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 WMO Airborne Dust Bulletin

Sand and dust storms (SDSs) have been recognized by recent United Nations General Assemblies and World Meteorological Congresses as severe hazards that can affect weather,climate,the environment,health and economies in many parts of the world.To combat these hazards,operational SDS forecasting,warning advisory and information assessment services need to be provided for various regions of the world in a globally coordinated and harmonized manner.Since 2004,and at the request of more than 40 countries,WMO has taken the lead in this area and established the Sand and Dust Storm Warning Advisory and Assessment System (SDSWAS) to develop,refine and provide a basis for distributing the global community products that can be used to reduce the adverse impacts of SDSs and to assess the effects of SDSs on societies and on the environment.In June 2019,the Eighteenth World Meteorological Congress approved Resolution 19 (Cg-18)—enhancing cooperation for monitoring and forecasting sand and dust storms.Congress noted the progress made regarding the implementation of SDS-WAS and suggested that Member Countries promoted international cooperation to combat SDSs through the exchange of knowledge,experiences and best practices and by offering training courses.The congress also suggested that Member Countries enhance their capacity-building efforts and their provision of technical assistance in order to monitor and forecast SDSs and to support the implementation of the national,regional and global action plans of affected countries.(Zhang Xiaoye)

1.2 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 longterm reanalysis that incorporates interactions between aerosols and physical processes.(Zhang Xiaoye)

1.3 Efficient conversion of NO to NO2 on SO2-aged MgO under atmospheric conditions

NO-NO2cycle determines the formation of O3and hence plays a critical role in the oxidizing capacity of the troposphere.Traditional view concluded that the heterogeneous oxidation of NO to NO2was negligible due to the weak reactivity of NO on aerosols compared to homogeneous oxidation process.However,the results here reported the first time that SO2can greatly promote the heterogeneous transformation of NO to NO2and HONO on MgO particles under ambient condition.The uptake coefficients of NO were increased by 2−3 orders of magnitudes on SO2-aged MgO compared to fresh sample.Based on spectroscopic characterization and density functional theory (DFT) calculations,the active sites for the adsorption and oxidation of NO were determined to be sulfates,where an intermediate [SO4-NO] complex was formed during the adsorption.The decomposition of this species led to the formation of NO2and the change of sulfate configuration.The formed NO2could further react with surface sulfite to form HONO and sulfate.The conversion of NO to NO2and HONO on SO2-aged MgO surface under ambient condition contributes a new formation pathway of NO2and HONO,and could be quite helpful for understanding the source of atmospheric oxidizing capacity as well as the formation of air pollution complex in polluted regions such as the northern China.（Liu Chang）

1.4 Long-term changes of regional ozone in China:implications for human health and ecosystem impacts

The first Tropospheric Ozone Assessment Report (TOAR) provides information on present-day distributions and long-term trends of ozone metrics relevant for climate change,human health,and vegetation.However,only few results are available in TOAR for China due to limited long-term ozone observations.Here,we present an integrated analysis of long-term measurements of surface ozone from eight sites distributed in the North China Plain (NCP) and Yangtze River Delta (YRD),the relatively underdeveloped region in Northeast China,and the remote regions in Northwest and Southwest China.Trends and present-day values for seven annual and five seasonal ozone metrics were calculated following the TOAR methodologies.We compare the seasonal and diurnal cycles of ozone concentrations as well as the present-day values of ozone among sites and discuss the long-term trends in the ozone metrics.Large and significant increases of ozone are detected at the background site in the NCP,moderate increases at the global baseline site in the western China,significant decreases at the northwestern edge of China,and nearly no trend at other sites.Extremely high values of ozone occurred in the NCP and YRD,particularly in warmer seasons.The present-day levels of summer ozone metrics in the NCP are much higher than the thresholds set in TOAR for the highest value groups of ozone metrics.The summer ozone metrics at the Shangdianzi background site in the NCP indicate increases at rates of more than 2% per year during 2004−2016.In contrast,ozone at the Lin’an background site in the YRD was constant over the period 2006−2016.Our results fill some knowledge gaps in spatiotemporal changes of ozone in China and may be useful in the assessment of ozone impacts on human health and vegetation.（Xu Xiaobin）

1.5 Understanding the formation of high-ozone episodes at Raoyang,a rural site in the North China Plain

An observational field campaign was carried out from 20 June to 31 July 2016 at Raoyang,a rural site in the North China Plain (NCP).In-situ measurements of O3and related gases were obtained and analyzed in this work.Heavy photochemical pollution episodes lasting for a few days were observed in the campaign.The episodes could be divided into two categories,with one being influenced by plume from northeast (NE)via Beijing-Tianjin cities and the other from southeast (SE) via Shandong Province.The SE episodes were more frequent with higher O3concentrations and lagged daily peaks of O3despite of lower concentrations of precursors.Such episodes could last for several days or even a week.In contrast,the NE episode was associated with relatively lower O3levels and earlier daily O3peaks though the levels of precursors were higher.The WRF-Chem model was used to study the differences between typical SE and NE episodes in regional distributions of O3concentrations,chemical regimes of O3formation,ozone production efficiency(OPE),photochemical age of plume,and evolution of O3concentration within the boundary layer.Our results suggest that the O3production regimes were different for the Raoyang site,with the NE episode being VOCssensitive and the SE episode in the VOC-sensitive to NOx-sensitive transition regime.In both episodes,a major part of the NCP was under the VOCs-sensitive regime.Plumes in the SE episodes were more aged than those in the NE episode.The OPE values were higher in more aged SE plumes,resulting in higher O3concentrations despite of lower precursor concentrations.Our results also indicate that downward mixing of O3in the nocturnal residual layer played an important role in rapid buildup of surface O3in the next morning in both NE and SE episodes.The formation of photochemical smog seems to be particularly effective in the SE episode since O3was rapidly produced during daytime,stored aloft at night,and mixed downward to the groundlevel,and the process can circulate for several consecutive days.Our study highlights the complicated role of chemical reaction,transport and boundary layer processes in the formation of severe photochemical smog in the region.（Xu Xiaobin）

1.6 Recent advances in studies of ozone pollution and impacts in China:A short review

Surface ozone (O3) is a photochemical pollutant detrimental to human health and vegetation.Anthropogenic emissions of O3precursors have enhanced the levels of O3over many areas of the world.Measurements from a large number of sites in China show that the country has been experiencing severe O3pollution for a few years.The concentrations of O3in many Chinese cities have been increasing in recent years,against the decreasing trends of particulate matters.Exposure to high levels of O3has been found to cause significant premature mortality,crop yield losses,and damages to trees in the country.This short review summarizes recent advances in studies of O3pollution in China,focusing on spatiotemporal variations as well as impacts on human health and vegetation,and provides some recommendations for future studies.（Xu Xiaobin）

1.7 Acidity and inorganic ion formation in PM2.5 based on continuous online observations in a South China megacity

To investigate the evolutionary characteristics and formation mechanisms of PM2.5in areas with high environmental capacity,Shenzhen,a representative megacity on the southeast coast of China,was chosen for analysis in this study.Continuous quantitative observations of the mass concentrations of PM2.5,watersoluble inorganic ions (WSIIs) in PM2.5and relevant reactive gaseous precursors were conducted for one year at an urban site in Shenzhen.The differences in the meteorological factors and in the major air pollutants between the polluted and clean periods were compared in detail.Daily air quality index (AQI) values were used to distinguish these two periods.Using the ISORROPIA-II thermodynamic equilibrium model,aerosol pH and liquid water content measurements were obtained for the first time during both the polluted period and the clean period.The aerosol pH values during the polluted period and the clean period were determined to be 7.55 ± 0.58 and 4.23 ± 2.99,respectively.The secondary conversion efficiencies of SO2and NO2to sulfate and nitrate,respectively,were found to increase during the polluted period.The polluted period exhibited a combination of both high PM2.5and high O3pollution.In the clean period,the ozone concentrations remained at high levels despite a significant drop in PM2.5levels.The potential sources and transportation modes of air pollution in Shenzhen were also discussed.（Wu Lingyan）

