Characteristics and source of aerosols at Shiyi Glacier，Qilian Mountains， China

XiaoYu Zhang， ZhongQin Li， Ping Zhou， ShengJie Wang

1. State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions Environment and Engineering Research Institute; Tianshan Glaciological Station, Chinese Academy of Sciences, Lanzhou, Gansu 730000, China

2. College of Geography and Environment Science, Northwest Normal University, Lanzhou, Gansu 730070, China



Characteristics and source of aerosols at Shiyi Glacier，Qilian Mountains， China

XiaoYu Zhang1*， ZhongQin Li1，2， Ping Zhou1， ShengJie Wang2

1. State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions Environment and Engineering Research Institute; Tianshan Glaciological Station, Chinese Academy of Sciences, Lanzhou, Gansu 730000, China

2. College of Geography and Environment Science, Northwest Normal University, Lanzhou, Gansu 730070, China

ABSTRACT

Aerosol samples were collected in the Shiyi Glacier， Qilian Mountains from July 24 to August 19， 2012 and analyzed for major water-soluble ionic species （F-， Cl-， NO2-， NO3-， SO42-， Na+， NH4+， K+， Mg2+and Ca2+） by ion chromatography. SO42-and NH4+were the most abundant components of all the anions and cations， with average concentrations of 94.72 and 54.26 neq/m3， respectively， accounting for 34% and 20% of the total water-soluble ions analyzed. These mean ion concentrations were generally comparable with the background conditions in remote sites of the Qilian Mountains， but were much lower than those in certain cities in China. The particles were grouped into two dominant types according to their morphology and EDX signal: Si-rich particles and Fe-rich particles. Backward air mass trajectory analysis suggested that inland cities may contribute some anthropogenic pollution to this glacier， while the arid and semi-arid regions of central Asia were the primary sources of the mineral particles.

aerosol； glacier； particles； scanning electron microscopy （SEM）

1　Introduction

Aerosols are an important atmospheric constituent that influences climate， and they are related to human activities （Buseck and Pósfai， 1999）. The climatic influence of aerosols is complex. Light scattering by aerosols decreases penetration of solar radiation through the atmosphere and absorption at the surface，increases cloud reflectance， enhances cloud lifetimes，and suppresses precipitation， thereby exerting a cooling influence （Schwartz and Andreae， 1996； Ramanathan et al.， 2007）. Although a considerable amount of attention has been paid to the radiative properties of aerosols， there are still uncertainties in qualifying their net effect on the global climate， largely due to the high spatial and temporal variability of aerosol concentrations and properties （Cong et al.， 2010）. Therefore，better understanding of the various effects of aerosols in the atmosphere requires more detailed information on aerosol chemical and physical properties from diverse geographic locations.

Water-soluble ions comprise a large part of aerosol particles and play an important role in the atmospheric chemistry （Deshmukh and Deb， 2013）. It was reported that water-soluble ions such as sulfate， nitrate， and other acid-rain-related pollutants have severe effects on human health （Raizenne et al.， 1996）.

Individual-particle analysis through scanning electron microscopy and energy dispersive X-ray analysis （SEM-EDX） can provide a great amount ofinformation on the differences in morphology， elemental composition， and particle density of aerosols to further investigate their potential sources as well as the transport of pollutants， which cannot be obtained through bulk ionic analysis （Paoletti et al.， 2002； Reid et al.， 2003； Srivastava et al.， 2009）.

Glaciers of the Qilian Mountains are sensitive indicators of climate change and are natural archives of variations in atmospheric processes. The Qilian Mountains are located at the center of the arid and semi-arid regions of central Asia， in the northeastern edge of the Tibetan Plateau which is connected to the arid region of northwest China. It is the major water source and climate regulation zone of the Hexi Corridor， and also the interaction zone of the Qinghai-Tibet Plateau and Asian dust activities.

Asian dust storms originating from this area as well as from the larger deserts on the planet， which include the Gobi， Taklimakan， and Badain Jaran deserts of Asia， are the primary sources of mobilized desert topsoils that move great distances through the atmosphere each year. Transport and deposition of this dust plays a significant role in the biogeochemical cycles and atmospheric chemistry of the Northern Hemisphere （Arimoto et al.， 1996； Griffin， 2007）.

