Abstract -- Aerosol samples were collected at four high elevation sites (> 5000 m asl) in the mountains of central Asia. The sites extend from the southern slopes of the Himalayas to the northern margin of the Tibetan Plateau and are representative of the natural landscape variations in the highlands of central Asia. Daily samples were collected over periods of four days to two weeks in late summer or early autumn. This period is typically one of relatively low levels of dust in the Asian troposphere. Here we discuss the water soluble chemical composition of the aerosol samples.

Tropospheric aerosols from the south slope of the Himalayas and the southern/central portions of the eastern Tibetan plateau are dominated (in order of importance) by NH₄⁺, SO₄⁼, NO₃^- and Ca⁺⁺. Concentrations of these species are comparable to previously reported measurements in the remote troposphere. Tropospheric aerosol from the north-eastern region of the Tibetan Plateau shows very high levels of Ca⁺⁺, SO₄⁼, Cl^-, and Mg⁺⁺ due to the influx of evaporite mineral rich dust derived from the Qaidam Basin and/or Taklamakan Desert. Our results confirm that high elevation mountain sites in the Himalaya and southern/central regions of the eastern Tibetan Plateau provide isolated platforms above the planetary boundary layer from which to investigate the composition of the remote continental troposphere. Fresh and surface snow samples were also collected. The results show that the general composition and spatial pattern in summer snow chemistry is similar to that for aerosols.
 

Key word index: Aerosol sampling and analysis, Asian dust, remote troposphere, nitrate, sulfate

INTRODUCTION

REGIONAL SETTINGS

EXPERIMENTAL METHODS

RESULTS

DISCUSSION

CONCLUSIONS

ACKNOWLEDGMENTS

REFERENCES
 
 

INTRODUCTION

Windblown dust derived from arid regions of the Earth is a major source of tropospheric aerosol particles (Junge 1979; Prospero et al., 1983). While the arid regions of central Asia are a major source area for windblown dust in the northern hemisphere, most of our knowledge concerning Asian dust originates from atmospheric aerosol and ocean sediment sampling programs distant from this source. For example, the production and long-range transport of Asian dust peaks in spring when large quantities of dust are transported to eastern China (Liu et al., 1981; Winchester et al., 1981; Gao et al., 1992a), and the north Pacific (e.g., Parrington et al., 1983; Uematsu et al, 1983; Prospero et al., 1985; Merrill et al., 1989; Gao et al., 1992b). Dust derived from the deserts in central Asia is also a significant source of deep sea sediments in the north Pacific ocean (e.g., Blank et al., 1985). Large quantities of mineral aerosol in the Arctic regions have also been traced to Asian sources (Rahn et al., 1977; Welch et al., 1991).

Despite the global importance of mineral dust derived from the large desert regions of central Asia, very little is known concerning atmospheric concentration and chemical composition of atmospheric aerosol close to the source regions during either low dust (i.e., July to January) or high dust (i.e., February to June) periods. Isolated atmospheric aerosol samples have been collected in central Asia in the territories of the former USSR (Mészáros, 1978) and on the south slopes of the Himalayas (Ikegami et al., 1978; Kapoor and Paul, 1980; Davidson et al., 1986). More recently a multi-disciplinary Soviet/American team investigated the production, chemical and physical properties, and meteorological and climatic effects of eolian dust in the Tajikistan (e.g., Gillette and Dobrowolski, 1991).

Over the past several years we have conducted a regional survey of snow chemistry throughout the glaciated regions in the mountains of central Asia (Mayewski et al., 1984; Wake 1989; Wake et al., 1990; 1992; 1993). In conjunction with this program, we have collected aerosol samples at four remote, high elevation sampling sites; one on the south slope of the Himalayas and three on the Tibetan Plateau. Daily samples were collected over four day to two week periods in late summer or early autumn and represent aerosol composition of the central Asian troposphere during typically low dust periods. Here we discuss the regional distribution in the water soluble chemical composition of the aerosol samples and compare our results with other remote tropospheric aerosol chemistry data.

REGIONALSETTING

Site Description

The four sample sites roughly lie along a south-west to north-east trending transect which extends from the southern slopes of the Himalayas to the northern margin of the Tibetan Plateau (Fig. 1). The sample collection sites cover a wide geographic area and can be separated into three distinct regions on the basis of their landscape, as defined by Alekseyev et al., (1988), and/or on the basis of their climatic regime (i.e., relative influence of monsoonal versus westerly air masses) as reviewed by Ramage (1981), Barry and Chorley, (1982), and Hastenrath (1985):

(i) Ngozumpa Glacier (site 1 in Fig. 1) lies on the southern slopes of the eastern Himalaya. This region is characterized by extreme relief. Mixed forest and small-scale agriculture dominate below 4500 meters above sea level (m asl); while glaciers and mountains dominate above this elevation. Precipitation is derived primarily from monsoonal air masses in summer and from westerly depressions during the winter (Barry, 1981).

