold center and dominated by anomalous zonal westerlies, the northern China was intruded by warm southerly winds. The warm regional advection increased positive anomalies in low-level temperature differences, and in turn, enhanced geopotential thickness throughout the troposphere. The adverse impact on pollution was also confirmed in comparison to the vortex pattern in January. The vortex changed from the elongated one with a split-flow pattern in January to the annular vortex with a major center over Novaya Zemlya in March. Correspondingly, Beijing was less affected by cold air masses and thereby under relatively stable stratification, which is unfavorable for pollution dispersion in March. Additionally, this aerosol pollution was further worsened by aerosol-induced deteriorating meteorological conditions. (Zhong Junting, Zhang Xiaoye)

1.5 Three-dimensional climatology, trends, and meteorological drivers of global and regional tropospheric type-dependent aerosols: Insights from 13 years (2007–2019) of CALIOP observations

Globally gridded aerosol extinction data from the Cloud-Aerosol Lidar with Orthogonal Polarization(CALIOP) during 2007–2019 are utilized to investigate the three-dimensional (3D) climatological distribution of tropospheric type-dependent aerosols and to identify the trends in column aerosol optical depth (AOD),partitioned within different altitude regimes, and their meteorological drivers. Using detection samples of layer aerosols, we also yield a 3D distribution of the frequency of occurrence (FoO) of aerosol subtypes classified by CALIOP. The results show that the aerosol extinction coefficient (AEC) shows contrasting vertical distribution patterns over land and ocean, with the former possessing significant geographical dependence,while the enhancement of AEC in the latter is mainly located below 1 km. The vertical structures of the typedependent AECs, however, are strongly dependent on altitude. When the total AOD (TAOD) is partitioned into the planetary boundary layer (PBL) and the free troposphere (FT), results demonstrate that the PBL and FT contribute 62.08% and 37.92%, respectively, of the global tropospheric TAOD averaged over daytime and nighttime. Yet this CALIOP-based partitioning of the different aerosol subtypes in the PBL and FT varies significantly. Among all 12 typical regions of interest analyzed, more than 50% of TAOD is located in the lower troposphere (0‒2 km), while the contribution is less than 2% above 6 km. In global average terms, we found the aerosol FoO averaged over all layers is 4.45%, with the largest contribution from “clean marine”(1.79%) and the smallest from “clean continental” (0.05%). Overall, the FoO vertical structures of the aerosol layer exhibit a distribution pattern similar to that of AEC. The resulting trend analyses show that CALIOP accurately captures significant regional anomalies in TAOD, as observed in other satellite measurements and aerosol reanalysis. Our correlation analysis between meteorological factors and TAOD suggests the interannual variability of TAOD is related to the variability of precipitation (PPT), volumetric soil moisture (VSM), and wind speed (WS) in the particular regions. For instance, the positive TAOD trend over the equatorial central Pacific is mainly attributable to the increased PPT and decreased WS. In contrast, in dry convective regions dominated by dust and smoke, the interannual variability/trend in TAOD is largely modified by the VSM driven by the PPT. Additionally, we further found that these significant regional correlations are more robust within the PBL and significantly weakened or even reversed within the FT. This highlights the superiority of using the TAOD partitioned within the PBL as a proxy variable for the widely applied TAOD to explore the relationships between atmospheric pollution and meteorology. (Gui Ke)

1.6 Seasonal variability and trends in global type-segregated aerosol optical depth as revealed by MISR satellite observations

This study utilized a long-term (2001–2018) aerosol optical component dataset retrieved from the Multiangle Imaging Spectroradiometer (MISR), Version 23, to perform comprehensive analyses of the global climatology of seasonal AODs, partitioned by aerosol types (including small-size, medium-size, large-size,spherical, and non-spherical). By dividing eight different AOD bins and performing trend analysis, the seasonal variability and trends in these type-segregated AODs, as well as in the frequency occurrences (FOs) for different AOD bins, globally and over 12 regions of interest, were also investigated. In terms of particle size,small-size aerosol particles (diameter < 0.7 μm) contribute the largest to global extinction in all three seasons except winter. A similar globally dominant role is exhibited by spherical aerosols, which contribute 68.5%,73.3%, 71.6% and 70.2% to the global total AOD (TAOD) in spring, summer, autumn and winter, respectively,on a global average scale. FOs with different aerosol loading levels suggested that the seasonal FOs tend to decrease progressively with increasing aerosol loading, except for Level 1 (TAOD < 0.05). Examination of the seasonal distribution of FOs revealed that the FO at Level 1 (Level 2, 0.05 < TAOD < 0.15) is much larger in summer/winter (winter/autumn) than in spring/autumn (spring/summer) over most areas of the world.However, the FOs for Level 3 (0.15 < TAOD < 0.25) to Level 8 (TAOD > 1.0) generally exhibit greater intensity in spring/summer than in autumn/winter. Temporal trend analyses showed that the seasonal TAOD experiences a significant decline during 2001‒2018 in most regions globally, except in South Asia, the Middle East, and North Africa. Opposite seasonal trends in the above regions are closely related to the increase in FOs in the range of 0.4 < TAOD < 1.0. The global average TAOD shows the most pronounced decline in spring,falling by −10.4% (P < 0.05). Examination of the trends in type-segregated AODs further revealed that the decreases in size-segregated (shape-segregated) AODs all contribute to the decrease in seasonal TAOD, with small-size AOD (spherical AOD) contributing most significantly. (Gui Ke)

1.7 A global-scale analysis of the MISR Level-3 aerosol optical depth (AOD) product: Comparison with multi-platform AOD data sources

This study analyses the applicability of the recently released Level-3 (L3) daily and monthly aerosol optical depth (AOD) products (version F15_0032) over land retrieved from the Multiangle Imaging Spectroradiometer (MISR) instrument. For this purpose, daily AOD data from 427 Aerosol Robotic Network(AERONET) sites worldwide during 2001–2018 and 39 China Aerosol Remote Sensing Network (CARSNET)sites across China during 2002–2014 were selected for comparison. Also, MISR-based size-segregated AODs were collected to compare with the coarse- and fine-mode AODs retrieved from AERONET to reveal the particles size modes that mainly contribute to the offset of MISR total AOD relative to observations.By comparing with five other monthly AOD datasets, including two MODIS (Moderate resolution Imaging Spectroradiometer) retrieval products, a multi-satellite merged product, and two aerosol reanalysis, MERRA-2(Modern-Era Retrospective Analysis for Research and Applications) and CAMS (Copernicus Atmosphere Monitoring Service), we then explored the applicability of MISR for characterizing the climatology,interannual variations, and long-term (2003–2017) trends in regional aerosol loadings over the 12 regions of interest. Overall, about 80% (60%) of the daily AOD values retrieved by MISR fall within expected error bounds of ± [0.05 + 0.2 × AOD] (± [0.03 + 0.1 × AOD]), with a high correlation (R = 0.87). Our comparison results show that although the V23 algorithm addresses several issues in L2 AOD retrieval relative to the previous version, the L3 data aggregated from L2 data tends to on average overestimate low AOD values and underestimate high AOD values. These offsets relative to observations are mainly attributed to the overestimation of coarse-mode AOD and the underestimation of fine-mode AOD by MISR. Intercomparison of 2003–2017 AOD trends from multiple data sources indicates that MISR can capture well the increases occurring in South Asia, as well as the decreases occurring in the eastern China, eastern United States, and western Europe. Our study confirmed that MISR L3 product performs reliably at regional scales, particularly in typically polluted areas, but caution is still needed when applied to areas where sparse sampling is encountered.(Gui Ke)

1.8 Identifying the dominant local factors of 2000–2019 changes in dust loading over East Asia

East Asian dust aerosols play a vital role in the local and regional climate through its direct, indirect,and semidirect effects, but the dominant factors affecting the interannual variation of dust aerosols over East Asia and their regional differences remain unclear. This study verified the accuracy of MEERA-2 dust data in East Asia, analyzed the interannual trends of dust in East Asia from 2000 to 2019 using the MERRA-2 dust column mass density (DCMD) and identified the dominant factors affecting the interannual variation during the dusty season (March–July) by developing the regional multiple linear regression models, combined with correlation and partial correlation analysis. The comparison with the dust index (DI) calculated from groundbased observations of dust events frequency indicated that MERRA-2 DCMD exhibited high spatial agreement(R >0.8) with ground-based observations in most regions (especially in the dust source region of North China).The trend analysis revealed that DCMD in East Asia decreased significantly after 2000, particularly in the dusty season (March–July). These significant decreases were generally highly correlated with increases in normalized differential vegetation index (NDVI), volumetric soil moisture (VSM), and precipitation (PPT)and with decreases in wind speed (WS). Furthermore, WS dominated the interannual variation in the dust concentration over the East Asian dust source regions and their downstream. By contrast, PPT, through its wet deposition effect, dominated the variation in the rest of the regions away from the dust source regions. The study findings may help clarify the associations between local meteorological and surface factors and longterm variations in dust aerosols over East Asia. (Gui Ke)

1.9 Aerosol optical properties and its type classification based on multiyear joint observation campaign in North China Plain megalopolis

Since haze and other air pollution are frequently seen in the North China Plain (NCP), detail information on aerosol optical and radiative properties and its type classification is demanded for the study of regional environmental pollution. Here, a multiyear ground-based synchronous sun photometer observation at seven sites on North China Plain megalopolis from 2013 to 2018 was conducted. First, the annual and seasonal variation of these characteristics as well as the intercomparsion were analyzed. Then the potential relationships between these properties with meteorological factors and the aerosol type classification were discussed. The results show: Particle volume exhibited a decreasing trend from the urban downtown to suburban and the rural region. The annual average aerosol optical depth at 440 nm (AOD440) varied from 0.43 to 0.86 over the NCP.Annual average single-scattering albedo at 440 nm (SSA440) varied from 0.89 to 0.93, indicating a moderate to slight absorption capacity. Average absorption aerosol optical depth at 440 nm (AAOD440) varied from 0.07 to 0.10. The absorption Ångström exponent (AAE) (0.89–1.40) indicated the multi-types of absorptive matters originated form nature and anthropogenic emission. The discussion of aerosol composition showed a smaller particle size of aerosol from biomass burning and/or fossil foil consumption with enhanced aerosol scattering and enlarged light extinction. Aerosol classification indicated a large percentage of mixed absorbing aerosol(20%‒49%), which showed increasing trend between relative humidity (RH) with aerosol scattering and dust was an important environmental pollutant compared to the southern China. (Zheng Yu)

1.10 Simultaneous measurements of PM1 and PM10 aerosol scattering properties and their relationships in urban Beijing: A two-year observation

The aerosol scattering properties of submicron (PM1) and sub-10-μm particles (PM10) under dry conditions(RH < 30%) were investigated in Beijing from 2018 to 2019. Using the simultaneous measurement of PM1and PM10, the scattering properties of super micron (PM10–1) particles were also calculated. At 550 nm, the average of scattering coefficient (σsp) and asymmetry parameter (g) were 208.7 ± 204.9 Mm−1and 0.61 ± 0.04 for PM10,140.6 ± 130.2 Mm−1and 0.60 ± 0.04 for PM1, and 69.8 ± 82.2 Mm−1and 0.62 ± 0.04 for PM10–1, respectively,while the backscattering ratio (b) values were 0.13 ± 0.02 for PM10and PM1, and 0.12 ± 0.02 for PM10–1. The mass scattering efficiencies (MSE) for PM10, PM1and PM10–1were 2.43 ± 2.37, 3.67 ± 0.96, and 1.73 ± 1.82 m2g−1,respectively. In 2019, σspdecreased by approximately 18.4% for PM10, and 16.7% for PM1compared with those in 2018, which was quite similar to the decrease of 17% and 19% for PM10and PM2.5mass concentrations during the same time period. The scattering Ångström exponent (SAE450/700), which was 1.88 ± 0.29 for PM1and 1.50 ± 0.27 for PM10indicated size distributions dominated by fine mode aerosols. This is also evidenced by the high submicron scattering ratio (Rsp) (71.1% ± 7.9%). The high SAE, Rsp, and high PM1σspin the study suggest that control of fine particle pollution is important to reduce overall PM pollution in urban Beijing. In addition, with an increase in σsp, b, Rsp, and SAE gradually decreased, while g and MSE increased. The clearly scattering coefficient-dependent MSE suggests that high aerosol loading and high MSE both play an important role in degraded visibility during heavy pollution periods. (Sun Junying)