1.8 Explosive morning growth phenomena of NH3 on the North China Plain:Causes and potential impacts on aerosol formation

Atmospheric ammonia (NH3) as the most important alkaline gas in the atmosphere has attracted much attention in recent years due to its critical role in haze formation,especially on the North China Plain (NCP).Comprehensive studies are needed for investigating diurnal variations of NH3and underlying mechanisms in different seasons and their potential impacts on atmospheric chemistry.In this study,continuous long-term observation (March 2016 to May 2017) of NH3at a rural site in the NCP was used to characterize the diurnal variation of NH3in different seasons and to unveil its causes and potential impacts on atmospheric chemistry.NH3concentrations displayed rapid increases during the morning,reaching very prominent peaks mostly between 08:00 and 11:00 LT.Such frequent (55%) morning peaks were mainly caused by the evaporation of dew and guttation water droplets.Average dew and guttation water volume concentrations of 750 mL m−2was estimated for spring,which resulted in approximate NH3emissions of 800 ng m−2s−1.Such high emission fluxes from dew and guttation water evaporation have never been reported before,suggesting dew and guttation droplets to be significant night-time reservoirs and strong morning sources for NH3.In light of recent studies putting forward that NH3can promote the heterogeneous formation of HONO and nitrate under high humidity conditions,we investigated the differences in HONO and aerosol chemical composition diurnal variations between days with and without NH3morning spikes during November.HONO,nitrate and sulfate concentrations were significantly higher for days with NH3morning spikes,with HONO displaying a morning peak near that of NH3.These results demonstrate that the prevailing NH3morning spikes on the NCP have significant influences on aerosol formation and atmospheric chemistry.NH3emission mitigation strategies and regulations are urgently needed.（Xu Wanyun）

1.9 Changes in ammonia and its effects on PM2.5 chemical property in three winter seasons in Beijing,China

NH3,SO2,NOxand the inorganic ions of PM2.5in winter 2009,2014 and 2016 were examined to investigate the change in NH3and aerosol chemistry in Beijing,China.NH3concentrations showed an increase by 59% on average,in contrast to the decrease of SO2by 63% from winter 2009 to 2016.The mean mass ratio of NH3/NHxwas 0.83 ± 0.12 in 2016,which is higher than those obtained in 2009 and 2014,implying more NHxremaining as free NH3in 2016 winter.Our findings suggest that vehicles exhaust emissions associated with contributions are an important NH3source in the urban central atmosphere of Beijing in winter.Despite the observed NOxpresenting declining trends from 2014 to 2016,nitrate concentrations even exhibited a significant increasing trend,which may be largely attributable to high NH3levels.An in-depth analysis of measured NH3and aerosol species in a heavy pollution episode in December 2016,combined with the acidity predicted by ISORROPIA II model,demonstrated abundant NH3most of the time in air,where NH3is not only a precursor for NH4+but also effect the neutralization of SO42−and NO3−in PM2.5.With high RH and low photochemical activity,elevated NO3−concentration was attributed to an enhanced heterogeneous conversion of NOxto HNO3to form NH4NO3in the pollution transport stage.The decrease in NOxfrom high level and the increase in NH3,with peaks of SO42−occurring,were observed in the pollution cumulative stage.The aqueousphase oxidation of SO2by NO2to sulfate might play an important role with high pH values.Our results suggested that the simultaneous control of NH3emissions in conjunction with SO2and NOxemissions would be more effective in reducing particulate matter PM2.5formation（Meng Zhaoyang）

1.10 Retrievals of fine mode light-absorbing carbonaceous aerosols from POLDER/PARASOL observations over East and South Asia

A new aerosol component approach that infers aerosol species fractions from satellite observations was developed and incorporated by Li et al.(2019) into the Generalized Retrieval of Aerosol and Surface Properties(GRASP) algorithm.In the current work we employ this aerosol component approach for derivation and analysis of temporal and spatial characteristics of black and brown carbon (BC and BrC) concentrations in East and South Asia using POLDER/PARASOL satellite observations during 2005–2013.The inter-comparison of the satellite retrieved and in situ measured BC in China shows a reasonably good agreement for both monthly averaged and daily values.The distribution of light-absorbing carbon (BC and BrC) shows a significant temporal variation cycle with increases matching the months known for strong biomass burning or fossil fuels combustion emissions.The main concentrations of BC are found near the fire or anthropogenic emission regions,while concentrations of BrC are predominantly distanced from the emission sources.Seasonal means of BC/(BC+BrC) ratio in East and South Asia are also presented.High ratios (above 0.5) in Northeast India and North China during December,January,and February can be attributed to fossil fuel combustion and anthropogenic emissions.Medium ratios (around 0.2–0.3) over the Indo-China Peninsula and Northeast China during March,April,and May are associated with the biomass burning (e.g.,forest fire,agriculture waste burning).The provided observationally-based retrievals of BC,BrC,and BC/(BC+BrC) are believed to be of high importance for constraining the aerosol modeling and reducing the uncertainties of the model estimations.（Li Lei,Che Huizheng ）

1.11 Climatology of fine and coarse mode aerosol optical thickness over East and South Asia derived from POLDER/PARASOL satellite

The climatological characteristics of total aerosol optical thickness (AOT) together with fine and coarse mode AOT (fAOT,cAOT),and fine mode fraction (FMF= fAOT/AOT) over East and South Asia are presented here for the period March 2005 to October 2013.The characteristics are retrieved from POLDER/PARASOL measurements using the recently developed aerosol component module of the GRASP algorithm.The satellite retrievals were validated and presented a good agreement with the AERONET products (e.g.,Rof about 0.8–0.9 for AOT,fAOT,Ångström Exponent and FMF).The results show a seasonal cycle and strong relation to the emission sources.For example,FMF has a peak of nearly 1.0 in March in the Indo-China Peninsula region,reaches a minimum of about 0.4 in April in the northwestern China region.These patterns can be explained by dominance of anthropogenic pollution or desert dust emissions.Maps of fine,coarse,and total AOT,as well as of FMF for the East and South Asia region,are presented and analyzed in scope of seasonal and interannual variabilities（Li Lei）

1.12 Exploring the inconsistent variations in atmospheric primary and secondary pollutants during the 2016 G20 summit in Hangzhou,China:Implications from observations and models