The Shiyi Glacier in the Qilian Mountains （Figure 1）， with its highest elevation of 4，711 m a.s.l.， provides a unique opportunity to improve our understanding of aerosols transport in the free troposphere， which has potentially significant impacts on regional and global climates， and can deepen our knowledge of physical processes in the cryosphere （Wang et al.， 2013）.

Studies on atmospheric chemistry have been very limited to date on both temporal and spatial scales in this area. Ion chemistry studies of aerosols have suggested that the atmospheric environment is mainly controlled by natural sources； however， during summer monsoons， man-made pollution emissions cannot be ignored （Wang et al.， 2013）. Nevertheless， there is no data reported to reflect the air conditions directly. Therefore， in order to obtain updated knowledge of the air conditions， an intensive aerosol sampling experiment was conducted during a scientific expedition to Shiyi Glacier in the Qilian Mountains from July 24 to August 19， 2012.

The purposes of this study were to understand the inorganic ion chemistry properties of the aerosols，determine the elemental compositions and morphological properties of individual aerosol particles，identify their sources of origin， and evaluate the influence of anthropogenic pollutants on the atmosphere over the Qilian Mountains. The 13-day intensive sampling was intended to elucidate the characteristics of aerosols in summer time. This field experiment was the first-ever investigation of inorganic ion chemistry and individual particle analysis of aerosols.

Figure 1　Location map of the Shiyi Glacier with geographic environment

2　Aerosol sampling

Aerosol samples were collected for a 26-day period from July 24 to August 19， 2012. Aerosol samples were recovered on Zefluo TeflonTMfilters （2.0-μm pore size，47-mm diameter； Gelman Sciences） using a 12-V diaphragm pump powered by solar cells. The filters were loaded in the field and mounted face-down about 1.5 m above the ground surface. Air volume through each filter was measured by an in-line meter and then converted into standard conditions according to the local ambient pressure and temperature. The particle collection efficiency （for particles as small as 0.035 μm） was estimated to be ＞97% based on the mean flow rate of 1.27m3/h over the filter （Liu et al.， 1984； Zhao et al.， 2011）.

The sampling period for each aerosol sample was 3-10 h， with samples taken from 09:00 to 18:00 （Beijing time） depending on weather conditions. No samples were collected during rain or snow events. After sampling， the filters were removed from the filter holder and placed in cleaned airtight plastic containers and stored at 4 °C before analysis. Meteorological parameters， including air temperature， wind speed and direction， precipitation， air pressure， and relative humidity were measured simultaneously by an automatic weather station that was placed near the aerosol sampling site. The actual measured meteorological parameters are summarized in Table 1.

Table 1　Meteorological conditions during the sampling period

3　Sample analysis

Eight major inorganic ions （Cl-， NO3-， SO42-， Na+，NH4+， K+， Mg2+， and Ca2+） of the aerosol samples were analyzed in a class 100 clean room， using a Dionex ion chromatograph （model DX-320； Thermo Fisher Scientific Inc.）. To efficiently extract the water-soluble ions from the TeflonTMaerosol filters， each filter was first wetted with 200 μL of ultra-pure methanol and then extracted with 25 mL of deionized water for about 30 min using an ultrasonic water bath device （Sun et al.， 1998； Zhao and Li， 2004； Zhao et al.， 2011）.

The cation chromatography conditions were as follows: a Dionex IonPac CS12A （4mm×250mm）analytical column， 15 mmol/L and 1.0 mL/min methane eluent， and 200 μL sample injected； for anions，an AS11-HC （4mm×250mm） analytical column， 15 mmol/L and 1.0 mL/min NaOH eluent， and 200 μL sample injected （Zhao and Li， 2004）.

The average concentrations of 10 laboratory blanks were: Cl-（8.50 ng/g）， NO3-（48.43 ng/g）， SO42-（6.07 ng/g）， Na+（7.76 ng/g）， NH4+（11.71 ng/g）， K+（7.11 ng/g）， Mg2+（2.36 ng/g）， and Ca2+（13.65 ng/g）. These were obviously lower than the concentrations detected in the aerosol samples. Mean blank values were subtracted from the sample concentrations and then the sample concentrations were divided by the sample volumes （at standard conditions） of each sample and converted into micrograms per cubic meters， which are listed in Table 2. Detailed sampling methods and analytical techniques of aerosols are described by Zhao and Li （2004） and Zhao et al. （2011）.