1.11 Reduced volatility of aerosols from surface emission to the top of planetary boundary layer

Aerosols from surface emission can be transported upwards through convective mixing in the planetary boundary layer (PBL), which subsequently interact with clouds, serving as important sources to nucleate droplets or ice particles. However, the evolution of aerosol composition during this vertical transport has yet to be explicitly understood. In this study, simultaneous measurements of detailed aerosol compositions were conducted at both sites, urban Beijing (50 m a.s.l.) and Haituo mountain (1344 m a.s.l.) during wintertime,representing the anthropogenically polluted surface environment and the top of the PBL respectively. The pollutants from surface emissions were observed to reach the mountain site on daily basis through daytime PBL convective mixing. From surface to the top of PBL, we found efficient transport or formation for lowervolatile species, black carbon, sulfate and low-volatile organic aerosol (OA); however notable reduction of semi-volatile substances, such as the fractions of nitrate and semi-volatile OA reduced by 74% and 76%respectively, during the upward transport. This implied the mass loss of these semi-volatile species was driven by the evaporation process, which repartitioned the condensed semi-volatile substances to gas-phase, when aerosols were transported and exposed to a cleaner environment. In combination with the oxidation processes,these led to enhanced oxidation state of OA at the top of the PBL compared to surface environment, with an increase of oxygen to carbon atomic ratio by 0.2. Such reduction of aerosol volatility during vertical transport may be important in modifying its viscosity, nucleation activity and atmospheric lifetime. (Liu Quan)

1.12 Aerosol component retrievals from satellite observations

The multi-angle polarimetric satellite observations are helpful for improving the retrievals of aerosol parameters. However, practical applications of polarization technology are still limited because of complexity of measurement and interpretation of polarimetric observations. In this study, we analyze the performance of a new component approach developed in the frame of the Generalized Retrieval of Atmosphere and Surface Properties (GRASP) algorithm. In addition to aerosol optical properties including particle size distribution,non-sphericity and index of refraction that are commonly derived from multi-angle radiance and polarization measurements, the GRASP/Component approach also provides some information about aerosol composition.Specifically, in this approach aerosol is modeled as an internal mixture of several components with distinctly different chemical compositions and known refractive indices. The approach is intended not only to provide additional insight on aerosol composition but also to improve retrieval of basic aerosol optical properties. This study presents comprehensive validation and evaluation of aerosol optical depth (AOD)s, Ångström exponent(AE), fine mode AOD (AODF), coarse mode AOD (AODC), and single scattering albedo (SSA) as retrieved by the GRASP/Component approach. The GRASP/Component products include aerosol retrievals using two different aerosol component mixing rules, i.e., Maxwell-Garnett (MG) effective medium approximation and a simple Volume-Weighted averaging (VW). The differences between the results obtained using these two assumptions are also discussed. The obtained results show that the aerosol optical property products of GRASP/Component approach have good agreement with the ground-based AERONET measurements,which is comparable to other PARASOL/GRASP approaches. Specifically, the AOD retrieved by GRASP/Component approach show high correlation and nearly no bias both over land and ocean, as compared with AERONET. The more detailed aerosol properties such as AE, AODF, AODC and SSA also show one of the best comparisons with AERONET. These improvements can probably be attributed to the use of the additional physical constraints on spectral dependence of the complex refractive index and the reduction of total number of aerosol parameters directly retrieved in the GRASP/Component approach. In addition, the choice of mixing rules had no significant effect on optical retrievals. With the exception of SSA, the results obtained based on the MG mixing rule were found to be slightly better over those obtained using VW mixing rule, especially for bias. (Li Lei)

1.13 The influence of stagnant and transport type weather on heavy pollution in the Yangtze-Huaihe Valley, China

The ambient atmospheric PM2.5concentrations in Anhui Province, China, which links the Yangtze River Delta region, China’s fastest growing economy area, with the Beijing-Tianjin-Hebei (BTH) region, China’s most polluted region, are influenced not only by local emissions, but also by changes in regional circulation.During the period 2013−2017, when China adopted a series of pollution abatement measures, there were still occasional pollution episodes with significant increases in PM2.5concentrations. PM2.5rose instead during the period 2013−2017 in Anhui (the Center of the Yangtze-Huaihe, YH), when pollution emissions continued to decrease. What is the controlling mechanism behind these? By analyzing elements such as ground-based PM2.5concentration and the planetary boundary layer (PBL) structure affecting it as well as larger scale circulation, combined with the analysis of a parameterized index, one can find that aerosol pollution in the YH region can usually be classified into three types. (1) There is a short-term transport stage (TS) in the initial stage of pollution; then as the pollutant concentrations increase, the PBL height decreases; the temperature inversion is gradually formed or strengthened; the wind speed decreases and the relative humidity of the lower layer increases, forming a two-way feedback mechanism in the cumulative stage (CS). (2) The pollutant concentrations will not drop rapidly in the later stage of CS, while a short-term TS will occur again. (3) The explosive rise (ER) events are mainly affected by transportation in the YH. The first of these types tends to be accompanied by the emergence and maintenance of heavy pollution periods (HPEs), and in some phases is accompanied by explosive rises (ERs) in PM2.5that at least double in a short period of time. To sum up,deterioration of meteorological conditions explains approximately 68% to the increase in PM2.5in the ER. (Jia Wenxing, Zhang Xiaoye)

1.14 Assessing the pollutant evolution mechanisms of heavy pollution episodes in the Yangtze-Huaihe Valley: A multiscale perspective

The Yangtze-Huaihe (YH) region experiences heavy aerosol pollution, characterized by high PM2.5concentration. To unravel the pollutant evolution mechanism during the heavy pollution episodes (HPEs), this study combined observational data analysis and three-dimensional WRF-Chem simulations. From 2 December 2016 to 15 January 2017, the YH region experienced 4 HPEs under the control by synoptic system, normally associated with a transport stage (TS) and a cumulative stage (CS). During the TS, pollutants are transported to the north of the YH region through the near-surface, and then transported to the “mountain corridor”through the residual layer (RL) under the influence of prevailing wind. For the RL transport mechanism, the change of pollutant concentration can consider not only the net flux in the horizontal direction, but also the role of the vertical movement, which is extremely important and cannot be ignored. By analyzing the mass conservation equation of pollutant, the results show that the advection transport and turbulent diffusion have a synergistic effect on the change of pollutant in the CS of three HPEs. The change of turbulence characteristics is also affected by topography. The “mountain corridors”, which is accompanied by variable wind direction and turbulence diffusion, is easily affected by wind shear. In addition, the turbulence characteristics are different during the TS and CS, especially the strong stable conditions in the CS at nighttime. The turbulence,is intermittent, and the model has insufficient performance for turbulence, which will lead to differences for the simulation of pollutant concentration. In short, as the PM2.5concentration linearly increases, the friction velocity (turbulent diffusion coefficient) decreases by 63% (80%), 61% (78%) and 45% (68%), respectively.Therefore, the change of pollutants is less sensitive to the change of turbulence during the HPEs. The contribution of regional transport (local emissions) reaches 43% (47%), thus we need pay attention to the contribution of each part during the HPEs, which will help us to build a certain foundation for the emission reduction work in the future. (Jia Wenxing, Zhang Xiaoye)

1.15 A novel method of retrieving low visibility during heavily polluted episodes in the North China Plain

The prediction of visibility is an ongoing problem in air quality models, particularly that of low visibility during heavily polluted episodes. In this study, a new method of calculating visibility based on the data of particle mass concentration of PM2.5(particles with diameter≤2.5 μm) and relative humidity (RH), which are generally available in most regions of China, is developed. The method also considers the particle number size distribution (PNSD) and hygroscopic parameter (κ), and focuses on visibility below 10 km. First, the PNSD was re-constructed under dry condition (PNSDdry) based on the relationship between PM2.5and the particle volume size distribution modal parameters obtained in a previous study conducted in the North China Plain.Then, the ambient PNSD was retrieved based on the PNSDdry and κ, and the light extinction was calculated by applying the Mie code (σext,amb). Finally, the visibility was calculated based on the Koschmieder experimental equation, and denoted as Viscal. A parameterization scheme was proposed based on the σext,amb, PM2.5, and RH to simulate the visibility (Vissimu), which is more applicable than the theoretical calculation described above. This method was validated at different locations in different regions in China. The values calculated by the scheme showed agreeable well with the observed data in general, especially for low visibility of≤5 km, associated with severe haze. Although a large bias occurred at some sites, both the hourly and daily averages for almost every event with visibility lower than 5 km were captured. The method reported in this work exhibited smaller bias below 2 km than the other visibility parameterization scheme, and will be available for improving the prediction of visibility in air quality models. (Shen Xiaojing)

1.16 Drivers of the rapid rise and daily-based accumulation in PM1

Submicron particle matter (PM1) that rapidly reaches exceedingly high levels in several or more hours in the North China Plain (NCP) has been threating about 400 million individuals’ health for decades. The precise cause of the rapid rise in PM1remains uncertain. Based on sophisticated measurements in PM1characterizations and corresponding boundary-layer (BL) meteorology in the NCP, it is demonstrated that this rising is mainly driven by BL meteorological variability. Large increases in near-ground inversions and decreases in vertical heat/momentum fluxes during the day-night transition result in a significant reduction in mixing space. The PM1that is vertically distributed before accumulates at the near-ground and then experiences a rapid rise. Besides meteorological variability, a part of the rise in organics is ascribed to an increase of coal combustion at midnight. The daily-based accumulation of PM1is attributed to day-to-day vertical meteorological variability, particularly diminishing mixing layer height exacerbated by aerosolradiation feedback. Resolved by a multiple linear regression model, BL meteorological variability can explain 71% variances of PM1. In contrast, secondary chemical reactions facilitate the daily-based accumulation of PM1rather than the rapid rise. Our results show that BL meteorological variability plays a dominant role in PM1rising and day-to-day accumulation, which is crucial for understanding the mechanism of heavy pollution formation. (Zhong Junting, Zhang Xiaoye, Zhang Yangmei)

1.17 Enhancement of nanoparticle formation and growth during the COVID-19 lockdown period in urban Beijing

Influenced by the spread of the global 2019 novel coronavirus (COVID-19) pandemic, primary emissions of particles and precursors associated with anthropogenic activities decreased significantly in China during the Chinese New Year of 2020 and the lockdown period (24 January–16 February 2020). The 2-month measurements of the number size distribution of neutral particles and charged ions showed that during the lockdown (LCD) period, the number concentration of particles smaller than 100 nm decreased by approximately 40% compared to the pre-LCD period in January. However, the accumulation mode particles increased by approximately 20% as several polluted episodes contributed to secondary aerosol formation.In this study, new particle formation (NPF) events were found to be enhanced in the nucleation and growth processes during the LCD period, as indicated by the higher formation rate of 2 nm particles (J2) and the subsequent growth rate (GR). The relevant precursors, e.g., SO2and NO2, showed a clear reduction, and O3increased by 80% during LCD period, as compared with pre-LCD. The volatile organic vapors showed different trends due to their sources. The proxy sulfuric acid during the LCD period increased by approximately 26%, as compared with pre-LCD. The major oxidants (O3, OH, and NO3) of VOCs were also found to be elevated during LCD. That indicated higher J2 and GR (especially below 5 nm) during the LCD period were favored by the increased concentration level of condensing vapors and decreased condensation sink. Several heavy haze episodes have been reported by other studies during the LCD period; however, the increase in nanoparticle number concentration should also be considered. Some typical NPF events produced a high number concentration of nanoparticles that intensified in the following days to create severe aerosol pollution under unfavorable meteorological conditions. Our study confirms a significant enhancement of the nucleation and growth process of nanoparticles during the COVID-19 LCD in Beijing and highlights the necessity of controlling nanoparticles in current and future air quality management. (Shen Xiaojing)