(ii) Xixabangma Peak (site 2) and Mt. Geladaindong (site 3) fall within the grassland steppes which dominant in the southern and central regions of the eastern Tibetan Plateau. Mt Geladaindong lies close to the transition zone between grassland steppes and arid regions to the north and west. The eastern Tibetan Plateau receives most of its precipitation from summer plateau monsoon circulation (Murakami, 1976).

(iii) Meikuang Glacier (site 4) lies in the arid regions which characterize the northern and western Tibetan Plateau. The Qaidam Basin and Taklamakan Desert, which contain calcareous soils and extensive salt deposits (Dregne, 1968; Chen and Bowler, 1986) lie north and northwest, respectively, of Meikuang Glacier (Fig. 1).
 
 
 

Atmospheric Circulation

Atmospheric circulation in central Asia is characterized by a marked seasonal shift in wind systems associated with the Asian monsoon (Ramage, 1971; Barry and Chorley, 1982; Hastenrath, 1985). During the winter the westerly jet stream is split into two branches, one passing to the north and one to the south of the Tibetan Plateau. In May and June the southern jet stream slowly weakens, and by mid-June is altogether replaced by an easterly jet stream, as the summertime high level anticyclone develops over the Tibetan Plateau. During the summer, Indian summer monsoon circulation transports moisture from the Bay of Bengal to the eastern and central Himalaya (site 1). Moisture from the Bay of Bengal is also carried into the eastern portions of the Tibetan plateau (site 2, 3, and 4) via summer plateau monsoon circulation (Murakami, 1976; Domrös and Peng, 1988). The peak in dust storm activity throughout China occurs from mid-February until late May, with a strong maximum in late April-early May (Merrill et al., 1989).

Aerosol Samples

Aerosol samples were collected during late summer or autumn at four different high elevation sites in the mountains of central Asia (Table 1) during periods when dust storm activity in central Asia is typically minimal (Merrill et al., 1989; Gao et al., 1992b). All sampling sites were in remote regions distant from anthropogenic emissions. The aerosol samples were recovered on 2 micron Zefluor Teflon filters using a lightweight (= 30 kg), portable 12 volt system powered by photo voltaic cells. Two micron Zefluor filters have a collection efficiency greater than 97% for aerosol particles in the size range 0.035-1.0 microns (Liu et al., 1984), and also exhibit minimal positive artifact problems with respect to the collection of gaseous species such as SO₂ and HNO₃ (Appel et al., 1979, 1984). Aerosol samples during the 1990 sampling season (i.e. samples from Ngozumpa Glacier, Mt. Geladaindong and Meikuang Glacier) were collected on 47 mm diameter Zefluor filters using a diaphragm pump. The filters were loaded into filter holders in the field adjacent to the sampling site and then mounted face down inside an open faced, 0.20 m diameter polyethylene housing two meters above the surface. Daily aerosol samples were collected over periods of 4 to 8 hours during daylight. Sample volumes were measured using an in-line totalizing flow meter. Measured volumes were converted to standard cubic meters (scm) using corrections for ambient temperature and atmospheric pressure. Mean volume for 12 samples collected on 47 mm filters was 5.4 scm. Immediately following sample collection the 47 mm filters were removed from the filter holder, folded with the exposed surface facing inwards and packed into a clean polyethylene bag.

The low flow diaphragm pump was replaced with a higher flow pump to increase the volume of samples collected in 1991 (Xixabangma Peak). Samples were collected on 90 mm diameter Zefluor filters. Each filter was loaded into a polyethylene cassette in a class 100 clean room at the University of New Hampshire, and then sealed inside a clean polyethylene bag. Just prior to sample collection the filter was removed from the sealed bag and mounted, once again, face down inside an open faced, 0.20 m diameter polyethylene housing two meters above the snow surface. Daily samples were collected over periods of 3 to 6 hours; mean volume for 7 samples was 11.2 scm. After sampling, the filters, still in their cassettes, were returned to the original clean polyethylene bag.

All loading and unloading of filters was performed wearing a non particulating clean suit, hood, face mask and plastic gloves. Field filter blanks (4 each of the 47 and 90 mm diameter filters) were loaded and unloaded in the same manner as the respective sample filters.

Sample and blank filters were wetted with methanol (=200 µl for 47 mm filter and =500 µl for the 90 mm filter) and extracted using two aliquots of 10 and 5 ml each, for the 47 and 90 mm diameter filters, respectively. Analysis for anions (Cl^-, NO₃^- and SO₄⁼) and cations (Na⁺, NH₄⁺, K⁺, Mg⁺⁺, Ca⁺⁺) were performed on a Dionex Ion Chromatograph using AS4A, and Fast CAT-I and CAT-II columns, respectively. The mean ion concentrations of the field blanks were subtracted from the ion concentrations in the samples.