For each membrane， a section of the total filter was cut and mounted onto the electron microprobe stub，and coated with a thin gold film （10 nm） for a higher-quality secondary electron image. The signal of Au in the EDX diagrams appeared at the fixed opposition，and the gold film was too thin to prevent proper identification of the other elemental compositions. In our study， individual particles were analyzed using a scanning electron microscopy-energy dispersive X-ray spectrometer （SEM-EDX）. A section of each filter was cut and mounted onto the electron microprobe stub，and coated with a thin gold film （16 nm） for a higher-quality secondary electron image. The operating conditions were: accelerating voltage in range of 5-10 kV； spectral acquisition time is 60 s. NoranTMSystem software （Thermo Fisher Scientific Inc.） for energy-dispersive microanalysis was used for the quantitative analysis of individual particles.

4　Results and discussion

4.1Mass loading of chemical composition of aerosols over Shiyi Glacier

4.1.1Concentrations of water-soluble inorganic ions in total aerosols

A total of 13 atmospheric aerosol samples were collected during the scientific expedition to Shiyi Glacier. To provide an overview of the chemical composition characteristics， the daily concentrations of the major inorganic ions （F-， Cl-， NO2-， NO3-，SO42-， Na+， NH4+， K+， Mg2+， and Ca2+） are shown in Table 2. Of all the anions， sulfate was the dominant component， followed by nitrate； of the cations， ammonia had the highest concentration， while the concentrations of chloride and nitrite were much lower than the others.

The concentration of SO42-ranged from 17.05 to 315.40 ng/m3， with a mean value of 94.72 ng/m3， while the concentration of NO3-varied from 55.43 to 8.82 ng/m3， with a mean value of 27.41 ng/m3. The mean concentration of NO2-was about 3.09 ng/m3and Clwas about 6.41 ng/m3. Ammonia and calcium were the most prominent components. The concentration of Ca2+ranged from 13.81 to 133.03 ng/m3with an average value of 41.32 ng/m3， and the concentration of ammonia varied from 4.06 to 176.13 ng/m3with anaverage value of 54.26 ng/m3； K+and Na+had lower levels of concentration than the other analyzed cationic species， averaging about 6.85-14.49 ng/m3.

The following ion balance calculations are commonly used to evaluate the acid-base balance of aerosol particles:

∑+= Na+/23 + NH4+/18 + K+/39 + Mg2+/12 + Ca2+/20 (1)

∑-= F-/19 +Cl-/35.5 + NO2-/46 + NO3-/62 + SO42-/48 (2)

The ratios of the sum of the equivalent concentrations of cations to anions （∑+/∑-） were calculated and presented in Table 2. The ion balance expressed by the sum of the concentrations of anion-to-cation is a good indicator to study the acidity of the environment（Verma et al.， 2010）.

The ratios calculated for all aerosol samples ranged from 0.675 to 2.033. Such variation of ion balance can probably be attributed to the different air masses that arrived at the sampling site， which meant there were different origins with different air conditions.

The ∑+/ ∑-ratios for the aerosol samples on July 31 and August 3-5 were ＜1.0， which indicated that these samples were acidic. This was possibly due to the NO3-and SO42-increasing much more than the cations. The ∑+/∑-ratios for the aerosol sample collected on July 29 was 1.02， suggesting that aerosol particles on that day were almost neutral. The ratios of the other samples were ＞1.0， indicating that they were more alkaline. The total equivalents of anions were plotted against the total equivalents of cations， and the slope of the regression line was slightly lower than unity （Figure 2， slope = 0.97， R2= 0.77）， which implied cation deficiencies.

High temperature is probably the primary reason for the volatilization of NH4+（Khoder and Hassan，2008）. However， the lower temperature over this glacierized region （Table 1） may have contributed to the very low vaporization or volatilization of NH4+in our samples. Thus， the cation deficiencies could mostly be attributed to H+， which was not counted in Equations (1) and (2) above （Zhao et al.， 2011）.