1.18 Simultaneous observation of atmospheric peroxyacetyl nitrate and ozone in the megacity of Shanghai, China: Regional transport and thermal decomposition

Atmospheric peroxyacetyl nitrate (PAN) and ozone (O3) are two typical indicators for photochemical pollution that have adverse effects on the ecosystem and human health. Observation networks for these pollutants have been expanding in developed regions of China, such as North China Plain (NCP) and Pearl River Delta (PRD), but are sparse in Yangtze River Delta (YRD), meaning their concentration and influencing factors remain poorly understood. Here, we performed a one-year measurement of atmospheric PAN, O3,particulate matter with aerodynamic diameter smaller than 2.5 mm (PM2.5), nitrogen oxides (NOx), carbon monoxide (CO), and meteorological parameters from December 2016 to November 2017 in Shanghai. Overall,high hourly maximum PAN and O3were found to be 7.0 and 185×10−9(V) in summer, 6.2 and 146×10−9(V)in autumn, 5.8 and 137×10−9(V) in spring, and 6.0 and 76.7×10−9(V) in winter, respectively. Continental air masses probably carried atmospheric pollutants to the sampling site, while frequent maritime winds brought in less polluted air masses. Furthermore, positive correlations (R: 0.72−0.85) between PAN and O3were found in summer, indicating a predominant role of photochemistry in their formation. Unlike in summer, weak or no correlations between PAN and O3were featured during the other seasons, especially in winter, due to their different loss pathways. Unexpectedly, positive correlations between PAN and PM2.5were found in all seasons. During summer, moderate correlation could be attributed to the strong photochemistry acting as a common driver in the formation of secondary aerosols and PAN. During winter, high PM2.5might promote PAN production through HONO production, hence resulting in a good positive correlation. Additionally, the loss of PAN by thermal decomposition (TPAN) only accounted for a small fraction (about 1%) of the total(PAN+TPAN) during a typical winter episode, while it significantly reached 14.4×10−9(V) (71.1% of the total) in summer. (Zhang Gen)

1.19 Measurement report: Chemical characteristics of PM2.5 during typical biomass burning season at an agricultural site of the North China Plain

Biomass burning activities are ubiquitous in China, especially in North China, where there is an enormous rural population and winter heating custom. Biomass burning tracers (i.e., levoglucosan, mannosan and potassium (K+)), as well as other chemical components were quantified at a rural site (Gucheng, GC) in North China from 15 October to 30 November, during a transition heating season, when the field burning of agricultural residues was becoming intense. The measured daily average concentrations of levoglucosan,mannosan and K+ in PM2.5during this study were 0.79 ± 0.75 μg m−3, 0.03 ± 0.03 μg m−3and 1.52 ± 0.62 μg m−3, respectively. Carbonaceous components and biomass burning tracers showed higher levels at nighttime than daytime, while secondary inorganic ions were enhanced during daytime. An episode with high levels of biomass burning tracers was encountered at the end of October, 2016, with high levoglucosan at 4.37 µg m−3. Based on the comparison of chemical components during different biomass burning pollution periods,it appeared that biomass combustion can obviously elevate carbonaceous components levels, whereas no essentially effect on secondary inorganic aerosols in the ambient air. Moreover, the levoglucosan/mannosan ratios during different biomass burning pollution periods remained at high values (in the range of 18.3−24.9);however, the levoglucosan/K+ ratio was significantly elevated during the intensive biomass burning pollution period (1.67) when air temperatures decrease, substantially higher than those in other biomass burning periods(averaged at 0.47). (Liang Linlin)

1.20 Characteristics and potential sources of wintertime air pollution in Linfen, China

Linfen in China’s Shanxi Province suffers severe air pollution in winter. Understanding the characteristics of air pollution and providing scientific support to mitigate such pollution are urgent matters. This study investigated the variations of PM2.5, PM10, NO2, SO2, O3, and CO in Linfen between December 1, 2019 and February 29, 2020. The mean concentrations of PM2.5, PM10, NO2, SO2, MDA8 (the maximum daily 8-h average) O3, and CO were 106.2, 139.4, 47.2, 41.0, 57.0 μg m−3, and 1.8 mg m−3, respectively. Large amounts of pollutants emitted by coal burning, industry, vehicles, and residents contributed to air pollution.Unfavorable meteorological conditions, such as lower temperature, weaker wind, higher relative humidity,and reduced planetary boundary layer height, made the situation worse. Fireworks and firecrackers set off to celebrate traditional Chinese festivals caused the concentration of PM pollutants to spike, with the maximum daily mean concentration of PM2.5reaching 314 μg m−3and the peak hourly value reaching 378.0 μg m−3.Suspensions of commercial and social activities due to COVID-19 reduced anthropogenic emissions, mainly from industry and transportation, which decreased the level of air pollutants other than O3. Analyses involving backward trajectory cluster, the potential source contribution function, and concentration weighted trajectory demonstrated that PM2.5pollution mainly came from local emissions in Shanxi Province and regional transport from Inner Mongolia, Shaanxi, Hebei, Henan, and Gansu provinces. Shanxi and its surrounding provinces should adopt measures such as tightening environmental management standards, promoting the use of renewable energy, and adjusting the transportation structure to reduce regional emissions. This study will help policy-makers draft plans and policies to reduce air pollution in Linfen. (Liu Lei)

1.21 Aerosol promotes peroxyacetyl nitrate formation during winter in the North China Plain

Peroxyacetyl nitrate (PAN) is an important indicator for photochemical pollution, formed similar to ozone in the photochemistry of certain volatile organic compounds (VOCs) in the presence of nitrogen oxides, and has displayed surprisingly high concentrations during wintertime that were better correlated to particulate rather than ozone concentrations, for which the reasons remained unknown. In this study, wintertime observations of PAN, VOCs, PM2.5, HONO, and various trace gases were investigated to find the relationship between aerosols and wintertime PAN formation. Wintertime photochemical pollution was affirmed by the high PAN concentrations (average: (1.2 ± 1.1)×10−9, maximum: 7.1×10−9), despite low ozone concentrations.PAN concentrations were determined by its oxygenated VOC (OVOC) precursor concentrations and the NO/NO2ratios and can be well parameterized based on the understanding of their chemical relationship. Data analysis and box modeling results suggest that PAN formation was mostly contributed by VOC aging processes involving OH oxidation or photolysis rather than ozonolysis pathways. Heterogeneous reactions on aerosols have supplied key photochemical oxidants such as HONO, which produced OH radicals upon photolysis,promoting OVOC formation and thereby enhancing PAN production, explaining the observed PM2.5-OVOCPAN intercorrelation. In turn, parts of these OVOCs might participate in the formation of secondary organic aerosol, further aggravating haze pollution as a feedback. Low wintertime temperatures enable the long-range transport of PAN to downwind regions, and how that will impact their oxidation capacity and photochemical pollution requires further assessment in future studies. (Zhang Gen)

1.22 Unexpected deep mixing layer in the Sichuan Basin, China

In the Sichuan Basin (SCB), one of the four major basins in China, a one-year continuous observation study was performed using a ceilometer in Chengdu (November 2018–December 2019). The results show that the mixing layer height (MLH) in the SCB exhibits a bimodal seasonal variation pattern, with peaks in June (842 m) and October (704 m) and valleys in September (535 m) and January (607 m). Stable atmospheric conditions in September were the main reason for the decrease in MLH. Through comparison to other regions in China,it is found that the seasonal evolution of the MLH in the SCB is similar to that in the Yunnan-Guizhou Plateau region and more pronounced than that in the central and eastern plain regions of China. This indicates that the change in MLH is influenced by both the Western Pacific subtropical high and South Asian high. Combined with an analysis of the fine-particulate matter concentration, it is found that the main influence factor of the air pollution in the SCB is not the atmospheric dilution capability, and the local contribution should be paid more attention. The MLH as a meteorological index for air quality prediction varies from place to place. This study is of great importance to the understanding of the mixing layer structure in the basin and its influence on air pollution. (Liu Yusi)

1.23 Impact of residual layer transport on air pollution in Beijing, China

The residual layer (RL) stores a large amount of pollutants, but its effect on near-surface pollution is unknown. In this study, a two-year continuous observation was performed in Beijing using a ceilometer. The generalized boundary layer includes the mixing layer and RL. The results showed that there is no significant seasonal difference in the generalized boundary layer height (GBLH). The average GBLHs in spring, summer,autumn and winter are 1155, 1139, 1036 and 1195 m, respectively. The diurnal variation characteristics of spring, summer and autumn are similar, and the RL disappears when the mixing layer height reaches its peak in the afternoon. In winter, the development of the mixing layer is weak, and there is a 33.8% chance that the RL cannot be breached, thus making the mixing layer height at noon much lower than the GBLH. The concentrations of PM2.5in the mixing layer and RL are 89 and 52 mg m−3, respectively, and the probability that the PM2.5concentration in the RL was higher than that near the ground was 38.9%. RL transport represents an important beginning of the pollution event during the winter mornings and afternoons in Beijing. This study is helpful to better understand the structure of the RL and its influence on air pollution. (Liu Yusi)

1.24 基于航测的云底气溶胶活化率和过饱和度估算

2016年11月13日在北京地区上空存在持续稳定的层状云天气背景下，利用飞机开展了气溶胶粒径谱、化学组成、云滴谱等参量的垂直观测，研究了本次个例中云底气溶胶的活化能力。结果表明，探测期间北京地区为轻度污染天气，地面气溶胶浓度（0.11～3 μm）达到了4600 cm-3。云层高度为800～1200 m，云底气溶胶数浓度相对于近地面大幅度降低，而有效粒径显著增大（0.3～0.6 µm）。同时，近地面气溶胶中疏水性的一次有机气溶胶POA贡献显著，而云底气溶胶中POA的贡献大幅降低，无机组分和二次有机气溶胶SOA的贡献明显增大，造成吸湿性参数κ由0.25（地面）增大至0.32（云底）。云中气溶胶和云滴的谱分布可很好地衔接，且两者的数浓度之和与云底气溶胶浓度一致，可分别代表未活化和已活化的粒子。基于云底气溶胶粒径谱和吸湿性参数计算得到不同过饱和比下云凝结核（CCN）的活化率，通过与实测的云中结果对比，反推得到此次个例的云中的过饱和度约为0.048%。（刘全）