We define the detection limit for major ions as two times the standard deviation of all the field blank extracts divided by the mean volume of all the samples (after Talbot et al., 1986). The detection limit for atmospheric aerosol species collected on 47mm filters (mean volume = 5.4 scm) were (in neq scm-1): Cl^- (0.50), NO₃^- (0.66), SO₄⁼ (0.54), NH₄⁺(1.0), Mg⁺⁺ (0.07) and Ca⁺⁺ (0.24). As a result of high variability in blank values, Na⁺ and K⁺ in 11 of the 12 samples collected on the 47mm filters were below detection. The limit of detection for atmospheric aerosol species collected on 90 mm filters (mean volume = 11.2 scm) were (in neq scm-1): Cl^- (0.31), NO₃^- (0.03), SO₄⁼ (0.06), NH₄⁺(0.08), K⁺(0.21), Mg⁺⁺ (0.07) and Ca⁺⁺ (0.29). Due to high variability in the blank values, Na⁺ concentrations in all samples collected on 90mm filters were below detection. Below detection levels of K⁺ and Mg⁺⁺ at Xixabangma Peak, and Cl^- at Ngozumpa Glacier, were due predominantly to low atmospheric concentrations.

The uncertainty in atmospheric ion concentrations associated with variability in the blank values and precision of the ion chromatograph analyses were determined based on the propagation of errors (Miller and Miller, 1988). The overall mean uncertainty for samples collected on the 47 mm filters were (in neq scm-1): Cl^- (0.27), NO₃^- (0.35), SO₄⁼ (0.26), NH₄⁺ (0.61), Mg⁺⁺ (0.18) and Ca⁺⁺ (0.68). The overall mean uncertainty for samples collected on the 90 mm filters were (in neq scm-1): Cl^- (0.16), NO₃^- (0.02), SO₄⁼ (0.03), NH₄⁺ (0.15), Mg⁺⁺ (0.06) and Ca⁺⁺ (0.17).

The detection limits and uncertainty in atmospheric ion concentration is equivalent or lower in the samples collected on the 90 mm filters. This is due to a combination of factors which include the doubling of mean sample volume and halving the extract volume in the samples collected in 1991 (site 2) as well as improving the handling techniques of the 90 mm filters (i.e., loading filters in class 100 clean room and leaving filters in cassettes until just prior to analysis).
 

Post-Dust Season, Summer Snow Samples

Details of the fresh and surface snow sample collection are provided in Table 2. Only at Meikuang Glacier did a snowfall event occur during the period that aerosol samples were collected. At Xixabangma Peak, fresh snow fell on 11 September, six days before aerosol samples were collected. Fresh snow samples were collected within twelve hours after the end of the snowfall event. At Ngozumpa Glacier and Mt. Geladaindong, surface snow samples were collected which represent summer snow deposited after the end of the major dust storm activity in Asia. All snow samples were collected using clean techniques described elsewhere (Wake et al., 1992). Quantification of major ion concentrations in snow were performed using ion chromatography as described above.

RESULTS

Aerosol Samples

The concentrations of water soluble aerosol species in the central Asian troposphere are presented in Table 3. The sample locations represent three distinct physiographic regions in central Asia which can be characterized by their landscape and climatic regimes, as well as by their snow chemistry characteristics (Wake et al., 1993). Ngozumpa Glacier represents the southern slopes of the eastern Himalaya; Xixabangma Peak and Mt. Geladaindong represent the southern and central regions of the eastern Tibetan Plateau; while Meikuang Glacier represents the north-eastern region of the Tibetan Plateau.

During low dust periods in the central Asian troposphere, the chemical characteristics of aerosol samples from Ngozumpa Glacier, Xixabangma Peak and Mt. Geladaindong are similar (Fig. 2). Aerosol from all three sites display relatively low ion concentrations; the sum of measured ions

(i.e., E = Cl^- + NO₃^- + SO₄⁼ + NH₄⁺ + Mg⁺⁺ + Ca⁺⁺) in all samples is less than 18 neq scm-1. The dominant ions at Ngozumpa Glacier are NH₄⁺, SO₄⁼, and NO₃^-. These three ions account for more than 85% of the total burden of measured ions. The mean SO₄⁼ to NO₃^- ratio at Ngozumpa Glacier is 1.6. At Xixabangma Peak, NH₄⁺ and SO₄⁼ account for more than 80% of the total burden of measured ions. NO₃^- levels here are 15 to 20 times lower compared to Ngozumpa Glacier and Mt. Geladaindong, increasing the mean SO₄⁼ to NO₃^- ratio to 14. NH₄⁺, SO₄⁼, and NO₃^- concentrations in aerosol samples from Mt. Geladaindong are similar to those from Ngozumpa Glacier and the mean SO₄⁼ to NO₃^- ratio is 1.1. However, Ca⁺⁺ concentrations at Mt Geladaindong show a five-fold enhancement compared to levels at Ngozumpa Glacier and Xixabangma Peak.