Because organic ions are another important constituent of aerosol particles， the lack of organic ions would cause some errors in estimation of the ion balances. Because ions like formate， acetate， and other organic acid ions will contribute to the sum of the equivalent concentration of anions ∑-， the real values of the ∑+/ ∑-ratios will become smaller. Therefore，the deviation to the upper side of the theoretical line indicates a deficiency of anions， since bicarbonate，organic ions （formate and acetate）， CO32-， HCO3-，PO43-and Br-were not determined in the present study （Khoder and Hassan， 2008）.

4.1.2Correlation between ionic species in aerosols

In order to identify and separate the impacts of various sources， correlation coefficients between the ionic species were calculated and are shown in Table 3. The sum mass concentration of SO42-， NH4+， NO3-and Ca2+accounted for about 80% of all the 10 ions measured， and significantly positive correlation coefficients were found between SO42-and NH4+（r = 0.98）， NO3-and SO42-（r = 0.79）， and Ca2+and NO3-（r = 0.79）.

Querol et al. （1998） proposed the presence of（NH4）2SO4·CaSO4·2H2O for the NH4+/SO42-molar ratio close to 1.0 based on XRD （X-ray diffraction）observations. In the present study， the NH4+/SO42-ranged from 0.24 to 0.66， with a mean value of 0.37，and all of the ratios were ＜1.0， indicating that sulfate mayhavebeenpresentasCaSO4and（NH4）2SO4·CaSO4·2H2O. Consistent with the observations of previous investigators， NH4+/SO42-molar ratios ＜1.0 could be explained by the combination of sulfate with calcium to form CaSO4， and with ammonium and calcium to form （NH4）2SO4·CaSO4·2H2O（Duan et al.， 2003； Khoder and Hassan， 2008）.

Figure 2　Ion balance of the major water-soluble inorganic ions in the aerosols of Shiyi Glacier， Qilian Mountains

Table 3　Inter-species correlation coefficients

To separate anthropogenic sources from natural sources of SO42-， we calculated the value of the equivalent concentration ratio of SO42-/Ca2+as described by Ming et al. （2007）. High values of SO42-/Ca2+（43.2 and 138.8） near Mount Qomolangma suggested that SO42-had primarily anthropogenic sources， while lower value （3.8） suggested primarily dust sources （Ming et al.， 2007）. The values of SO42-/Ca2+in our samples ranged from 0.97 to 5.70（Table 2）. This indicated that domestic house heating in winter， industrial coal burning， and other fossil fuel burning were the major potential pollution sources of SO42-in this region. This means the SO42-in our samples was controlled by both natural processes and anthropogenic sources.

Gaseous NH3can be either wet- or dry-deposited，or can neutralize H2SO4， HNO3and HCl to form ammonium bisulphate （NH4HSO4）， ammonium nitrate（NH4NO3）， and ammonium chloride （NH4Cl） via particle gas formation and gas-to-particle conversion（Baek and Aneja， 2005； Aneja et al.， 2009； Ianniello et al.， 2011）. They are usually formed in these formulae: H2SO4+NH3→NH4HSO4； HNO3+ NH3↔ NH4NO3；HCl + NH3↔ NH4Cl； therefore， some of the NH4+in this study could also exist as NH4HSO4， NH4NO3， and NH4Cl （Ianniello et al.， 2011）. The formations are significant in summer due to the conversion of gaseous precursors （Wang et al.， 2005）， while the potential sources for ammonia in the formula are human and agriculture activities such as animal farming， use of fertilizers， and organic decomposition （Sun et al.，1998； Kreutz et al.， 2001； Wu et al.， 2006； Verma et al.， 2010）.

Nitrogen oxides emissions from mobile sources are an important contributor to NO3-in the atmosphere. Therefore， the mass ratio of NO3-/SO42-has been used as an indicator of the relative importance of stationary versus mobile sources of sulfur and nitrogen in the atmosphere （Arimoto et al.， 1996； Khoder and Hassan 2008）. Arimoto et al. （1996） ascribed a high NO3-/SO42-mass ratio to the predominance of mobile sources over stationary sources of pollutants. In the present study， the average mass ratios of NO3-/SO42-were 0.37 on average， relatively similar to that found in some cities in China， such as Shanghai （0.43； Yao et al.， 2002）， Qingdao （0.35； Hu et al.， 2002）. This was slightly higher than those reported in Taiwan （0.2；Fang et al.， 2002） and Guiyang （0.13； Xiao and Liu，2004）， and lower than those found in Beijing during 2001-2003 （0.71； Wang et al.， 2005） and Shanghai in 2006 （0.83； Wang et al.， 2006）. Generally， the mass ratio of NO3-/SO42-in the aerosols of Shiyi Glacier was ＜1.0， suggesting that stationary source emissions were more predominant.