1.25 江西景德镇站大气CH4和CO季节变化及源解析

基于江西景德镇温室气体站2017年12月至2018年11月筛分获得的CH4及CO大气本底和污染浓度数据，对大气CH4和CO浓度季节变化及其排放源特征进行研究，结果表明，大气CH4和CO本底浓度季节变化特征与浙江临安本底站类似，即夏季低而冬季高,而夏季江西地区水稻田和湿地排放导致CH4污染浓度显著抬升，相比本底浓度抬升幅度可达133.9×10-9，冬季受西北部地区取暖排放的区域输送的影响，1月CO污染平均浓度较本底浓度抬升达227.2×10-9。基于本底数据及污染数据，结合后向轨迹模型分析发现景德镇站大气CO潜在排放源主要分布在湖北东南部（四季）、安徽（秋冬季）、山东中部（秋季）、长三角上海及杭州（夏秋季）、湖南东部和江西地区（冬季）等区域，其中冬季湖南东部和江西地区贡献率达53.7%，CH4排放源主要集中在江西地区（夏季）、长江三角洲杭州、南京及安徽南部覆盖区域（夏季）、湖北东南部（夏秋季）以及安徽（秋季）、山东中部（秋季）等区域，夏季南京、杭州及安徽南部覆盖区域的CH4排放对景德镇站CH4浓度抬升的贡献率达到69.5%。大气CH4及CO呈现较好的相关性，冬季其相关系数可达0.86，受CH4和CO源汇季节变化影响，CH4/CO排放比呈现冬季低值（0.31）、夏季高值（1.06）。（张根）

2 环境气象数值预报模式发展及大气成分与天气、气候相互作用研究

2 Development of environmental meteorological numerical prediction model and studies of interactions between atmospheric compositions and weather/climate

2.1 Biological crust in sand and dust storm source areas of Asia and its impact on dust emission

Even though the biological crusts are critical to dust emissions, no sand and dust forecast model have considered the impacts of the biological crust in dust emission scheme. This situation mainly comes from two scientific difficulties: there is no large scale regional biological crust data available that can be used in the forecast model; there is no quantification of how biological crusts impact on sand emission. In this way, we studied the distribution of biological soil crust in sand and dust storm (SDS) source areas of the Central and East Asia using moderate resolution imaging spectroradiometer satellite surface reflectance data collected in 2000−2019 to determine its potential impact on dust emission according to two empirical schemes. We further evaluated the relationships between soil crust coverage, roughness length, and dust emission to study SDS source areas. We found that biological crust is widely distributed in SDS source areas of the Central and East Asia, with coverage rates of 19.8% in Central Asian deserts, 23.1% in the Gobi Desert, and 17.3% to 32.8%in Chinese deserts ( p > 0.05). Cyanobacteria and lichen coverage has increased in Chinese deserts, reflecting the recent impacts of the Project of Returning Farmland to Grassland and Farmland to Forests. However,biological soil crust coverage has not increased in Central Asian deserts or the Gobi Desert, and that in Central Asian deserts continues to decrease, demonstrating the complexity of the combined effects of human activities and climate change on its distribution. Biological soil crust increased the roughness length of Central and East Asian SDS source areas by 0.14−0.62 mm. The suppression of dust emission due to biological soil crust did not change among years during the study period. The horizontal and vertical dust flux inhibition coefficient(DFIC) were 2.0−11.0 and 1.7−2.9 (p > 0.05), respectively, clearly showing a suppressive effect. Improvement of the ecological environment in some deserts can lead to the ability of these crusts to inhibit dust erosion errors that must be considered in the dust emission scheme for areas where crust coverage has improved. (Zhou Chunhong)

2.2 Development of key physicochemical mechanisms in CUACE model

The development of chemical transport models with advanced physics and chemical schemes could improve air-quality forecasts. In this study, the China Meteorological Administration unified atmospheric chemistry environment (CUACE) model, a comprehensive chemistry module incorporating gaseous chemistry and a size-segregated multicomponent aerosol algorithm, was coupled to the weather research and forecasting(WRF) framework with chemistry (WRF-Chem) using an interface procedure to build the WRF/CUACE v1.0 model. The latest version of CUACE includes an updated aerosol dry deposition scheme and the introduction of heterogeneous chemical reactions on aerosol surfaces. We evaluated the WRF/CUACE v1.0 model by simulating PM2.5, O3, NO2, and SO2concentrations for January, April, July, and October (representing winter,spring, summer and autumn, respectively) in 2013, 2015, and 2017 and comparing them with ground-based observations. Secondary inorganic aerosol simulations for the North China Plain (NCP), Yangtze River Delta(YRD), and Sichuan Basin (SCB) were also evaluated. The model captured well the variations of PM2.5, O3,and NO2concentrations in all seasons in the eastern China. However, it is difficult to accurately reproduce the variations of air pollutants over SCB, due to its deep basin terrain. The simulations of SO2were generally reasonable in the NCP and YRD with the bias at 15.5% and 24.55%, respectively, while they were poor in the Pearl River Delta (PRD) and SCB. The sulfate and nitrate simulations were substantially improved by introducing heterogeneous chemical reactions into the CUACE model (e.g., change in bias from 95.0% to 4.1%for sulfate and from 124.1% to 96.0% for nitrate in the NCP). Additionally, The WRF/CUACE v1.0 model was revealed with better performance in simulating chemical species relative to the coupled fifth-generation Penn State/NCAR mesoscale model (MM5) and CUACE model. The development of the WRF/CUACE v1.0 model represents an important step towards improving air-quality modeling and forecasts in China. (Zhang Lei, Gong Sunling)

2.3 Development of four-dimensional variational assimilation system based on the GRAPESCUACE adjoint model (GRAPES-CUACE-4D-Var V1.0) and its application in emission inversion

In this study, a four-dimensional variational (4DVar) data assimilation system was developed based on the GRAPES-CUACE (global/regional assimilation and prediction system-CMA unifified atmospheric chemistry environmental forecasting system) atmospheric chemistry model, GRAPES-CUACE adjoint model and L-BFGSB (extended limited-memory Broyden-Fletcher-Goldfarb-Shanno) algorithm (GRAPES-CUACE-4D-Var) and was applied to optimize black carbon (BC) daily emissions in the northern China on 4 July 2016,when a pollution event occurred in Beijing. The results show that the newly constructed GRAPES-CUACE-4D-Var assimilation system is feasible and can be applied to perform BC emission inversion in the northern China. The BC concentrations simulated with optimized emissions show improved agreement with the observations over the northern China with lower root-mean square errors and higher correlation coeffificients.The model biases are reduced by 20%‒46%. The validation with observations that were not utilized in the assimilation shows that assimilation makes notable improvements, with values of the model biases reduced by 1%‒36%. Compared with the prior BC emissions, which are based on statistical data of anthropogenic emissions for 2007, the optimized emissions are considerably reduced. Especially for Beijing, Tianjin, Hebei,Shandong, Shanxi and Henan, the ratios of the optimized emissions to prior emissions are 0.4–0.8, indicating that the BC emissions in these highly industrialized regions have greatly reduced from 2007 to 2016. In the future, further studies on improving the performance of the GRAPES-CUACE-4D-Var assimilation system are still needed and are important for air pollution research in China. (Wang Chao, An Xingqin)

2.4 Application of turbulent diffusion term of aerosols in mesoscale model

The presence of unfavorable meteorological conditions triggers pollution, and then further weakens turbulence, especially in the stable boundary layer (SBL), which is a frequent situation in heavy pollution episodes in China. The inapplicability of the classical Monin-Obukhov similarity theory (MOST) and the uncertainty of the planetary boundary layer height can lead to large deviation of turbulent diffusion in the SBL in numerical simulations. However, in current mesoscale models, no term has been used to accurately describe the turbulent diffusion of aerosols. Therefore, we use the Mixing-Length theory to obtain the turbulent diffusion term of aerosols based on high-resolution observational data, and, for the first time, embed this term into a mesoscale model, which makes the turbulent diffusion process of aerosols more truly depicted. Results from a two-way coupled atmospheric-chemistry mesoscale model demonstrate that the turbulent diffusion term of aerosols can improve the problem of overestimated PM2.5concentration in the eastern China. (Jia Wenxing,Zhang Xiaoye)

2.5 Impact of modified turbulent diffusion of PM2.5 aerosol in WRF-Chem simulations in eastern China

Correct description of the boundary layer mixing process of particle is an important prerequisite for understanding the formation mechanism of pollutants, especially during heavy pollution episodes. Turbulent vertical mixing determines the distribution of momentum, heat, water vapor and pollutants within the planetary boundary layer (PBL). However, what is questionable is that turbulent mixing process of particles is usually denoted by turbulent diffusion of heat in the WRF-Chem model. With mixing-length theory, the turbulent diffusion relationship of particle is established, embedded into the WRF-Chem and verified based on long-term simulations from 2013 to 2017. The new turbulent diffusion coefficient is used to represent the turbulent mixing process of pollutants separately, without deteriorating the simulation results of meteorological parameters.The new turbulent diffusion improves the simulation of pollutant concentration to varying degrees, and the simulated results of PM2.5concentration are improved by 8.3% (2013), 17% (2014), 11% (2015) and 11.7%(2017) in the eastern China, respectively. Furthermore, the pollutant concentration is expected to increase due to the reduction of turbulent diffusion in mountainous areas, but the pollutant concentration did not change as expected. Therefore, under the influence of complex topography, the turbulent diffusion process is insensitive to the simulation of the pollutant concentration. For mountainous areas, the evolution of pollutants is more susceptible to advection transport, because of the simulation of obvious wind speed gradient and pollutant concentration gradient. In addition to the PM2.5concentration, the concentration of CO as a primary pollutant,has also been improved, which shows that the turbulent diffusion process is extremely critical for variation of the various aerosol pollutants. Additional joint research on other processes (e.g., dry deposition, chemical and emission processes) may be necessary to promote the development of the model in the future. (Jia Wenxing,Zhang Xiaoye)

2.6 Development and application of a street-level meteorology and pollutant tracking system

A multi-model simulation system for street level circulation and pollutant tracking (S-TRACK) has been developed by integrating the weather research and forecasting (WRF), the STAR-CCM+ (computational fluid dynamics model, CFD) and the flexible particle (FLEXPART) models. The winter wind environmental characteristics and the potential contribution of traffic sources on nearby receptor sites in a city district of China are analysed with the system for January 2019. It is found that complex building layouts change the structure of the wind field and thus have an impact on the transport of pollutants. The wind speed inside the building block is smaller than the background wind speed due to the dragging effect of dense buildings. Ventilation is better when the dominant airflow is in the same direction as the building layout. Influenced by the building layout, the local circulations show that the windward side of the building is mostly the divergence zone and the leeward side is mostly the convergence zone, which is more obvious for high buildings. With the hypothesis that the traffic sources are uniformly distributed on each road and with identical traffic intensity, the potential contribution ratios (PCR) of four traffic sources to certain specific sites under the influence of the street-level circulations are estimated with the method of residence time analysis. It is found that the contribution ratio varies with the height of the receptor site. As a result of the generally upward motion in the airflow, the position with the greatest PCR from the four road traffic sources is located on a certain height which is commonly influenced by the distance of this location from the traffic source and the background wind field (about 15 m in this study). The potential contribution of a road to one of the receptor sites is also investigated under different wind directions. The established system and the results can be used to understand the characteristics of urban wind environment and to help the air pollution control planning in urban areas. (Zhang Huan, Gong Sunling)

2.7 A new parameterization of uptake coefficients for heterogeneous reactions on multi-component atmospheric aerosols

Based on laboratory studies and field observations, a new parameterization of uptake coefficients for heterogeneous reactions on multi-component aerosols is developed in this work. The equivalent ratio (ER)of inorganic aerosol is used to establish the quantitative relationship between the heterogeneous uptake coefficients and the composition of aerosols. Incorporating the new ER-dependent scheme, the WRF-CUACE model has been applied to simulate sulfate mass concentrations during December 2017 in the Beijing-Tianjin-Hebei region and evaluate the role of aerosol chemical components played in the sulfate formation. Simulated temporal variations and magnitudes of sulfate show good agreement with the observations by using this new scheme. From clean to polluted cases, although both dominant cations and anions increase significantly, the equivalent ratio decreases gradually and is closer to unity, representing the variation of aerosol compositions,which inhibits the heterogeneous uptake of SO2, with the uptake coefficient decreasing from 1×10−4to 5.3×10−5.Based on this phenomenon, a self-limitation process for heterogeneous reactions with the increasing secondary inorganic aerosol from clean to polluted cases is proposed. (Zhou Yike, Gong Sunling)