Complex aerosol and photochemical pollution (ozone and peroxyacetyl nitrate,PAN) frequently occur in the eastern China,and mitigation strategies to effectively alleviate both kinds of pollution are urgently needed.Although the effectiveness of powerful control measures implemented by the Chinese State Council has been comprehensively evaluated in terms of reducing atmospheric primary pollutants,the effectiveness in mitigating photochemical pollution is less assessed and therefore the underlying mechanisms are still poorly understood.The stringent emission controls implemented from 24 August to 6 September 2016 during the summit for the Group of Twenty (G20) provide us a unique opportunity to address this issue.Surface concentrations of atmospheric O3,PAN,and their precursors including volatile organic compounds (VOCs)and nitrogen dioxides (NOx),in addition to the other trace gases and particulate matter,were measured at the National Reference Climatological Station (NRCS) (30.22°N,120.17°E,41.7 m a.s.l) in urban Hangzhou.We found significant decreases in atmospheric PAN,NOx,total VOCs,PM2.5,and sulfur dioxide (SO2) under the unfavorable meteorological conditions during G20 (DG20) relative to the adjacent period before and after G20 (BG20 and AG20),indicating that the powerful control measures were effective in reducing the pollutant emissions in Hangzhou.Unlike the other pollutants,daily maximum 8-hour average (DMA8) O3exhibited a slight increase and then decrease from BG20 to AG20,which was mainly attributed to the variation in the solar irradiation intensity and regional transport in addition to the contribution from the implementation of stringent control measures.Results from an observation-based chemical model (OBM) indicated that acetaldehyde and methylglyoxal (MGLY) were the most important second-generation precursors of PAN,accounting for 37.3%-51.6% and 22.8%–29.5% of the total production rates including the reactions of OVOCs,propagation of other radicals,and other minor sources.Moreover,we confirmed the production of PAN and O3was sensitive to VOCs throughout the whole period,specifically dominated by aromatics in BG20 and DG20 but by alkenes in AG20.These findings suggested that reducing emissions of aromatics,alkenes,and alkanes would mitigate photochemical pollution including PAN and O3.Source appointment results attributed the reductions of VOC source and ozone formation potentials (OFPs) during G20 to the effective emission controls on traffic (vehicle exhaust) and industrial processes (solvent utilization and industrial manufacturing).However,fuel combustion and biogenic emissions both weakened such an effect with a sizable contribution to the VOC mixing ratios(18.8% and 20.9%) and OFPs (25.6% and 17.8%),especially during the latter part of G20 (G20 II) when anthropogenic VOCs were substantially reduced.This study highlights the effectiveness of stringent emission controls in relation to traffic and industrial sources,but a coordinated program related to controlling fuel combustion and biogenic emissions is also required to address secondary pollution.(Zhang Gen)

1.13 The abundance and inter-relationship of atmospheric peroxyacetyl nitrate (PAN),peroxypropionyl nitrate (PPN),O3,and NOy during the wintertime in Beijing,China

Although atmospheric peroxyacetyl nitrate (PAN) and O3have been extensively measured in Beijing during the summertime,the abundances of PAN,peroxypropionyl nitrate (PPN) and the total odd-reactive nitrogen budget (NOy) and their inter-relationship have been studied comparatively less in the winter.Here we measured atmospheric PAN,PPN,O3,NOx,and NOyin Beijing from November 2012 to January 2013.Compared with our previous results in the summertime,much lower levels were observed in the winter,with the mean and maximum values of 311.8×10−12v and 1465×10−12v for PAN,52.8×10−12and 850.6×10−12v for PPN,and 11.6×10−9and 36.7×10−9v for O3.In contrast,high levels were found as 94.2×10−9v and 374.9×10−9v for NOy,with a major constituent of NOx(75.9%).The source to the west and northwest made the significant contribution to the relatively high O3concentrations during nighttime.PAN concentrations were highly related to the PAN-rich air mass transported from the southeast during the nighttime,whereas predominated by local photochemical production during the daylight.The distributions of NOxand NOywere dominated by local emission and photochemical production during daylight but also influenced by air masses transported from south direction during nighttime.Significant positive correlation (R2=0.9,p0.0001) between PAN and PPN with a slope (ΔPPN/ΔPAN) of 0.17 indicated that anthropogenic volatile organic compounds (AVOCs)dominated the photochemical formation of PANs in Beijing,and the independent relationship between the PPN/PAN ratio and PAN ( 500×10−12v) implied a steady state between PAN and PPN achieving rapidly in the polluted air masses.Negative correlation and slopes between PAN and O3likely resulted from their weak photochemical productions in the winter,coupled with the large NO sources which acted as a local sink for O3,but much less so for PAN due to its enhanced thermal stability under low temperature.(Zhang Gen)

1.14 Methane emissions from oil and gas platforms in the Bohai Sea,China

Although oil and gas explorations contribute to atmospheric methane (CH4) emissions,their impact and influence along the shelf seas of China remain poorly understood.From 2012 to 2017,we conducted four ship-based surveys of CH4in the seawater column and boundary layer of the Bohai Sea,China,and further measured CO2and several meteorological parameters.The average observed CH4mixing ratios in the boundary layer and its concentrations in seawater column were (1950 ± 46)×10−9in November 2012 (dissolved CH4was not observed in this survey),(2222 ± 109)×10−9and 13.0 ± 5.9 nmol/L in August 2014,(2014 ± 20)×10−9and 5.4 ± 1.4 nmol/L in February 2017,and (1958 ± 25)×10−9and 5.3 ± 3.8 nmol/L in May 2017,respectively.The results demonstrated that the CH4emissions from the oil and gas platforms accounted for approximately 72.5 ± 27.0% of the increase in the background atmospheric CH4in the local area.The remaining emissions were attributed to land-sea air mass transportation.Conversely,the influence of the air-sea exchange was negligible,measuring within the 10−3×10−9range.For carbon balance calibration,the mean flaring efficiency of the oil-associated gas based on the enhancement of CO2(△CO2) and enhancement sum of CO2and CH4(△CO2+△CH4) was 98.5 ± 0.5%.Furthermore,the CH4emission rate from the oil and gas platforms was 0.026 ± 0.017 Tg/year,which was approximately 7.2 times greater than the sea-to-air CH4flux over the entire Bohai Sea area.Thus,oil and gas platforms must be recognized as important artificial hotspot sources of atmospheric CH4in the Bohai Sea.（Zhang Gen）

1.15 Seasonal variations of atmospheric CH4 at Jingdezhen station in central China:Understanding the regional transport and its correlation with CO2 and CO

Greenhouse gases observation networks have been extensively established in the eastern,southern,western,and northern China,but rare in the developing central China.We conducted in-situ measurement of atmospheric CH4mixing ratios with high precision at JDZ (Jingdezhen) station (29.37°N,117.22°E,87.3 m a.s.l)in the central China between December 2017 and November 2018.The “regionally representative data” and“polluted events” are separated from the complete dataset,based on the analysis of diurnal discrepancy among different levels,the impacts of wind directions and speeds,and the statistical identification procedure.The average CH4mixing ratio of regionally representative compositions during the study period was 2037.4×10−9,lower than that of polluted conditions (2086.3×10−9).The lowest levels of CH4for regionally representative conditions are observed in summer,while highest values are obtained during June–August for polluted events.Cluster analysis of backward trajectories indicated that air-mass transport arising from the northwestern China,eastern China and Jiangxi made the significant contribution to the variability in regionally representative air.Air masses passing through Jiangxi Province and Yangtze River Delta (YRD) regions carried the polluted air with higher CH4mixing ratios to the observational station in summer,probably owing to large quantities of CH4emission from rice cultivation over these areas.△CH4and △CO exhibit strong correlations in all the months during the study period.Moreover,the △CO/△CH4ratio showed the distinct seasonal characteristics,with the rapid decrease from the peak (2.74 ± 0.05) in January to the trough (0.40 ± 0.02) in July,and then the gradual increase to high levels in winter.It is possibly attributed to the maximum CH4emission during summertime and the maximum CO emission during wintertime,as well as the significantly faster reaction rate of CO with OH in summer.The enhancements of CH4(△CH4) and CO2(△CO2) in polluted conditions relative to the regionally representative conditions present strong correlation only in winter,with the △CH4/△CO2slope of (3.92 ± 0.08)×10−3,similar with the average△CH4/△CO2ratio observed in winter (4.15 ± 0.40)×10−3reported for LAN regional station during 2007 and 2013.（Zhang Gen）