NO3-is the most of the reaction product of NOxemission， while the NOxemission is mainly due to fossil fuel combustion and biomass burning （Wang et al.， 2006； Wu et al.， 2006）. Under an alkaline atmospheric environment， gaseous HNO3formed by the oxidation of NOxcan be absorbed on the surface of mineral particles and reacted to form salts （Mamane and Gottlieb， 1992）. This may be the reason why NO3-has significant correlations with Na+， NH4+and Ca2+（r = 0.75， 0.74， and 0.79， respectively）. Previous studies found that automobile exhaust is a major contributor of NO3-（Arimoto et al.， 1996； Wang et al.， 2005， 2006；Bhaskar et al.， 2010）. Glacio-chemical investigation from the Belukha Glacier （Olivier et al.， 2003） also pointed out that increased nitrates resulted from the growth of traffic and the associated rise of the emission of precursor gases NOx. Therefore， the higher content of NO3-in our study was likely related to anthropogenic pollutions， and traffic emissions from Qilian County may also be a potential source.

Significant correlations with Ca2+， Na+， SO42-and NO3-suggest that there are some other compounds originating from desert dust， existing as Na2SO4， NaNO3and Ca（NO3）2. Calcium is usually rich in desert and loess soils， and even in atmospheric aerosol particles collected from the desert and loess areas， Ca2+was always accepted as a proxy for dust in past studies of snow and ice chemistry （Mayewski et al.， 1993；Wolff， 1996； Zhang and Edwards， 2011）. Previous studies over the Tianshan Mountains found that thesurrounding desert and arid regions were usually considered to be the primary sources of Ca2+（Williams et al.， 1992； Sun et al.， 1998； Aizen et al.， 2004； Zhang et al.， 2008； Zhang et al.， 2012）. Okada and Kai （2004）also found that the calcium-rich mineral particles in deserts usually present as CaCO3， CaSO4， and internal mixtures of CaCO3and CaSO4or silicates. However， in this study the potential existence of Ca（NO3）2gave us a new revelation， that is， human activity has already affected this region by the use of chemical fertilizers，because calcium nitrate is a typical fast-acting fertilizer material used in agriculture. It is also used in the electronics industry for coating cathodes.

Halite （NaCl） particles were commonly detected in aerosol particles collected over the Taklimakan Desert（Okada and Kai， 2004）. Wake et al. （1990） and Sun et al.（1998） also pointed out that Na+and Cl-in the Tianshan Mountains represent an input of Na+and Cl-rich dust originating from the extensive evaporate deposits in the arid regions surrounding these mountain ranges.

Potassium is also probably derived from the evaporation of insecticides， chemical fertilizers， and dust， because it has higher correlations with F-and SO42-（r = 0.53 and 0.45， respectively）.

4.1.3Comparison with data of the other areas

In our study， mean ion concentrations of the aerosol samples were ordered as SO42-> NH4+> Ca2+> NO3-> Mg2+> K+> F-> Na+> Cl->NO2-. These mean ion concentrations of the Shiyi Glacier were generally comparable with the background conditions in remote sites in the Qilian Mountains and the Tianshan Bogda Glacier， except that SO42-and Ca2+were slightly higher， and the ion concentrations were much lower than those in cities such as Beijing of China and Raipur of India （Table 4）.

4.2Individual particles analysis

The particles were grouped into two dominant types according to their morphology and EDX signal（Table 5）. Figure 3 shows the typical individual particles loaded on the filters at different magnifications. The distinct characteristics and possible origins of each group are described in detail as follows.