2.8 Impact of Arctic oscillation anomalies on winter PM2.5 in China via a numerical simulation

The impact of Arctic oscillation (AO) anomalies on winter PM2.5variability in China was investigated using a numerical modeling system (WRF-CUACE). The model results showed that the influence of AO anomalies on winter PM2.5concentration was mainly concentrated in the eastern China, especially in the Central China (CEN), Beijing-Tianjin-Hebei (BTH), the Yangtze River Delta (YRD), and Pearl River Delta(PRD) and was mostly consistent with the conclusions of a previous analysis using haze data. Winter PM2.5concentrations in CEN and BTH increased under abnormally high AO and decreased under abnormally low AO due to the subsequent changes in specific meteorological conditions, such as temperature, wind speed, and boundary layer height. Winter PM2.5decreased in the YRD and PRD in both abnormally high and low AO years due to more favorable vertical transport conditions and regional transport capacity compared with those of other regions. In addition to meteorological factors, AO anomalies also impacted PM2.5depositions in winter,with more apparent effects in the southern China. It is found that AO had a larger impact on dry deposition than on wet deposition, and dry deposition was a dominant factor affecting PM2.5concentrations in CEN. (Lu Shuhua, Gong Sunling)

2.9 Impacts of long-range transports from Central and South Asia on winter surface PM2.5 concentrations in China

A quantitative analysis of the impacts of particulate matter transported from Central and South Asia on winter surface PM2.5concentrations in China is investigated from 2013 to 2017. The chemical boundary conditions generated by the MOZART4 global model (MOZ-CBC) are used to drive WRF-Chem regional model as the long-range transport inflow to China. The long-range transport effects of PM2.5were estimated by the difference caused by primary aerosol (PA), secondary aerosol (SIA), and dust separately from the corresponding component changes in MOZ-CBC. On the five-year average, the long-range transports of particulate matter increase the winter surface dust concentrations by 6‒30 μg m−3, PA concentrations by 0–2 μg m−3, and SIA concentrations by 0‒1.2 μg m−3. Except for Xinjiang, which is closest to the western border,North China Plain is the most significant region in the mainland of China that could be affected by the longrange transport, indicated by the average increase at 1.6 μg m−3(4.5%) in PA, 0.9 μg m−3(1.8%) in SIA,and 16 μg m−3(35%) in dust. The average increment in PA, SIA, and dust is decreased during El Niño and increased during La Niña. Wind anomalies in the El Niño event weaken prevailing westerly wind but favor the meridional circulation, increasing the precipitation and wet scavenging contribution in the eastern seaboard and resulting in the decrease of the PM2.5concentration in China caused by long-range transport, while these phenomena are opposite in La Niña events. However, the MOZART4 model might overestimate the frequency of dust storms in winter and the vertical height that dust could reach. (Mo Jingyue, Gong Sunling)

2.10 Assessment of meteorology vs. control measures in the China fine particular matter trend from 2013 to 2019 by an environmental meteorology index

A framework was developed to quantitatively assess the contribution of meteorology variations to the trend of fine particular matter (PM2.5) concentrations and to separate the impacts of meteorology from the control measures in the trend, based upon the environmental meteorology index (EMI). The model-based EMI realistically reflects the role of meteorology in the trend of PM2.5and is explicitly attributed to three major factors: deposition, vertical accumulation and horizontal transports. Based on the 2013–2019 PM2.5observation data and re-analysis meteorological data in China, the contributions of meteorology and control measures in nine regions of China were assessed separately by the EMI-based framework. Monitoring network observations show that the PM2.5concentrations have declined by about 50% on the national average and by about 35% to 53% for various regions. It is found that the nationwide emission control measures were the dominant factor in the declining trend of China PM2.5concentrations, contributing about 47% of the PM2.5decrease from 2013 to 2019 on the national average and 32% to 52% for various regions. The meteorology has a variable and sometimes critical contribution to the year-by-year variations of PM2.5concentrations, 5% on the annual average and 10%‒20% for the fall-winter heavy pollution seasons. (Gong Sunling)

2.11 Development and application of an automated air quality forecasting system based on machine learning

As one of the most concerned issues in the modern society, air quality has received extensive attentions from the public and the government, which promotes the continuous development and progress of air quality forecasting technology. In this study, an automated air quality forecasting system based on machine learning has been developed and applied for daily forecasts of six common pollutants (PM2.5, PM10, SO2, NO2, O3, and CO) and pollution levels, which can automatically find the best “Model + Hyperparameters” without human intervention. Five machine learning models and an ensemble model (Stacked Generalization) were integrated into the system, supported by a knowledge base containing the meteorological observed data, pollutant concentrations, pollutant emissions, and model reanalysis data. Then five-year data (2015‒2019) of Beijing,Shanghai, Guangzhou, Chengdu, Xi’an, Wuhan, and Changchun in China, were used as an application case to study the effectiveness of the automated forecasting system. Based on the analysis of seven evaluation criteria and pollution level forecasts, combined with the forecasting results for the next 3-days, it is found that the automated system can achieve satisfactory forecasting performance, better than most of numerical model results. This implied that the developed system unveils a good application prospect in the field of environmental meteorology. (Ke Huabin, Gong Sunling)

2.12 Incorrect Asian aerosols affecting the attribution and projection of regional climate change in CMIP6 models

Anthropogenic aerosol (AA) forcing has been shown as a critical driver of climate change over Asia since the mid-20th century. Here we show that almost all Coupled Model Intercomparison Project phase 6(CMIP6) models fail to capture the observed dipole pattern of aerosol optical depth (AOD) trends over Asia during 2006–2014, the last decade of CMIP6 historical simulation, due to an opposite trend over the eastern China compared with observations. The incorrect AOD trend over China is attributed to problematic AA emissions adopted by CMIP6. There are obvious differences in simulated regional aerosol radiative forcing and temperature responses over Asia when using two different emissions inventories (one adopted by CMIP6; the other from Peking University, a more trustworthy inventory) to drive a global aerosol-climate model separately.We further show that some widely adopted CMIP6 pathways (after 2015) also significantly underestimate the more recent decline in AA emissions over China. These flaws may bring about errors to the CMIP6-based regional climate attribution over Asia for the last two decades and projection for the next few decades,previously anticipated to inform a wide range of impact analysis. (Wang Zhili)

2.13 Reduction in European anthropogenic aerosols and the weather conditions conducive to PM2.5 pollution in North China: a potential global teleconnection pathway

Frequent and severe PM2.5pollution over China seriously harms natural environment and human health.Changes in meteorological conditions in recent decades have been recognized to contribute to the longterm increase in PM2.5pollution in North China (NC). However, the dominant climatic factors driving the interdecadal changes of the weather conditions conducive to PM2.5pollution remain unclear. Here we identify a potential global teleconnection mechanism: the decadal reduction in European aerosol emissions since the 1980s may have partially contributed to the interdecadal increase in weather conditions conducive to PM2.5pollution in NC, measured by an emission-weighted air stagnation index (ASIE) that increases at a rate of 6.2%per decade (relative to the 1981–1985 level). By regression analysis, we show that the decreased European aerosol loadings can warm the lower atmosphere and induce anomalous ascending motion in Europe, which potentially stimulates two anomalous Rossby wave trains in the upper troposphere travelling eastward across Eurasia. The teleconnection patterns project on NC by weakening the near-surface horizontal dispersion, which may be favorable to the increase in local ASIE and air pollution build-up. The suggested mechanism is further supported by the results from a set of large-ensemble simulations, showing that the European aerosol emission decline since the 1980s excites similar local heating and ascending motion and leads to increasing trends of 0.1‒0.5 μg m−3per 38 years in surface sulfate concentrations over most of NC. This proposed “West-to-East Aerosol-to-Aerosol” teleconnection mechanism helps resolve opposite views on the impact of global versus local aerosol forcing on PM2.5pollution weather in NC. The policy implication is that the sustained decline in European aerosol emissions in coming decades, in conjunction with unabated global and regional warming,could further exacerbate air pollution in NC, thus imposing stronger pressure to reduce local emission sources quicker and deeper. (Wang Zhili)

2.14 Responses of the East Asian summer monsoon to aerosol forcing in CMIP5 models: The role of upper-tropospheric temperature change

We quantitatively distinguish the fast and slow responses of the East Asian summer monsoon (EASM)to aerosol forcing using the outputs from 16 Coupled Model Intercomparison Project phase 5 (CMIP5)models. The mechanism of the EASM change due to aerosol forcing is then evaluated from the perspective of upper-tropospheric temperature change. The results show that aerosol forcing leads to the weakening of the EASM circulation and decreases in precipitation. The aerosol-induced fast atmospheric response dominates the weakening of EASM and the decreased precipitation over the eastern China. In the fast response, uppertropospheric cooling in the midlatitudes of East Asia during summer changes the circulation structure, thereby causing the weakening of the EASM. The formation of upper-tropospheric cooling is closely related to the eastward propagation of atmospheric cooling caused by aerosol forcing in Europe and the resulting change in local meridional heat transport. The slow ocean-mediated response to aerosol forcing partially offsets the weakening of the EASM over the eastern China in the fast response. In the slow response, southwesterly wind is enhanced and precipitation is increased over the eastern China, while southwesterly wind is weakened over the northwestern Pacific and the South China Sea, which dominates the decrease in precipitation over the oceans. The aerosol-induced changes of land‒sea thermal contrasts further confirm that the decrease of uppertropospheric land-sea thermal contrasts over East Asia in the fast response plays a key role in driving the weakening of the EASM. (Wang Zhili)

2.15 Simulation of the influence of a fine-scale urban underlying surface on the urban heat island effect in Beijing

In this study, the weather research and forecasting (WRF) model coupled with slab model and single-layer urban canopy model (SLUCM) was used to simulate the urban heat island effect in Beijing. The effects of the refined local climate zone (LCZ) urban underlying surface and Moderate resolution Imaging Spectroradiometer(MODIS) land use data on temperature simulated results were compared through sensitivity tests. These two types of underlying surface data present similar simulation error tendencies, showing an overestimation of the 2 m temperature at night. The simulated temperature results using MODIS land use data are closer to the observations than those simulated using the LCZ data. However, the MODIS land use data provide one classification for urban surface, which cannot reflect the complex urban morphology, whereas the LCZ concept divides urban land surface into ten categories that can reflect the surface characteristics more accurately. The sensitivity of different urban canopy parameters on temperature simulation provides priority parameters for model improvement. Real data were used to improve the parameter table setting in the urban canopy model and thus enhance the simulation ability of the LCZ test. The urban fraction, emissivity, and albedo in the parameter table were calculated and replaced. It was found that the use of the accurate emissivity provided the best simulation results. There are negative correlations between near-surface temperature and emissivity/albedo, and a positive correlation between near-surface temperature and urban fraction. The influence of the parameters on the simulated temperature at night was more evident than that during the daytime. (He Jianjun)