1.16 Impact of air-sea exchange on the spatial distribution of atmospheric methane in the Dalian Bay and adjacent coastal area,China

To date,the impact of air-sea exchange on spatial distribution of atmospheric methane (CH4hereafter)remains less understood in the coastal areas of China.Here we measured the dissolved and atmospheric CH4in the Dalian Bay and the adjacent area in July and August 2014,respectively.Results showed that the study area was a net significant source of atmospheric CH4,with a mean sea-to-air CH4flux of 170.6 ±149.5 mmol m−2day−1.We optimized a method to accurately quantify the elevated atmospheric CH4mole fraction (△CH4hereafter) caused by air-sea exchange.The calculated △CH4in the study area ranged from 15.4 to 102.1 nmol mol−1,1.5−10.2 nmol mol−1,and 0.03−0.22 nmol mol−1at the mixing height of 1,10,and 471 m,respectively.The △CH4mole fractions caused by air-sea exchange were positive with sea-to-air CH4flux and in situ observed atmospheric CH4mole fraction,while negative with altitude.Under the standard conditions,we defined 50.8 mmol m−2day−1.as the criteria value of sea-to-air CH4flux which could result in a detectable elevation atmospheric CH4mole fraction at a height of 10 m.（Zhang Gen）

1.17 Atmospheric CO2 and CO at Jingdezhen station in central China:Understanding the regional transport and combustion efficiency

Greenhouse gas observation networks have been extensively established in the eastern,southern,western,and northern China,but are rare in the developing central China.In this study,atmospheric CO2and CO were measured in-situ at Jingdezhen (JDZ) station (29.37°N,117.22°E,87.3 m a.s.l) in the central China during December 2017-November 2018.The observation data were filtered as regionally representative and polluted conditions,based on the analysis of diurnal CO2and CO variations and the impacts of wind directions and speeds.The average CO2and CO mixing ratios of regionally representative compositions during the study period were 416.2×10−6and 294.9×10−9,respectively,relative to 433.8×10−6and 408.9×10−9for the polluted events.Cluster analysis of HYSPLIT backward trajectories indicated that long-distance transport of air masses originating from the northwestern China,eastern China and Jiangxi made the main contributions to variability in regionally representative air.The trajectories passing through urban areas of YRD (Yangtze River Delta),Anhui Province in the central China,and Nanchang in Jiangxi Province carried polluted air masses that probably lead to the increase of CO2and CO mixing ratios.The enhancements of CO2(△CO2) and CO (△CO)in polluted conditions relative to the regionally representative conditions presented stronger correlations(R=0.78,p0.001) in wintertime relative to the other seasons at JDZ Station,suggesting anthropogenic emissions from fossil fuel and biomass combustion significantly affected CO2atmospheric loading in the cold season.Lower △CO2/△CO ratio (39.0) was observed in December 2017 than other months in winter,and the air mass transport mostly originated from the northern China in December,implying the strong impact of biomass burning emission sources with low combustion efficiency from the northern China.The particular polluted event during 18–21 January in 2018 with high slope value (70.7) of △CO2vs △CO was also investigated,and the analysis of air-mass transport routes indicated high combustion efficiency might be found in the central China such as Jiangxi and Hunan provinces.（Zhang Gen）

1.18 Biomass burning impacts on ambient aerosol at a background site in East China:Insights from a yearlong study

Biomass burning activities are ubiquitous in China.In order to better understand the biomass burning sources and their impacts on aerosols components in the background air of East China,biomass burning tracers(i.e.,levoglucosan (LG),mannosan (MN) and non-sea-salt potassium (nss-K+)) were quantified at the Lin’an site.The annual average concentrations of LG,MN and nss-K+in PM2.5were 0.13 ± 0.14 μg m−3,0.009 ± 0.010 μg m−3and 0.65 ± 0.38 μg m−3,respectively.The yearly variations of anhydrosugar tracers,i.e.,LG and MN were fairly consistent,as well as nss-K+.However,due to extensive fireworks activity,nss-K+was observed during a three-day episode of 7−9 February 2016,with abnormally high levels (average at 5.38 ± 3.55 μg m−3).The highest levels of biomass burning tracers were associated with the strong continental outflow from inland China in winter,while lower levels of biomass burning tracers were observed in summer season,influenced by maritime air masses.Combined with the analysis of fire activity data,clear seasonal trends of biomass burning pollution in Lin’an are apparent,ranging from high to low in winter,spring,autumn and summer.By analyzing the ratios of LG/MN and LG/nss-K+,biomass burning aerosols in China were identified to be predominately associated with burning of rice crop residues.Moreover,based on the relationship between biomass burning tracers and components in aerosols,biomass burning plays an important role in regulating chemical properties of aerosols in the background site in East China,especially for carbonaceous components.（Liang Linlin）

1.19 Wet inorganic nitrogen deposition at the Daheitin reservoir in North China:Temporal variation,sources,and biomass burning influences

Atmospheric nitrogen deposition is of great concern to both air quality and the ecosystem,particularly in the northern China,which covers one-quarter of China’s cultivated land and has many heavily air polluted cities.To understand the characteristics of wet N deposition at rural sites in the northern China,one-year wet deposition samples were collected in the Daheitin reservoir region.Due to the intense emissions of gaseous nitrogen compounds from heating activities during cold seasons and distinct dilution effects under different rainfall intensities and frequencies,the volume weighted mean concentrations of wet N deposition showed higher levels in dry seasons but lower levels in wet seasons.In contrast,the wet N deposition rates varied consistently with precipitation,i.e.,high during the wet season and lower during the dry season.The annual wet deposition rate of total inorganic ions (the sum of NO3−–N and NH4+–N) at the rural site in North China from July 2019 to June 2020 was observed at 18.9 kg hm−1year−1,still remained at a relatively high level.In addition,biomass burning activities are ubiquitous in China,especially in the northern China; however,studies on its impact on wet N deposition are limited.Non-sea salt potassium ion (nss-K+) was employed as a molecular tracer to investigate the characteristics of biomass burning activities as well as their impact on the chemical properties of wet N deposition.Three precipitation events with high nss-K+levels were captured during the harvest season (June to July).The variations in the patterns of nss-K+,deposited N species,and ratios of nss-K+to nitrogen species as well as their relationships all indicated that biomass burning emissions contributed remarkably to NO3−-N but had a minor influence on NH4+–N.(Liang Linlin）

1.20 Significant changes in chemistry of fine particles in wintertime Beijing from 2007 to 2017:Impact of clean air actions