4.2.1Si-rich particles

Mineral particles， comprising silicates （Na2SiO3，CaMg3Si4O12， K2Al2Si6O16， Al2（OH）4Si2O5）， quartz（SiO2）， feldspars （KAlSi3O8， NaAlSi3O8， CaAl2Si2O8），gypsum （CaSO4）， calcite （CaCO3）， andpyroxene （Ca，Mg， Fe， Al）2（Si，Al）2O6， etc.， were the richest particles over this region. The sizes of the mineral particles generally ranged from 0.6 to 14.1 μm. Most of these particles had irregular shapes （Figure 3）， suggesting that they were from natural sources. Particles containing predominantly silicon were classified as quartz（SiO2）， with the irregular shapes shown in Figures 3a，b，d. As an important constituent of many rock types and dust， quartz is almost ubiquitous on land areas.

In our study， particles containing Si were the major composition， with an average abundance of 38.3%. These particles can mainly be attributed to the surrounding dust sources or aeolian dispersion of soil particles， which originated from local deserts or through long-range transport from the west. This was similar to the results of aerosols collected from the high Himalayas （Cong et al.， 2010）.

About 11.5% of the particles were identified with high amounts of Ca. These particles could be classified into three types in theory: calcium sulfates characterized by high and approximately equal amounts of Ca and S，and calcium carbonates and calcium nitrates characterized by only relatively high X-ray intensities of Ca. The primary natural source of calcium sulfate is gypsum， a typical mineral in the earth crust but generally much less abundant than CaCO3（Gao et al.， 2007）. Since the lighter elements such as C， N， and O are very difficult to quantify from EDS spectra， calcium nitrate and carbonate cannot be distinguished in an automated analysis， which is why calcium sulfate is easily identified.

4.2.2Fe-rich particles

Fe-rich particles are considered to be oxides（hematite， magnetite） or oxyhydroxides （goethite）（Cong et al.， 2010）. Fe-rich particles accounted for 27.2% of the findings in the Shiyi Glacier. Besides soil dispersion， Fe-rich particles can also be produced by coal-fired boilers， metal industries， and power plants. Fe-rich particles emanated from a high-temperature furnace usually show a spherical shape. However， Fe particles in this study had irregular （non-spherical）morphology （Figure 3c）， and were thus identified as being of natural origin.

There were also some very small spherical-shaped particles with smooth edges and surfaces， shown in Figures 3a，c； these were likely emitted by a high-temperature process. They may have originated from the steel industry or from coal combustion. These unrecognized particles could contain organic particles with light elements （such as C， N， and O） or biological particles.

In summary， most of the large particles detected over the Shiyi Glacier were probably from natural mineral dust， while very small particles may have been emitted by anthropogenic activities. This suggests that both natural processes and anthropogenic pollution were the aerosol sources over this glacierized site during the sampling period.

Table 4　Water-soluble inorganic ions （μg/m3） of aerosols at different sites all over the world

Table 5　Individual particle types determined by clustering and their relative abundance

Figure 3　SEM images of typical aerosol particles. （a）， （b）， （d）: Si-rich particles； （c）: Fe-rich particles

4.2.3Particle shapes indicated by particle circularity

We used particle circularity as the shape parameter to characterize the degree of shape complexity of the particles. The circularity was defined as:

where C is circularity （non-dimensional）， L is particle perimeter （μm）， and A is the two-dimensional area of the particle in the SEM image （μm2） （Gao et al.， 2007）.

A circle has a circularity of 1.0， and a square has a circularity of 1.27. As the shape gets more complex，the circularity value increases. The circularity of the Si-rich type of dust particles （Figures 3a，b，d） ranged from 2.9 to 4.4， with a mean value of 3.77， suggesting the non-spherical nature of dust particles over the region.