2.16 The impact of the variation in weather and season on WRF dynamical downscaling in the Pearl River Delta region

In this study, the National Centers for Environmental Prediction (NCEP) final operational global analysis data and meteorological observation data from 2013 to 2017 were used to evaluate the impact of seasonal changes and different circulation classifications on the dynamical downscaling simulation results of weather research and forecasting (WRF) in the Pearl River Delta (PRD) region. The results show that the dynamical downscaling method can accurately simulate the time variation characteristics of the nearsurface meteorological field and the hit rates of a 2-m temperature, 2-m relative humidity, 10-m wind speed,and 10-m wind direction are 92.66%, 93.98%, 26.78%, and 76.78%, respectively. The WRF model slightly underestimates the temperature and relative humidity, and overestimates the wind speed and precipitation. For precipitation, the WRF model can better simulate the variation characteristics of light rain and heavy rain, with the probability of detection are 0.59 and 0.69, respectively. For seasonal factors, the WRF model can conduct a perfect simulation in autumn and winter, followed by spring, while summer is vulnerable to extreme weather,so the result of the simulation is relatively poor. The circulation type is an important parameter of downscaling assessment. When the PRD is controlled by high pressure, the simulated results of WRF are good, and when the PRD is affected by low pressure or extreme weather, the simulation results are relatively poor. (He Jianjun)

2.17 Simulation study on regional atmospheric oxidation capacity and precursor sensitivity

In this study, an evaluation index of atmospheric oxidation capacity (AOIe) was established. A typical pollution event from October 3 to 8, 2015, and the WRF-Chem air quality model were used to simulate the distribution characteristics of atmospheric oxidation capacity (AOC) in Beijing and the influence of reducing emissions (NOxand VOCs) on regional AOC. The results showed that urban areas have stronger (approximately 11% higher on average) AOC than suburban areas. AOIe_G (the process of valence change in gaseous oxidants) contributed more to the regional AOC during the clean period. AOIe_G also contributed more in the daytime whereas AOIe_P (the process of valence change in particulate state) contributed more at nighttime,with the highest contribution accounting for 72% and 69%, and the corresponding peak concentrations appearing at 14:00 and 06:00, respectively. In addition, compared with NOxreduction, the effect of reducing VOCs on reducing AOC was more significant. (Li Jiangtao, An Xingqin)

2.18 Simulated sensitivity of ozone generation to precursors in Beijing during a high O3 episode

This study uses the WRF-Chem model combined with the empirical kinetic modeling method (EKMA curve) to study the compound pollution event in Beijing that happened in 13−23 May 2017. Sensitivity tests are conducted to analyze ozone sensitivity to its precursors, and to develop emission reduction measures. The results suggest that the model can accurately simulate the compound pollution process of photochemistry and haze. When VOCs and NOxwere reduced by the same proportion, the effect of O3reduction at peak time was more obvious, and the effect during daytime was more significant than at night. The degree of change in ozone was peak time > daytime average. When reducing or increasing the ratio of precursors by 25% at the same time, the effect of reducing 25% VOCs on the average ozone concentration reduction was most significant.The degree of change in ozone decreased with increasing altitude, the location of the ozone maximum change shifted westward, and its range narrowed. As the altitude increases, the VOCs-limited zone decreases, VOCs sensitivity decreases, and NOxsensitivity increases. The controlled area changed from near-surface VOCslimited to high-altitude NOx-limited. Upon examining the EKMA curve, we have found that suburban and urban are sensitive to VOCs. The sensitivity tests indicate that when VOCs in suburban are reduced about by 60%, the O31 h concentration could reach the standard, and when VOCs of the urban decreased by about 50%,the O31 h concentration could reach the standard. Thus, these findings could provide references for the control of compound air pollution in Beijing. (An Xingqin)

2.19 Influence of East Asian winter monsoon on particulate matter pollution in typical regions of China

This study uses the NCEP/NCAR monthly average reanalysis and number of haze day data during 1958–2017, and the average daily PM2.5mass concentration data during 2013–2017, to calculate the East Asian winter monsoon index (EAWMI) and statistically analyze the correlation between the winter monsoon index and air quality in China, particularly for the five typical regions (Beijing-Tianjin-Hebei, Fen-Wei Plain,Sichuan Chongqing Delta, Yangtze River Delta, and Pearl River Delta). Thereafter, the strong and weak winter monsoon years were classified based on the EAWMI, and the atmospheric circulation and temperature fields over China and the five regions in different winter monsoon years were spatially compared. Finally, the study also investigated the various distribution features of the climatic circulation background responsible for the strong and weak winter monsoons and their impact mechanisms on air quality in the five typical regions in China. The results show that the effect of the winter monsoon on the air quality of China may be represented by a north-south boundary line located at approximately 30°N. During strong winter monsoon years, pollution was lower in the area north of the boundary but higher to its south. By contrast, the opposite phenomenon was observed during the weak monsoon years. During the strong winter monsoon years, the Beijing-Tianjin-Hebei region and Fen-Wei Plain to the north of the boundary line were less polluted, while the Yangtze River, Chengdu-Chongqing, and Pearl River delta regions to the south of the boundary were more polluted.Diagnostic analysis of the circulation field indicated that during the strong winter monsoon years, an abnormal downward airflow occurred to the south of the boundary, limiting convective diffusion and thereby causing the increased pollution. However, during the weak winter monsoon years, ascending airflows occurred, which favored pollutant diffusion. Furthermore, during the strong winter monsoon years, an abnormal southeast airflow with weak horizontal wind speed occurred in the lower atmosphere of the Chengdu-Chongqing region,causing localized pollutant accumulation, thereby aggravating the pollution. In the Pearl River Delta region,a descending abnormal westerly flow inhibited the local uplift and diffusion of air. Moreover, importing pollutants occurred from the north, aggravating the pollution in the region. (Li Yanjun, An Xingqin)

2.20 Key factors determining heterogeneous uptake kinetics of NO2 onto alumina: Implication for the linkage between laboratory work and modeling study

Heterogeneous reactions on mineral dust play pivotal roles in the removal/production of gaseous pollutants and the formation of secondary particles. However, the uptake coefficient (γ), a key kinetic parameter for a heterogeneous reaction, could vary by several orders of magnitude in different laboratory analyses and give rise to great uncertainties in modeling studies. Thus, a detailed understanding of heterogeneous uptake kinetics is vital to accurately evaluate the impacts of heterogeneous reactions on atmospheric chemistry. In order to reveal the key factors affecting uptake kinetics, heterogeneous reaction of NO2on surfaces of typical mineral component alumina (α-Al2O3, γ-Al2O3, δ-Al2O3, AlOOH) was comprehensively studied using two widely used methods, including diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and flow tube reactor. The discrepancy between the true uptake coefficients (γBET) obtained via two techniques was within 1 to 2 orders of magnitude for alumina samples. The γBETdepended positively on NO2concentration in the DRIFTS measurements but negatively on NO2concentration in the flow tube experiments, and the discrepancy might be attributed to different calculation methods of γBET, which was calculated based on nitrate formation kinetics in DRIFTS experiments while based on NO2consumption kinetics in flow tube experiments.The results implied that an accurate selection of the uptake coefficient for modeling studies should base on the consideration of factors such as the measurement method, the concentration range of the reaction gas, and the characteristics of the sample such as crystal structure and effective surface area. ( Liu Chang)

2.21 Relationship between summertime concurring PM2.5 and O3 pollution and boundary layer height differs between Beijing and Shanghai, China

The rapid development in the economy during past decades has caused serious air pollution issues in China with high concentrations of PM2.5and O3, particularly in the densely populous cities. To integrate PM2.5and O3controls, it is necessary to understand the impacts of meteorology on both pollutants. Thereby,the complex linkages between planetary boundary layer (PBL), synoptic forcing, regional transport, and heavy pollution in Beijing and Shanghai during summer were investigated using long-term measurements,simulations, and reanalysis. Influenced by the unfavorable meteorological conditions, PM2.5pollution and O3pollution often simultaneously occurred. In Beijing, the heavy concurring pollutions usually happened on the days with shallow afternoon PBL and southerly/southwesterly prevailing winds. Within the PBL, the pollutants emitted from the southern plains can be transported to Beijing and accumulated on the windward side of the mountains. At the top of PBL, the synoptic southerly warm advections can strengthen the elevated thermal inversion layer and suppress the development of PBL, leading to worse pollution. Contrarily, the heavy pollutions in Shanghai usually occurred on the days with deep afternoon PBL and southwesterly warm advections within the PBL. Although the warm advections were more favorable to the PBL development than the movements of cool marine air mass, the input of pollutants from the southwest can overweigh this advantage, resulting in poor air quality in Shanghai. The occurrence of heavy pollution or clean condition in Shanghai was primarily determined by the synoptic forcing rather than the local PBL structure. This comparative study indicates that the relationship between PBL height and pollution level is changeable and complicated, which needs to be elucidated from the synoptic perspective. (Miao Yucong)

2.22 On the heavy aerosol pollution and its meteorological dependence in Shandong Province,China

Partly due to the lack of fine-resolution measurements of the planetary boundary layer (PBL), the impacts of PBL on the aerosol pollution in the densely populous Shandong Province were not well understood. On the basis of long-term PM2.5measurements, fine-resolution radiosonde data, and meteorological reanalysis from April 2016 to March 2019, the aerosol pollution in Jinan and Qingdao and its complex relationships with the multi-scale meteorological conditions were investigated in this study. During an annual cycle, prominent seasonal variations of PM2.5concentrations can be observed in both cities, with heaviest pollution in the heating season and relatively low concentrations in summer. Significant positive correlation was found between the monthly PM2.5concentrations and thermal stability of the lower troposphere, indicating that the seasonal shifts of PBL play an important role in regulating the variations of aerosol pollution, in addition to the seasonal changes in the emissions. In the heating season, influenced by unfavorable synoptic patterns, heavy pollution often simultaneously happened in Jinan and Qingdao. Utilizing an objective synoptic classification approach with reanalysis data, two dominant synoptic types led to heavy pollution in Jinan and Qingdao were identified,which were featured by 900-hPa warm advections from the west or southwest with weaker prevailing winds.These synoptic types not only strengthened the elevated thermal inversion and inhibited the vertical dilution of pollutants locally, but also caused the regional transports of pollutants to Jinan and Qingdao from highemission upstream regions, such as the Beijing-Tianjin-Hebei region, Henan Province and Jiangsu Province.Therefore, to prevent heavy pollution in Jinan and Qingdao, regional joint measures should be implemented with full consideration of synoptic impact. (Miao Yucong)

2.23 Impacts of synoptic forcing and topography on aerosol pollution during winter in Shenyang,Northeast China

Northeast China frequently experiences aerosol pollution episodes in winter. In addition to the pollutant emissions, synoptic pattern and topography can impact the air quality in complex ways, which are still not well understood in Northeast China. Therefore, the impacts of synoptic forcing and topography on aerosol pollution in Shenyang were investigated combining surface observations, sounding measurements, and three-dimensional air quality simulations. The studied pollution episode occurred from January 1 to 5, 2020, along with poor meteorological dispersion conditions characterized by weak winds, strong thermal stabilities, and shallow planetary boundary layers (PBLs). During the formation of pollution, strong elevated thermal inversion layers were observed over Shenyang, induced by the large-scale synoptic pattern, which suppressed the PBL growth and the vertical dispersion of aerosols. Moreover, the blocking effect of mountains to the east of Shenyang further worsened the pollution when northwesterly/westerly flows prevailed in shallow PBLs. A numerical sensitive experiment was conducted to estimate the contribution of blocking effect of mountains to the nearsurface PM2.5concentration in Shenyang, and it was found that around one third of PM2.5concentration during January 1‒4 was relevant to the terrain effect. These findings can facilitate a comprehensive understanding of the physical formation of aerosol pollution in Northeast China and be helpful for the pollution controls. (Miao Yucong)

2.24 Study on the variation of air pollutant concentration and its formation mechanism during the COVID-19 period in Wuhan