The Beijing government implemented a number of clean air action plans to improve air quality in the last 10 years,which contributed to changes in the concentration of fine particles and their compositions.However,quantifying the impacts of these interventions is challenging as meteorological factors masks the real changes in observed concentrations.Here,we applied a machine learning technique to decouple the effect of meteorology and evaluate the changes in the chemistry of non-refractory PM1(particulate matter less than 1 μm) in winter 2007,2016,and 2017 as a result of the clean air actions.The observed mass concentrations of PM1were 74.6,90.2,and 36.1 μg m−3in the three winters,while the de-weathered concentrations were 74.2,78.7,and 46.3 μg m−3,respectively.The de-weathered concentrations of PM1,organics,sulfate,ammonium,chloride,SO2,NO2,and CO decreased by 38%,46%,59%,24%,51%,89%,16%,and 52% in 2017 in comparison to 2007.On the contrary,the de-weathered concentration of nitrates increased by 4%.Our results indicate that the clean air actions implemented in 2017 were highly effective in reducing ambient concentrations of SO2,CO,and PM1organics,sulfate,ammonium,and chloride,but the control of nitrate and PM1organics remains a major challenge.（Zhang Yangmei）

1.21 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 demonstrates 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.（Zhang Xiaoye,Zhang Yangmei）

1.22 Significant changes in autumn and winter aerosol composition and sources in Beijing from 2012 to 2018:Effects of clean air actions

A seven-year long-term comprehensive measurement of non-refractory submicron particles (NR-PM1)in autumn and winter in Beijing from 2012 to 2018 was conducted to evaluate the effectiveness of the clean air actions implemented by the Chinese government in September 2013 on aerosols from different sources and chemical processes.Results showed that the NR-PM1concentrations decreased by 44.1% in autumn and 73.2% in winter from 2012 to 2018.Sulfate showed a much larger reduction than nitrate and ammonium in both autumn (55%) and winter (86%) and that nitrate even slightly increased by 15.8% in autumn.As a result,aerosol pollution in winter gradually changed from sulfate-rich to nitrate-rich with a sudden change after 2016 and the dominant role of nitrate in autumn was also strengthened after 2016.Among primary organic aerosol(OA) types,biomass burning OA and coal combustion OA exhibited the largest decline in autumn and winter,with reductions of 87.5% and 77.3%,respectively,while hydrocarbon-like OA (HOA) exhibited the smallest decline in both autumn (24.4%) and winter (37.1%).These significant changes in aerosol compositions were highly consistent with the much faster reduction of SO2(75%−85%) than NOx(36%−59%) and were mainly due to the clean air actions rather than the impact of meteorological conditions.What’s more,the enhanced atmospheric oxidizing capacity,which was indicated by increased O3,altered the chemical processes of oxygenated OA (OOA),especially in autumn.Both of less-oxidized OOA (LO-OOA) and more-oxidized OOA showed elevated contributions in OA by 4% in autumn.The increased oxygen-to-carbon ratios of LO-OOA in autumn (from 0.42 to 0.58) and winter (from 0.44 to 0.52) indicated the enhanced atmospheric oxidizing capacity strengthened photochemical reactions and resulted in the increased oxidation degree of LO-OOA.This study demonstrates the effectiveness of the clean air actions for air quality improvement in Beijing.（Zhang Yangmei）

1.23 Source identification of trace elements in PM2.5 at a rural site in the North China Plain

An intensive sampling of PM2.5was conducted at a rural site (Gucheng) in the North China Plain from 22 October to 23 November 2016.A total of 25 elements (Al,Na,Cl,Mg,P,S,K,Ca,Ti,V,Cr,Mn,Fe,Co,Ni,Cu,Zn,As,Se,Br,Sr,Cd,Ba,Pb,and Sb) from PM2.5filter samples collected daily were measured using a wavelength dispersive X-ray fluorescence spectrometer.Cl,S,and K were the most abundant elements,with average concentrations of 2077.66 ng m−3(range 118.88–4638.96 ng m−3),1748.78 ng m−3(range 276.67–4335.59 ng m−3),and 1287.07 ng m−3(range 254.90–2748.63 ng m−3),respectively.Among noncrustal trace metal elements,the concentration of Zn was the highest,with an average of 397.74 ng m−3(range 36.45–1602.96 ng m−3),followed by Sb and Pb,on average,of 299.20 ng m−3and 184.52 ng m−3,respectively.The morphologies of PM2.5samples were observed using scanning electron microscopy.The shape of the particles was predominantly spherical,chain-like,and irregular.Positive matrix factorization analysis revealed that soil dust,following by industry,secondary formation,vehicle emissions,biomass and waste burning,and coal combustion,were the main sources of PM2.5.The results of cluster,potential source contribution function,and concentration weighted trajectory analyses suggested that local emissions from Hebei Province,as well as regional transport from Beijing,Tianjin,Shandong,and Shanxi Province,and long-range transport from Inner Mongolia,were the main contributors to PM2.5pollution.(Liu Lei)

1.24 Retrieving tropospheric NO2 vertical column densities around the city of Beijing and estimating NOx emissions based on Car MAX-DOAS measurements

We carried out 19 city-circle-around Car MAX-DOAS experiments on the 6th Ring Road of Beijing in January,September,and October 2014.The tropospheric vertical column densities (VCDs) of NO2were retrieved from measured spectra by the multi-axis differential optical absorption spectroscopy (MAX-DOAS)technique and used to estimate the emissions of NOx(NO+NO2) from urban Beijing during the experimental periods.The offline LAPS-WRF-CMAQ model system was used to simulate the wind fields by assimilation of observational data and calculate the NO2-to-NOxconcentration ratios,both of which are also needed for the estimation of NOxemissions.The NOxemissions in urban Beijing for the different months derived from the Car MAX-DOAS measurements in this study were compared to the multi-resolution emission inventory in China for 2012 (MEIC 2012).Our Car MAX-DOAS measurements showed higher NO2VCD in January than in the other two months.The wind field had obvious impacts on the spatial distribution of NO2VCD,with the mean NO2VCD along the 6th Ring Road typically being higher under the south wind than under the north wind.In addition to the seasonal difference,the journey-to-journey variations of estimated NOxemissions rates(ENOX) were large even within the same month,mainly due to uncertainties in the calculations of wind speed,the ratio of NO2and NOxconcentration,and the decay rate of NOxfrom the emission sources to the measured positions under different meteorological conditions.The ranges of ENOX during the heating and non-heating periods were 22.6×1025to 31.3×1025molecules s–1and 9.6×1025to 12.0×1025molecules s–1,respectively.The average ENOX values in the heating and non-heating periods were (26.9 ± 6.1)×1025molecules s–1and (11.0 ± 1.2)×1025molecules s–1,respectively.The uncertainty range of ENOX was 20%−52%.The monthly emission rates from MEIC 2012 are found to be lower than the estimated ENOX,particularly in January.Our results provide important information and datasets for the validation of satellite products and also show how Car MAX-DOAS measurements can be used effectively for dynamic monitoring and updating of the NOxemissions from megacities such as Beijing.（Cheng Xinghong）

1.25 深圳市城区大气颗粒物及主要水溶性无机离子的污染特征

基于2015年深圳市大气颗粒物和主要水溶性无机离子的观测数据，深入分析了大气颗粒物的浓度变化及二次污染特征。结果表明，2015年深圳的大气颗粒物(PM10、PM2.5、PM1)浓度虽然低，但其中细粒子占比高，PM2.5/PM10的比值高达0.744，甚至大于广州典型灰霾过程中的粗细粒子比。大气颗粒物浓度季节变化明显，秋冬高，春夏低。其日变化特征明显受到交通高峰的影响，汽车尾气可能是污染来源之一。SO42-、NO3-和NH4+（SNA）质量浓度在PM2.5中的占比超过1/3（37.7%），且全年硫转化率都大于0.1，这说明深圳市细颗粒物主要来自于二次转化。深圳大气颗粒物浓度受气象要素影响显著，与气压正相关，与气温、相对湿度、降水及风速负相关；若将风速、气温、气压、相对湿度和降水作为一个整体考虑，这些气象要素对深圳大气颗粒物浓度的影响大小是PM1PM10PM2.5。本工作不仅对深圳的大气环境管理和经济可持续发展有着重要参考价值，还对空气相对清洁地区的大气颗粒物和雾霾治理具有指导意义。(王郁，吴玲燕）