4.3Relationships with air mass transport

The Hybrid Single-Particle Lagrangian Integrated Trajectory （HYSPLIT） model of the NOAA Air Resources Laboratory has been widely used in previous studies （e.g.， Ming et al.， 2007； Cong et al.， 2010）. We used this model to simulate five-day backward air trajectories with a daily resolution to identify potential transport pathways and possible source regions of the aerosols at the Shiyi Glacier. Figure 4 presents the daily backward trajectories terminated at the sampling site（38.21°N， 99.88°E； 4，637 m a.s.l.） during the sampling periods. Most of the air masses were transported from the west， northwest， and south. Combined with the ion concentrations listed in Table 2， air masses arriving at the sampling site could be classified into three types:

1） The air masses which arrived on August 3， 15，and 19 originated from or were transported through the arid regions of Kazakhstan， Kyrgyzstan， Uzbekistan，and the northern region of the Taklimakan Desert. They then passed through the urban area of Yumen City， Dunhuang City， Qinghai Lake， and Qilian County，which may have resulted in the input of desert dust and some anthropogenic pollution. These air masses caused the higher concentrations of cations. Also， the concentrations of SO42-， NO3-， and NH4+were very high.

2） The air masses which arrived on July 24 and July 31 represented the short-range aerosol transport of air masses originating from the western edge of the Badan Jaran Desert. They passed through the north slope of the Qilian Mountains and arrived at the glacier. The relatively lower concentrations of SO42-，NO2-， NO3-， and NH4+at the Shiyi Glacier may be attributed to these cleaner air masses.

3） The air masses which arrived on July 27 and July 29， and August 4 and 5 represented the short-range aerosol transport of air masses originating from the south， specifically the Sichuan Hengduan Mountains. They crossed over two other provinces （Gansu and Qinghai） and finally reached our sampling site.

Our backward air mass trajectory analysis suggested that inland cities may have contributed some anthropogenic pollution to this glacier， while the arid and semi-arid regions of central Asia were the primary sources of the mineral particles.

5　Conclusions

Intensive aerosol sampling experiments were conducted from July 24 to August 19， 2012， providing the first-ever information about the atmosphere over the Shiyi Glacier in the Qilian Mountains. Inorganic ion chemistry and individual particle characteristics of the aerosols were analyzed through ion chromatography and SEM-EDX， respectively.

The results showed that SO42-and NH4+were the most abundant components of all the anions and cations， respectively. Of all the anions， sulfate was the dominant component， followed by nitrate. Ammonia had the highest concentration of the cations， while concentrations of chloride and nitrite were much lower than the others.

The relationship between the eight inorganic ion species and their origins indicated that domestic house heating in winter， industrial coal burning， and burning of other fossil fuels were the potential pollution sources of SO42-in this region. The SO42-in our samples was controlled by both natural processes and anthropogenic sources.

The mass ratio of NO3-/SO42-in the aerosols of the Shiyi Glacier was ＜1.0， suggesting that the stationary source emissions were more predominant， while the potential existence of Ca（NO3）2suggested that human activity has already affected this region by the use of chemical fertilizers.

Based on morphology， elemental compositions，and EDX signal， the particles were grouped into two dominant types: Si-rich particles and Fe-rich particles. Most of the large detected particles over this region were probably from natural mineral dust， while very small particles may have been emitted by anthropogenic activities. This suggests that both natural processes and anthropogenic pollution were the aerosol sources over the Shiyi Glacier during the sampling period.

Acknowledgments:

This research was jointly supported by the National Natural Science Foundation of China （Nos. 41201065，41121001， 41261017， 41171057， 41161012）； Funds for Creative Research Groups of China （No. 41121001）， the Foundation for Excellent Youth Scholars of CAREERI， CAS （No. 51Y251B51）； the SKLCS Foundation （No. SKLCSZZ-2012-01-01）； the National Basic Research Program of China （Nos. 2010CB951003， 2010CB951404）， and the KnowledgeInnovation Project of the Chinese Academy of Sciences （No. KZCX2-EW-311）. We would like to thank WenBin Wang， MingYing Xia， YuanYang She， XuLiang Li， and Shuang Jin for the sampling， and YuMan Zhu， QiuFang Bao， and YaPing Liu for the chemical analyses.

Figure 4　Daily backward trajectories arriving at the sampling site （38.21°N， 99.88°E； 4，637 m a.s.l.）

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*Correspondence to: Ph.D.， XiaoYu Zhang， Cold and Arid Regions Environmental and Engineering Research Institute，Chinese Academy of Sciences. No. 320， West Donggang Road， Lanzhou， Gansu 730000， China. Tel: +86-931-4967381；E-mail: zhangxiaoyu@lzb.ac.cn

October 16， 2015Accepted: December 16， 2015