To prevent the spread of COVID-19 (2019 novel coronavirus), from January 23 to April 8 in 2020, the highest Class 1 Response was ordered in Wuhan, requiring all residents to stay at home unless absolutely necessary. This action was implemented to cut down all unnecessary human activities, including industry,agriculture and transportation. Reducing these activities to a very low level during these hard times meant that some unprecedented naturally occurring measures of controlling emissions were executed. Ironically,however, after these measures were implemented, ozone levels increased by 43.9%. Also worthy of note, PM2.5decreased by 31.7%, which was found by comparing the observation data in Wuhan during the epidemic from 8 February to 8 April in 2020 with the same periods in 2019. Utilizing CMAQ (the community multiscale air quality modeling system), this article investigated the reason for these phenomena based on four sets of numerical simulations with different schemes of emission reduction. Comparing the four sets of simulations with observation, it was deduced that the emissions should decrease to approximately 20% from the typical industrial output, and 10% from agriculture and transportation sources, attributed to the COVID-19 lockdown in Wuhan. More importantly, through the CMAQ process analysis, this study quantitatively analyzed differences of the physical and chemical processes that were affected by the COVID-19 lockdown. It then examined the differences of the COVID-19 lockdown impact and determined the physical and chemical processes between when the pollution increased and decreased, determining the most affected period of the day. As a result, this paper found that (1) PM2.5decreased mainly due to the reduction of emission and the contrary contribution of aerosol processes. The northeast wind was also in favor of the decreasing of PM2.5. (2) O3increased mainly due to the slowing down of chemical consumption processes, which made the concentration change of O3pollution higher in 16:00–19:00. of the day, while increasing the concentration of O3at night during the COVID-19 lockdown in Wuhan. The higher O3concentration in the northeast of the main urban area also contributed to the increasing of O3with unfavorable wind direction. (NiuTao)

2.25 Study on the causes of heavy pollution in Shenyang based on the contribution of natural conditions, physical processes, and anthropogenic emissions

This study investigated the impacts of meteorological and geographical conditions, the roles of individual physical processes, and the contribution of emissions during severe PM2.5(fine particulate matter) pollution events in Shenyang, one of the largest industrial cities in China. Simulations of six severe PM2.5pollution events that occurred in 2015 and 2016 revealed that unfavorable meteorological conditions, including high relative humidity, low depth of the planetary boundary layer, low wind speed, changes in wind direction,and unfavorable geographical conditions underly severe PM2.5pollution events in Shenyang. Regarding individual physical processes, emission and aerosol processes increased the concentration of PM2.5, while horizontal advection, vertical advection, and vertical diffusion processes dominated the decrease in PM2.5.Analysis of source apportionment found that residential (37%) and transportation (30%) dominated PM2.5pollution as categories, and Shenbei New District contributed 75% of regional emissions. The main causes of the severe PM2.5pollution events in Shenyang are complex, including local emissions associated with transportation emissions from Shenbei New District, local emissions associated with agricultural emissions and low northwesterly winds, long-range transport with a southeasterly wind, and middle-range transport with a northerly wind. (NiuTao)

2.26 Application of morrison cloud microphysics scheme in GRAPES_Meso model and the sensitivity study on CCN’s impacts on cloud radiation

The Morrison double-moment cloud microphysics scheme is implemented into the GRAPES_Meso model. Sensitivity experiments of different cloud condensation nuclei (CCN) values are conducted to study the impacts of CCN on cloud microphysical processes and radiation processes in East China. The model evaluations illustrate the effectiveness (R = 0.6) of the Morrison scheme for the cloud processes simulations in East China. For the study period of 8 to 12 October 2017, with initial CCN number concentration (CCN0)increasing from 10 to 3000 cm−3, the maximum value of the daily average mixing ratio of cloud water, cloud liquid water path (CLWP), and cloud optical depth (COD) increases by 133%, 100%, and 150%, respectively.However, the maximum value of the daily average mixing ratio of rain decreases by 44%. These impacts on the cloud result in about a 65% increase of the maximum value of daily average cloud downward shortwave radiative forcing (CDSRF). This study indicates the significant impacts of CCN on cloud properties and radiation effects. (Wang Hong)

2.27 Aerosol impacts on cloud physical characteristics and radiation effect

The radiative forcing caused by aerosol-cloud interaction (ACI) is one of the most critical factors that lead to climate research uncertainty. In East China and the adjacent sea areas, the severe air pollution makes the ACI effect stronger than in other regions, but few observational studies focus on the effect of different aerosol components. This study estimates the shortwave radiation effect at the top of the atmosphere (TOA)caused by the interaction between the increased four aerosol components (black carbon, dust, organic carbon,and sulfate) and the warm liquid cloud in East China and the East China Sea, by applying the multiple linear regression to two ACI effect calculating methods proposed by Quaas et al. (2008; Method 1) and Chen et al.(2014; Method 2). The results suggest a cooling effect of aerosol, with sulfate showing the strongest cooling effect and dust showing the strongest warming effect. The meteorological factor has a significant influence on the distribution of the aerosol effect. Regions with higher relative humidity and low-tropospheric stability have a more significant cooling effect due to the suppression of droplet evaporation and entrainment. Meanwhile, in the heavily polluted East China, a relatively higher warm cloud altitude could reduce the in-cloud aerosol and avoid the droplet evaporation caused by absorbing aerosol and the over much cloud condensation nuclei. This research contributes to a better understanding of the aerosol-cloud radiative effect and its mechanism in East China and the East China Sea. (Wang Hong).

2.28 Comparative analysis of PM2.5 and O3 source in Beijing using a chemical transport model

For many years, Beijing has suffered from severe air pollution. At present, fine particulate matter (PM2.5)pollution in the winter and ozone (O3) pollution in the summer constitute serious environmental problems. In this study, the combination of a comprehensive air quality model with particulate matter source apportionment technology (CAMx-PAST) and monitoring data was used for the high-spatial resolution source apportionment of secondary inorganic components (SNA: SO42−, NO3−, and NH4+) in PM2.5and their corresponding precursor gases (SO2, NO2, and NH3) and O3in the winter and summer over Beijing. Emissions from residents, industry,traffic, agriculture, and power accounted for 54%, 25%, 14%, 5%, and 2% of PM2.5in the winter, respectively.In the summer, the emissions from industry, traffic, residents, agriculture, and power accounted for 42%,24%, 20%, 10%, and 4% of PM2.5, respectively. The monthly transport ratio of PM2.5was 27% and 46% in the winter and summer, respectively. The regional transport of residential and industrial emissions accounted for the highest proportion of PM2.5. The regional transport of emissions had a significant effect on the SO42−and NO3−concentrations, whereas SO2and NO2pollution were mainly affected by local emissions, and NH4+and NH3were mainly attributed to agricultural emissions. Industrial and traffic sources were two major emission sectors that contributed to O3pollution in Beijing. The monthly transport ratios of O3were 31% and 65% in the winter and summer, respectively. The high-spatial resolution regional source apportionment results showed that emissions from Langfang, Baoding, and Tangshan had the greatest impact on Beijing’s air pollution. This work’s methods and results will provide scientific guidance to support the government in its decision-making processes to manage the PM2.5and O3pollution issues. (Liu Lei)

2.29 A reliability assessment of the NCEP/FNL reanalysis data in depicting key meteorological factors on clean days and polluted days in Beijing

In this study, based on the National Centers for Environmental Prediction (NCEP) final analysis (FNL)data, the reliability and performances of their application on clean days and polluted days (based on the PM2.5mass concentrations) in Beijing were assessed. Conventional meteorological factors and diagnostic physical quantities from the NCEP/FNL data were compared with the L-band radar observations in Beijing in the autumns and winters of 2017−2019. The results indicate that the prediction reliability of the temperature was the best compared with those of the relative humidity and wind speed. It is worth noting that the relative humidity was lower and the near-surface wind speed was higher on polluted days from the NCEP/FNL data than from the observations. As far as diagnostic physical quantity is concerned, it was revealed that the temperature inversion intensity depicted by the NCEP/FNL data was significantly lower than that from the observations, especially on polluted days. For example, the difference in the temperature inversion intensity between the NCEP/FNL data and the observation ranged from −0.56 to −0.77 on polluted days. In addition,the difference in the wind shears between the NCEP/FNL reanalysis data and the observations increased to 0.40 m/s in the lower boundary layer on polluted days compared with that on clean days. Therefore, it is suggested that the underestimation of the relative humidity and temperature inversion intensity, and the overestimation of the near-surface wind speed should be seriously considered in simulating the air quality in the model,particularly on polluted days, which should be focused on more in future model developments. (Liu Chao, Guo Jianping, Zhang Bihui)

2.30 Characteristics of surface energy balance and atmospheric circulation during hot-and-polluted episodes and their synergistic relationships with urban heat islands over the Pearl River Delta region

This study analyzed the nature, mechanisms and drivers for hot-and-polluted episodes (HPEs) in the Pearl River Delta, China. Numerical model simulations were conducted for the summer and autumn of 2009−2011. A total of eight HPEs were identified, mainly occurring in August and September. k-means clustering was applied to group the HPEs into three clusters based on their characteristics and mechanisms. We found three HPEs were driven by weak subsidence and convection induced by approaching tropical cyclones (TC-HPE) and two HPEs were controlled by calm (stagnant) conditions (ST-HPE) with low wind speed in the lower atmosphere,whereas the remaining three HPEs were driven by the combination (hybrid) of both aforementioned systems(HY-HPE). A positive synergistic effect between the HPE and urban heat island (UHI; similar to 1.1 increase)was observed in TC-HPE and ST-HPE, whereas no discernible synergistic effect was found in HY-HPE. Total aerosol radiative forcing (TARF) caused a reduction in temperature (0.5−1.0 ) in TC-HPE and ST-HPE but an increase (0.5 ) in HY-HPE. (Nduka Ifeanyichukwu C., Tam Chi-Yung, Guo Jianping)

2.31 Distinct spatiotemporal variation patterns of surface ozone in China due to diverse influential factors

A better knowledge of surface ozone variations and the relevant influential factors is of great significance for controlling frequent ozone pollution events. In this study, we first examined the primary variation patterns of surface ozone in space and time across China via a clustering analysis on the basis of daily maximum 8 h average surface ozone (MDA8) between 2015 and 2018. Statistical models were then established between MDA8 and a set of influential factors to pinpoint dominant factors contributing to regional MDA8 variations.The clustering results revealed four typical variation patterns of MDA8 in China given distinct pollution levels,seasonality, and long-term trends. Statistical modeling results indicated that the seasonal variability of MDA8 was closely associated with UV radiation and meteorological factors like boundary layer height, temperature and relative humidity. In contrast, the long-term trends of MDA8 were largely linked to ozone precursors and meteorological variables including temperature, relative humidity, and total cloud cover. Moreover, the phenomenal increasing trends of MDA8 in North China were found to be statistically associated with the depletion of nitrogen dioxide (NO2) and carbon monoxide (CO). Specifically, substantial increases in volatile organic compounds (VOCs) along with depletions in NO2and CO significantly boosted the photochemical ozone formation chain process in a VOC limited regime like the North China Plain. Overall, the inferred linkage in this study provides evidence and clues to help control increasing ozone pollution events in North China. (Ma Mingliang, Yao Guobiao, Guo Jianping)

2.32 Impacts of biomass burning in Peninsular Southeast Asia on PM2.5 concentration and ozone formation in southern China during springtime—A case study