1.26 大气气溶胶监测与分析方法综合评述

大气气溶胶不仅会通过吸收或散射太阳光影响大气能见度产生直接气候效应，对云凝结核、降水的形成等产生间接气候效应，还会影响人类健康。对大气气溶胶特性的监测与分析是认识大气气溶胶、了解污染物形成与老化机理、追溯污染物来源的主要途径。随着大气科学技术的不断发展，大气气溶胶的监测分析方法和仪器层出不穷。不同分析方法和测量手段的工作原理不同，测量的参数不同，不同仪器之间的差别很大。为此，文章整理了目前针对大气气溶胶特性测量广泛使用的监测仪器及主要分析方法，介绍这些监测仪器和分析方法的基本原理及其在实际应用中的优点和缺点，并对未来大气颗粒物及特性监测分析方法进行展望，旨在提升科研工作者对这些监测手段和方法的理解和认知。文章给出了大气气溶胶监测与分析方法未来的发展方向的建议。我国正在实施的PM10和PM2.5国家标准在防治大气污染方面起到了关键作用。由于亚微米粒子（PM1） 对大气能见度、人体健康和云-雾形成的作用更大，效果更明显，随着治理进程的不断深入，开展多点实时监测PM1质量浓度具有一定的现实意义。大气气溶胶中有机组分的特性复杂，对大气颗粒物中有机物的在线识别和监测技术将成为未来的必然需求。水汽在气溶胶测量，气溶胶形成、增长和老化过程以及凝结核形成中的作用不容忽视，值得未来在气溶胶生成机理及模式研究中进一步考虑。发挥多学科融合优势，在对大气气溶胶特性研究中，引进其他领域广泛使用的分析技术和方法（例如同位素分析技术、机器学习模型），有利于促进大气科学领域的长足发展。（张养梅）

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 Reexamining the mechanisms of East Asian summer monsoon changes in response to non-East Asian anthropogenic aerosol forcing

This study examines the mechanisms by which the East Asian summer monsoon (EASM) changes in response to non-East Asian (NEA) anthropogenic aerosol forcing by distinguishing the fast direct atmospheric response and slow ocean-mediated response to forcing using a global aerosol-climate coupled model.The results show that NEA aerosol forcing significantly exacerbates the weakening of the EASM due to local aerosol forcing.The fast response is dominant in the weakening of the EASM and an anomalous precipitation pattern over the eastern China resembling the so-called “southern flood and northern drought’’ in the total response to NEA aerosol forcing.Changes in upper-tropospheric temperature caused by the fast response play a major role in the impact of NEA aerosol forcing on the EASM.Anomalous cooling occurs during summer in the upper troposphere (at about 40°N) over East Asia caused by the fast response.This is due to the combined effects of strong eastward cold advection in the Northern Hemisphere mid-latitudes caused by increased aerosol loading in Europe and the resulting change in local meridional heat transport in East Asia.Subsequently,the zonal wind speed changes on either side of the anomalous cooling,and the East Asian subtropical jet shifts equatorward,thereby weakening the EASM.The changes in atmospheric temperature and the local Hadley cell caused by the slow response to NEA aerosol forcing are conducive to strengthening the southwesterly winds over the eastern China.Our study suggests the importance of NEA aerosol forcing in driving changes in the EASM on a fast timescale.(Wang Zhili)

2.2 Contrasting impacts of two types of El Niño events on winter haze days in China’s Jing-Jin-Ji region

El Niño events differ widely in their patterns and intensities.The regional climate anomalies caused by different types of El Niño events likely lead to various impacts on winter haze pollution in China.Based on long-term site observations of haze days in China from 1961 to 2013,this study explores the effects of the eastern Pacific (EP) and central Pacific (CP) types of El Niño events on the number of winter haze days (WHDs)in China’s Jing-Jin-Ji (JJJ) region and the physical mechanisms underlying WHD changes.The results show statistically significant positive and negative correlations,respectively,between WHDs in the JJJ region and EP and CP El Niño events.At most sites in the JJJ region,the average WHD increased in all EP El Niño years,with the maximum change exceeding 2.0 days.Meanwhile,the average WHD decreased at almost all stations over this region in all CP El Niño years,with the largest change being more than −2.0 days.The changes in large-scale circulations indicate obvious positive surface air temperature (SAT) anomalies and negative sea level pressure (SLP) anomalies over North China,as well as southerly wind anomalies at the middle to low troposphere over the eastern China in the winters of EP El Niño years.These anomalies are conducive to increases in WHDs in the JJJ region.However,there are significant northerly and northwesterly wind anomalies at the middle to low troposphere over the eastern China,as well as stronger and wider precipitation anomalies in the winters of CP El Niño years,which contribute to decreased WHDs over the JJJ region.Changes in local synoptic conditions indicate negative SLP anomalies,positive SAT anomalies,and weakened northerly winds over the JJJ region in the winters of EP El Niño years.The total occurrence frequency of circulation types conducive to the accumulation (diffusion) of aerosol pollutants increased (decreased) by 0.4%(0.2%) in those winters.However,the corresponding frequency decreased (increased) by 0.5% (0.6%) in the winters of CP El Niño years.Our study highlights the importance of distinguishing the impacts of these two types of El Niño events on winter haze pollution in the China’s JJJ region.（Wang Zhili）

2.3 Integrated impacts of synoptic forcing and aerosol radiative effect on boundary layer and pollution in the Beijing-Tianjin-Hebei region,China

Rapid urbanization and industrialization have led to deterioration of air quality in the Beijing-Tianjin-Hebei (BTH) region due to high loadings of PM2.5.Heavy aerosol pollution frequently occurs in winter,in close relation to the planetary boundary layer (PBL) meteorology.To unravel the physical processes that influence PBL structure and aerosol pollution in BTH,this study combined long-term observational data analyses,synoptic pattern classification,and meteorology-chemistry coupled simulations.During the winter of 2017/2018,Beijing and Tangshan often experienced heavy PM2.5pollution simultaneously,accompanied by strong thermal inversion aloft.These concurrences of pollution in different cities were primarily regulated by the large-scale synoptic conditions.Using principal component analysis with geopotential height fields at the 850 hPa level during winter,two typical synoptic patterns associated with heavy pollution in BTH were identified,One of which is characterized by a southeast-to-north pressure gradient across BTH,and the other is associated with high pressure in the eastern China.Both synoptic types feature warmer air temperature at 1000 m a.g.l.,which could suppress the development of the PBL.Under these unfavorable synoptic conditions,aerosols can modulate PBL structure through the radiative effect,which was examined using numerical simulations.The aerosol radiative effect can significantly lower the daytime boundary layer height through cooling the surface layer and heating the upper part of the PBL,leading to the deterioration of air quality.This PBL-aerosol feedback is sensitive to the aerosol vertical structure,which is more effective when the synoptic pattern can distribute more aerosols to the upper PBL.（Miao Yucong）

2.4 Impacts of regional transport and boundary layer structure on the PM2.5 pollution in Wuhan,central China

Wuhan,one of the most developed cities in central China,has been experiencing frequent heavy haze pollution.To understand the impacts of large-scale synoptic patterns and the local-scale planetary boundary layer (PBL) structures on the PM2.5pollution in Wuhan,this study applied an objective approach to the classifying of the daily synoptic patterns in 2017.It then combined observational analyses and meteorologychemistry coupled simulations to investigate a typical pollution episode at the end of November 2017.The synoptic type associated with the heaviest PM2.5pollution in Wuhan was characterized by high pressure to the northwest and low pressure to the northeast at the 850-hPa level,which can support northwesterly prevailing winds towards Wuhan.As a result,the aerosols from the highly polluted northern regions can be transported to Wuhan,leading to a high PM2.5concentration.Also,when there was high pressure located to the east/southeast of Wuhan at the 850-hPa level,southerly warm advections could be induced.The warming of upper air can significantly suppress the development of PBL by enhancing thermal stability,favoring the accumulation of aerosols.This study elucidated the multi-scale physical mechanisms underlying the aerosol pollution in Wuhan,and has important implications for the forecasting and the mitigating of pollution.（Miao Yucong）

2.5 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 demonstrates 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.(Zhang Xiaoye)

2.6 Filling the gaps of in situ hourly PM2.5 concentration data with the aid of empirical orthogonal function analysis constrained by diurnal cycles

Data gaps in surface air quality measurements significantly impair the data quality and the exploration of these valuable data sources.In this study,a novel yet practical method called diurnal-cycle-constrained empirical orthogonal function (DCCEOF) was developed to fill in data gaps present in data records with evident temporal variability.The hourly PM2.5concentration data retrieved from the national ambient air quality monitoring network in China were used as a demonstration.The DCCEOF method aims to reconstruct the diurnal cycle of PM2.5concentration from its discrete neighborhood field in space and time firstly and then predict the missing values by calibrating the reconstructed diurnal cycle to the level of valid PM2.5concentrations observed at adjacent times.The statistical results indicate a high frequency of data gaps in our retrieved hourly PM2.5concentration record,with PM2.5concentration measured on about 40% of the days suffering from data gaps.Further sensitivity analysis results reveal that data gaps in the hourly PM2.5concentration record may introduce significant bias to its daily averages,especially during clean episodes in which PM2.5daily averages are observed to be subject to larger uncertainties compared to the polluted days (even in the presence of the same amount of missingness).The cross-validation results indicate that our suggested DCCEOF method has a good prediction accuracy,particularly in predicting daily peaks and/or minima that cannot be restored by conventional interpolation approaches,thus confirming the effectiveness of the consideration of the local diurnal variation pattern in gap filling.By applying the DCCEOF method to the hourly PM2.5concentration record measured in China from 2014 to 2019,the data completeness ratio was substantially improved while the frequency of days with gapped PM2.5records reduced from 42.6% to 5.7%.In general,our DCCEOF method provides a practical yet effective approach to handle data gaps in the time series of geophysical parameters with significant diurnal variability,and this method is also transferable to other data sets with similar barriers because of its self-consistent capability.(Bai Kaixu,Li Ke,Guo Jianping)

2.7 Higher risk of cardiovascular disease associated with smaller size-fractioned particulate matter

No nationwide studies have examined the associations between mortality risk and PM1(PM with an aerodynamic diameter of <1 µm) due to the scarcity of monitoring data of PM1.On the basis of newly released national scale PM1data,we performed a time series analysis to elucidate the cause-specific mortality risk caused by PM1exposure in China.During the period from January 2014 to December 2017,the PM1levels in 65 cities of China were on average 37 ± 32 µg m−3.Pooled results indicated a 10 µg m−3increase in the PM1level was associated with a 0.19% (95% confidence interval (CI) of 0.09%−0.28%) increased risk in nonaccidental mortality,which was almost the same as that for PM2.5(0.18%,95% CI of 0.08%−0.27%) and PM10(0.17%,95% CI of 0.01%−0.24%).By comparison,the magnitude increased to 0.29% (0.12%−0.47%)in cardiovascular disease for each 10 µg m−3uptick in PM1,which was significantly higher than that related to PM2.5and PM10exposure.This nationwide study supported the notion that PM1may be a higher risk factor for cardiovascular disease,which suggests rapid action is warranted to put more effort into mitigating the emissions of finer particulate matters.(Yin Peng,Guo Jianping,Wang Lijun)

2.8 Intra-seasonal differences in the atmospheric systems contributing to interannual variations of autumn haze pollution in the North China Plain

Our understanding of the factors influencing the interannual variation of autumn haze pollution in the North China Plain (NCP) remains quite limited.Here,we investigate interannual variations of atmospheric haze pollution and associated atmospheric anomalies in the NCP during autumn (September,October,and November).Pronounced anticyclonic anomalies tend to be observed around the northeastern Asia when more severe haze pollution occurs over the NCP during this season.However,the processes underlying the impact of the atmospheric anomalies on the NCP haze show considerable intra-seasonal differences.In September,anticyclonic anomalies impact the NCP haze via modulating the surface relative humidity (SRH) and boundary layer height (BLH),while changes in surface wind speed (SWS) are not found to be related to the NCP haze.In contrast,the interannual variation of the NCP haze has a close relationship with the changes of SWS,SRH,and BLH in both October and November.The factors responsible for formation of the anticyclonic anomalies around the northeastern Asia are found to differ greatly over the 3 months.In September,the formation of anticyclonic anomalies is related to the East Atlantic (EA) pattern,which triggers an eastward propagating atmospheric wave train extending from Europe to the northeastern Asia.In October,the Scandinavia teleconnection is crucial for generating anticyclonic anomalies.While in November,the formation of the anticyclonic anomalies is largely due to the joint effect of the Scandinavia and East Atlantic/Western Russian teleconnections,as well as the non-negligible influence caused by sea surface temperature anomalies in the tropical central-eastern Pacific.(Chen Shangfeng,Guo Jianping,Song Linye)

2.9 Temporal disparity of the atmospheric systems contributing to interannual variation of wintertime haze pollution in the North China Plain

Previous studies indicated that the meridional (northerly or southerly) wind anomalies over East China play an important role in modulating interannual variation of the winter haze pollution in the North China Plain(NCP) mainly via changing surface wind speed and humidity.Here,we report that the factors for the formation of the meridional wind anomalies over East China related to interannual variation of winter haze pollution experienced a significant interdecadal change around the mid-1990s.Before the mid-1990s,two upstream atmospheric wave trains contribute to generation of the meridional wind anomalies over East China via inducing significant geopotential height anomalies over northeast Asia.The first occurred over the mid-latitude Eurasia and propagated eastward into East Asia,resembling the East Atlantic-west Russia (EAWR) pattern.The second propagated eastward along the subtropical Asian jet.Furthermore,during this period,the change in the intensity of the East Asian trough (EAT) was closely linked with interannual variation of the winter haze variation in the NCP.By contrast,after the mid-1990s,the atmospheric wave train along the Asian subtropical jet was not observed.Furthermore,the connection between the EAT intensity and the winter time NCP haze variation was weak.The mid-latitude EAWR-like pattern and the El Niño-Southern Oscillation-related sea surface temperature anomalies in the tropical Pacific were possible factors that explain the meridional wind anomalies over East China.Understanding the change in the atmospheric anomalies contributing to interannual variation of the haze is essential for the prediction of haze in the NCP.(Chen Shangfeng,Guo Jianping,Song Linye)