Biomass burning (BB) affects fine particulate matter (PM2.5) concentration and ozone (O3) formation by emitting gaseous precursors and primary aerosols. The Impacts of BB in peninsular Southeast Asia (BBPSEA) on PM2.5concentration and O3formation in the southern China are evaluated using a source-oriented WRF-Chem model to simulate an air pollution episode from March 21 to March 25, 2015. The sourceoriented model separates the emissions from BB-PSEA and other sources and can evaluate the effects of aerosol-radiation interactions (ARIs) and aerosol-photolysis interactions (APIs) from BB-PSEA. Comparisons with observations reveal that the model performs well in simulating the air pollution episode. Sensitivity experiments show that BB-PSEA increases PM2.5concentrations on the regional average by 39.3 µg m−3(68.0%) in Yunnan Province (YNP) and 8.4 µg m−3(24.1%) in other downwind areas (ODAs) in the southern China, including the provinces of Guizhou, Guangxi, Hunan, Guangdong, Jiangxi, Fujian, and Zhejiang. PM2.5enhancement is mainly attributed to primary aerosols in YNP and to secondary aerosols in ODA. BB-PSEA increases O3concentrations by 18.1 µg m−3(19.4%) in YNP and decreases O3concentrations by 3.7 µg m−3(5.3%) in ODA. The O3increase in YNP is attributed to the gaseous emissions of BB-PSEA, and the O3decrease in ODA is caused by the effects of ARI and API from BB-PSEA. NH3emissions from BB-PSEA play a key role in enhancing secondary inorganic aerosols in the southern China and determining increases in PM2.5concentrations in ODA. (Xing Li, Bei Naifang, Guo Jianping)

2.33 Satellite-derived long-term estimates of full-coverage PM1 concentrations across China based on a stacking decision tree model

Fine particles with aerodynamic diameters less than 1 µm (PM1) often exert a greater threaten on human health, and thus it is highly imperative to accurately characterize the spatiotemporal variation of PM1concentrations and to assess the potential health risks. Our study attempted to predict the long-term fullcoverage PM1concentrations across China during 2004−2018 using a stacking decision tree model based on satellite data, meteorological variables, and other geographical covariates. The result suggested that the stacking model captured strong prediction capability with a higher cross-validation (CV) R2value (0.64),and the lower root-mean-square error (RMSE: 18.60 µg m−3) and mean absolute error (MAE: 11.96 µg m−3)compared with the individual model. The higher PM1concentrations were mainly concentrated on North China Plain (NCP), Yangtze River Delta (YRD), and Sichuan Basin due to intensive anthropogenic activities and poor meteorological conditions especially in winter. The annual mean PM1concentration in China exhibited a remarkable increase during 2004−2007 by 1.34 µg m−3year−1(p < 0.05), followed by a gradual decrease during 2007−2018 by 1.61 µg m−3year−1(p < 0.05). After 2013, the mean PM1 concentration at the national scale experienced a dramatic decrease by 2.96 µg m−3year−1(p < 0.05). The persistent increase of PM1concentration across China during 2004−2007 was mainly caused by the rapid increases of energy consumption and inefficient emission control measures, while the dramatic decrease since 2013 was attributed to increasingly strict control measures, particularly the implementation of the Air Pollution Prevention and Control Action Plan (the Action Plan). The long-term PM1estimates obtained here provide a key scientific basis and data support for epidemiological research and air pollution mitigation. (Li Rui, Guo Jianping, Geng Guannan)

2.34 温室气体对亚洲夏季风影响的数值研究

利用NCEP/NCAR再分析资料检验全球气候模式CAM5.1模拟亚洲夏季风的能力，CAM5.1模式能够较好再现亚洲夏季风的基本特征。通过工业革命前、后温室气体排放情景的敏感性试验探讨近现代温室气体增加对亚洲夏季风的影响机制，结果显示：温室气体增加导致亚洲大部分区域地面气温增加，印度半岛中部、中南半岛和中国东部地区季风增强，印度半岛中部及北部、中南半岛中北部和中国东部地区降水增加。分析大气能量收支和转换发现，温室气体增加通过增强大气对流凝结潜热释放的方式加强大气热源；夏季陆地为暖区，不均匀加热引起全位能增加，从而增强全位能向辐散风动能的转换和辐散风动能向无辐散风动能的转换，最终导致这些区域夏季风增强。其中，对流凝结潜热增加是温室气体增加造成大气稳定度降低，对流活动加强，对流云厚度加大，对流降水增加的结果；同时，对流降水增加是总降水增加的主要原因。（刘煜）

2.35 2016—2019年江西省臭氧污染特征与气象因子影响分析

本文利用2016—2019年生态环境部环境监测总站提供的江西省11个设区市的监测数据及同期的国家气象观测站常规观测资料，研究江西省臭氧污染特征与气象因子的关系。结果表明，江西省近几年臭氧污染日益严重，2016年全省臭氧（日最大8 h滑动平均值）平均浓度为80.1 μg/m3，到2019年上升至98.2 μg/m3，平均年增长率为6 μg/m3。2019年江西省11个设区市O3超标总天数为475 d，占总超标天数的72.6%。2016—2018年O3月平均浓度具有典型的季节变化特征：夏季＞春季＞秋季＞冬季，2019年秋季由于降水量显著减少、日照时数增多、气温升高等气象条件导致秋季近地面臭氧浓度异常升高，其平均浓度高于其他季节。臭氧质量浓度总体与气温、日照时数呈正相关，与相对湿度呈负相关，当气温高于30 ℃、相对湿度在20%～40%区间、风速在2～3 m/s区间时易出现高浓度臭氧污染。江西省臭氧浓度呈现一定的空间分布特征：赣东北城市低于其他地区，南部城市高于北部城市。其中，赣州市臭氧污染较为严重，其2019年平均浓度居全省最高，为104.2 μg/m3。基于后向轨迹HYSPLIT模型和潜在源解析PSCF对赣州市进行分析，研究结果表明赣州市臭氧污染的主要潜在贡献源区存在一定的季节差异：春季臭氧污染的外来输送源主要来自广东中部和江西北部地区，夏季主要来自江西北部地区，而秋季则主要来自广东北部和安徽中部地区。（张根）

2.36 基于XGBoost算法的WRF-Chem模式优化模拟研究

采用人工智能算法XGBoost结合大气化学模式WRF-Chem，利用北京地区大气污染物的模拟结果及站点监测数据，构建XGBoost统计预报算法模型，并对两种大气污染物PM2.5和O3进行优化模拟，同时分析其特征贡献要素。结果表明，该统计预报模型能够很好地优化大气化学模式模拟的大气污染物浓度，降低模拟误差，对于北京地区站点模拟浓度优化呈现出城区＞近郊＞远郊的优化特点，且算法模型对O3浓度优化程度更高，优化后相关系数提高达128%。此外，通过特征要素的贡献量分析表明，CO是影响O3优化的重要特征变量，城郊区特征贡献得分均高达1000以上，Q2（近地面2 m比湿）是影响PM2.5优化的重要气象特征变量，城郊区特征贡献得分分别为950和824。该研究结果为深入分析优化算法模型对大气污染物的优化原理及其影响因素的定量研究提供一种新思路。（李江涛，安兴琴）

2.37 精细化下垫面对海南地区气象场模拟的影响

使用WRF模式对2013年海南地区气象场进行了模拟研究，并对比分析使用不同精度的陆面资料［WRF默认的陆面资料（土地利用、植被覆盖度、地形和土壤类型），2013年MODIS土地利用和植被覆盖度资料，SRTM3地形资料，HWSD土壤类型资料］对WRF模式模拟结果的影响。结果表明：采用WRF模式默认或高精度的陆面资料都能够较为准确地模拟出当地气象场的时空分布特征；采用高精度且时效性更好的陆面资料可显著地改进WRF模式对2 m温度和2 m相对湿度的模拟，冬（夏）季的均方根误差（RMSE）分别降低了7.2%（6.5%）和6.1%（7.7%），准确率（HR）分别提高了3.7%（2.8%）和3.2%（2.9%）；陆面资料的分辨率及时效性对风场模拟的影响较不敏感，总体上WRF模式仍能较为准确地模拟出研究区域内风场的特征，更新陆面资料后WRF模拟冬（夏）季的风速的RMSE降低了3.8%（4.5%），HR提升了2.1%（2.9%）。（何建军）

2.38 北京边界层参数化敏感性模拟研究

利用中尺度数值天气预报模式WRF模拟晴天条件下北京边界层的气象场特征，并通过敏感性试验研究4组边界层参数化方案（YSU、 ACM2、 MYJ和BL）对辐射、地表能量、近地面气象要素以及边界层结构的模拟差异。结果表明，4种边界层参数化方案都可以准确模拟向下短波辐射，对长波辐射的模拟能力相似。YSU方案模拟的感热通量最低，4种参数化方案对地表净辐射通量的模拟差异主要受到短波辐射的影响。MYJ方案模拟的2 m温度效果最好，YSU方案对2 m比湿以及10 m风速的模拟效果最优，综合而言，YSU方案对近地面气象要素的模拟效果较好。与探空数据对比，得到4种边界层参数化方案模拟的高层温度廓线偏冷，湿度偏高，风速偏低。与气象铁塔观测数据对比，白天4组试验都能够较为准确地反映温度垂直廓线，YSU方案在15 m以上模拟的相对湿度结果最接近观测值。YSU方案模拟的边界层高度最高，非局地方案模拟的边界层高度相对局地方案更高，MYJ方案模拟的边界层高度误差较大。（何建军）

2.39 江西省冬季大气典型污染过程的气象成因研究

选取2019年1月江西省两次大气污染过程为研究对象，利用常规气象观测资料、美国国家环境预报中心（NCEP）再分析资料、全球资料同化系统（GDAS）气象数据和空气质量数据，分别从局地气象要素变化、地面天气形势、大气动力和热力条件及污染潜在源区等进行分析，对比两次污染过程形成机制。两次污染过程地面天气形势分别为冷锋前部型和低压倒槽型。冷锋前部型污染形成主要原因为冷空气南下在江西省减弱辐合导致上游细颗粒物输送并堆积，西北风增大细颗粒物浓度降低。低压倒槽型污染形成原因为较长时间处于高湿、小风或静风、逆温下的污染累积。对两次过程中污染较为严重的九江市进行分析，冷锋前部型九江市近地面主要受西风影响，低压倒槽型主要受东北风影响，低压倒槽型九江市风速明显低于冷锋前部型，风速多在2 m/s以下。两次污染期间大于3 m/s的风速有利于污染物清除。长时间高湿、小风（＜2 m/s）及风场辐合，是低压倒槽型九江市重污染维持较长时间的重要原因。低压倒槽型大气垂直结构较冷锋前部型稳定。低压倒槽型垂直湍流弱、低层风速小于2 m/s，且存在多层逆温和深厚的湿区，冷锋前部型存在明显下沉运动，逆温强度明显弱于低压倒槽。九江市PM2.5污染潜在贡献源主要来自河南东部、山东西部和安徽西北部；低压倒槽型九江市潜在源区主要位于江西省内及与江西省接壤的湖北东南部、安徽西南部。（张根）

2.40 新疆地区大气环境容量系数的气候特征及其在空气质量变化中的作用

新疆在我国属于冬半年大气环境容量系数（A值）显著偏低的区域。研究了1975—2019年新疆大气环境容量系数的时空变化特征。新疆的低A值区主要分布在塔里木盆地北部和新疆东部；研究期间新疆的大气环境容量系数在随机波动中整体呈下降趋势，这与新疆平均风速的变化较为一致，主要是由气候变化引起。新疆A值的季节差异大，春夏高、秋冬低；A值的冬季均值远小于年均值，说明新疆冬季的大气自净能力很弱。新疆4个地级市的月均A值与PM2.5浓度具有显著的负相关，A值能够较好地反映当地空气污染的气象条件特征。南疆城市则由于沙尘天气的影响，大气颗粒物浓度很高，但A值与PM2.5浓度的相关性较差。还基于A值，利用城市大气环境荷载指数对新疆4个地级市的大气污染排放变率进行了评估。（王郁）