2.10 The climatology of lower tropospheric temperature inversions in China from radiosonde measurements:Roles of black carbon,local meteorology,and large-scale subsidence

The variability of the lower tropospheric temperature inversion (TI) across China remains poorly understood.Using seven years’ worth of high-resolution radiosonde measurements at 120 sites,we compile the climatology of lower tropospheric TI in terms of frequency,intensity,and depth during the period from 2011 to 2017.The TI generally exhibits strong seasonal and geographic dependencies.Particularly,the TI frequency is found to be high in winter and low in summer,likely due to the strong aerosol radiative effect in winter.The frequency of the surface-based inversion (SBI) exhibits a “west low,east high” pattern at 08:00 Beijing time(BJT),which then switches to “west high,east low” at 20:00 BJT.Both the summertime SBI and elevated inversion (EI) reach a peak at 08:00 BJT and a trough at 14:00 BJT.Interestingly,the maximum wintertime EI frequency occurs over Southeast China (SEC) rather than over the North China Plain (NCP),likely attributable to the combination of the heating effect of black carbon (BC) originating from the NCP,along with the strong subsidence and trade inversion in SEC.Correlation analyses between local meteorology and TI indicate that larger lower tropospheric stability (LTS) favors more frequent and stronger TIs,whereas the stronger EI under smaller LTS conditions (unstable atmosphere) is more associated with subsidence rather than BC.Overall,the spatial pattern of the lower tropospheric TI and its variability in China are mainly controlled by three factors:local meteorology,large-scale subsidence,and BC-induced heating.These findings help shed some light on the magnitude,spatial distribution,and underlying mechanisms of the lower tropospheric TI variation in China.(Guo Jianping,Chen Xinyan,Su Tianning)

2.11 The impact of synoptic patterns on summertime ozone pollution in the North China Plain

Surface ozone pollution is a challenging environmental issue in most parts of China.In particular,the North China Plain (NCP) region suffers from the severest ozone pollution throughout the country.In addition to the emission of precursors,ozone concentration is closely related to meteorological conditions resulting from regional atmospheric circulation.In this study,we investigate the relationship between synoptic patterns and summertime ozone pollution in the NCP using the objective principal component analysis in T-mode (T-PCA)classification method.Four dominant synoptic patterns are identified during the summers of 2014-2018.The heaviest ozone pollution is found to be associated with a high pressure anomaly over the Northwest Pacific and a distinct low pressure center in Northeast China.The southwesterly wind surrounding the low pressure center brings dry,warm air from inland South China,resulting in a high temperature,low humidity environment in the NCP,which favors the chemical formation of surface ozone.Locally,this type is associated with a moderate planetary boundary layer height (PBLH) of about 860 m and a stronger warm anomaly within the boundary layer than the upper level.We also notice a non-linear relationship between surface ozone concentration and the PBLH,i.e.,ozone concentration first increases with PBLH till about 0.9 km,and then remains stable.This initial increase may relate to enhanced mixing with upper levels where ozone concentration is typically higher than that near the surface.However,when PBLH further increases,this downward mixing effect is balanced with the stronger upward turbulent mixing so that surface ozone shows little change.The synoptic patterns identified here,however,are unlikely responsible for the observed increasing trend in ozone concentration over the NCP region.Our study sheds light on the meteorological contribution to surface ozone pollution in North China and provides a reference for the pollution control and prediction.(Dong Yueming,Li Jing,Guo Jianping)

2.12 The urban-rural heterogeneity of air pollution in 35 metropolitan regions across China

Urbanization and air pollution are major anthropogenic impacts on Earth’s environment,weather,and climate.Each has been studied extensively,but their interactions have not.Urbanization leads to a dramatic variation in the spatial distribution of air pollution (fine particles) by altering surface properties and boundarylayer micrometeorology,but it remains unclear,especially between the centers and suburbs of metropolitan regions.Here,we investigated the spatial variation,or inhomogeneity,of air quality in urban and rural areas of 35 major metropolitan regions across China using four different long-term observational datasets from both ground-based and space-borne observations during the period 2001−2015.In general,air pollution in summer in urban areas is more serious than in rural areas.However,it is more homogeneously polluted,and also more severely polluted in winter than that in summer.Four factors are found to play roles in the spatial inhomogeneity of air pollution between urban and rural areas and their seasonal differences:(1) the urbanrural difference in emissions in summer is slightly larger than that in winter; (2) urban structures have a more obvious association with the spatial distribution of aerosols in summer; (3) the wind speed,topography,and different reductions in the planetary boundary layer height from clean to polluted conditions have different effects on the density of pollutants in different seasons; and (4) relative humidity can play an important role in affecting the spatial inhomogeneity of air pollution despite the large uncertainties.(Han Wenchao,Li Zhanqing,Guo Jianping)

2.13 Vertical wind shear modulates particulate matter pollutions:A perspective from radar wind profiler wbservations in Beijing,China

Vertical wind shear (VWS) is one of the key meteorological factors in modulating ground-level particulate matter with an aerodynamic diameter of 2.5 µm or less (PM2.5).Due to the lack of high-resolution vertical wind measurements,how the VWS affects ground-level PM2.5remains highly debated.Here we employed the wind profiling observations from the fine-time-resolution radar wind profiler (RWP),together with hourly ground-level PM2.5measurements,to explore the wind features in the planetary boundary layer (PBL) and their association with aerosols in Beijing for the period from December 1,2018,to February 28,2019.Overall,southerly wind anomalies almost dominated throughout the whole PBL or even beyond the PBL under polluted conditions during the course of a day,as totally opposed to the northerly wind anomalies in the PBL under clean conditions.Besides,the ground-level PM2.5pollution exhibited a strong dependence on the VWS.A much weaker VWS was observed in the lower part of the PBL under polluted conditions,compared with that under clean conditions,which could be due to the strong ground-level PM2.5accumulation induced by weak vertical mixing in the PBL.Notably,weak northbound transboundary PM2.5pollution mainly appeared within the PBL,where relatively small VWS dominated.Above the PBL,strong northerlies winds also favored the long-range transport of aerosols,which in turn deteriorated the air quality in Beijing as well.This was well corroborated by the synoptic-scale circulation and backward trajectory analysis.Therefore,we argued here that not only the wind speed in the vertical but the VWS were important for the investigation of aerosol pollution formation mechanism in Beijing.Also,our findings offer wider insights into the role of VWS from RWP in modulating the variation of PM2.5,which deserves explicit consideration in the forecast of air quality in the future.(Zhang Ying,Guo Jianping,Yang Yuanjian)

2.14 亚洲中部干旱气候研究综述与机理分析

综述了亚洲中部干旱气候若干研究进展，并对亚洲中纬度干旱区形成机理进行了深入分析。依据大尺度气候分类法将全球陆地划分为季风、地中海及西风带气候区，分别对应于季节循环中降水—气温同位相，反位相及不相关等水—热配置。亚洲中部干旱区南北部分属地中海和西风带气候区，其不同水-热配置又对应于不同水汽输送及其散度等特征。研究揭示了欧亚大陆中部干旱区形成于降水不足且季节性水—热配置不当，起因于大气环流平均槽脊季节变化引起的大气动力—热力配置不当，其根源是海-陆热力差异及大地形对大气环流强迫的结果。本文还讨论了季风区与非季风区之间的季节性互动，以及行星尺度上不同气候区之间平均气流与瞬变涡动水汽散度之间的配置等问题。（汪萍）