In contrast to the relatively low ion burden at the more southerly sites, the sum of measured ions at Meikuang Glacier ranges from 65 to 1154 neq scm-1. The dominant ions here are (in order of importance) Ca⁺⁺, SO₄⁼, Cl^- and Mg⁺⁺. These four ions account for more than 85% of the total measured ion burden while NO₃^- and NH₄⁺ together contribute less than 15% (Fig. 2). However, Meikuang Glaciers does show slightly elevated concentrations of NO₃^- compared to the more southerly sites. Concentrations of Ca⁺⁺, SO₄⁼, Cl^- and Mg⁺⁺ display considerable variation over the four day period samples were collected (Table 1). Linear regression analysis shows very good correlation between all four species (r > 0.95 at significance level p=0.05). The 11 August sample is noteworthy (Table 3) as it contains very high levels of Ca⁺⁺, SO₄⁼, Cl^- and Mg⁺⁺ and was the only aerosol sample from central Asia in which Na+ (205 neq scm-1) and K+ (6.7 neq scm-1) were above the detection limit. NO₃^- and NH₄⁺ do not vary concurrently with the other species.
 

Post-Dust Season, Summer Snow Samples

Post dust season, summer snow from Ngozumpa Glacier shows the lowest ion concentrations. The levels of major ions increase slightly moving northward to Xixabangma Peak and then to Mt. Geladaindong (Fig. 3). At all of the sites the ion composition of snow shows distinct similarities to that of the aerosol. At Ngozumpa Glacier, Xixabangma Peak and Mt. Geladaindong, NH₄⁺, SO₄⁼, NO₃^- and Ca⁺⁺ are the dominant ions. The snow from Mt. Geladaindong shows elevated concentrations of Ca⁺⁺ and Mg⁺⁺ compared to snow from Ngozumpa Glacier and Xixabangma Peak. The main difference between the chemical composition of snow and that of aerosol at these three sites is the relatively elevated levels of Cl^- in the snow, compared to that in the aerosol. This suggests that at least part of the Cl^- in snow originates from gaseous deposition of HCl to the snowpack (e.g., Legrand and Delmas, 1988).

In comparison to the three more southerly sites, summer snow from Meikuang Glacier shows very high ion concentrations (Fig. 3). The relative proportions of ions in fresh snow from Meikuang Glacier closely mimics those of the aerosol. As with the aerosol chemistry, the snow chemistry is dominated by salt related species (in order of importance) Ca⁺⁺, SO₄⁼, Cl^-, Mg⁺⁺. NH₄⁺ and NO₃^- contribute little to the overall ion burden.

DISCUSSION
 

Central Asian Aerosol Chemistry

The similar chemical composition of aerosols at Ngozumpa Glacier, Xixabangma Peak and Mt. Geladaindong, combined with relatively low concentrations of major ions, suggests that high elevation sites in the Himalayas and southern/central regions of the eastern Tibetan Plateau are representative of a well mixed, remote, central Asian troposphere during low dust periods. Differences in the concentration of individual species between sites can be explained by inputs from more local sources and/or problems associated with collection of aerosols on filter media.

The higher Ca⁺⁺ concentrations at Mt. Geladaindong, compared to those at Ngozumpa Glacier and Xixabangma Peak, reflect inputs of desert dust from the scrubgrass and unvegetated desert surfaces to the north and west of Mt. Geladaindong. The comparable SO₄⁼ concentrations at Ngozumpa Glacier, Xixabangma Peak, and Mt. Geladaindong suggest that desert dust is not a predominant source of tropospheric SO₄⁼ in the southern and central regions of the Tibetan Plateau during low dust periods.

While the use of teflon filters minimizes NO₃^- artifact problems, there is the potential to generate either positive (via interaction with HNO3; Appel et al., 1980) or negative (via interaction with strong acids; Appel and Tokiwa, 1981) artifact NO₃^-, depending on the pH of the particulate matter collected on the filter surface. Positive artifacts are particularly relevant to this study as a large fraction of NO₃^- can be present as HNO3 in the free troposphere (e.g., Norton et al., 1992; Talbot et al., 1992). If positive artifact NO₃^- was in fact related to the pH of particulate matter on the filter surface in our samples, we would expect to see some variation in NO₃^- concentrations, and SO₄⁼ to NO₃^- ratios, between samples collected at Ngozumpa Glacier and Mt. Geladaindong. At Mt. Geladaindong, calcium concentrations are five times higher (and therefore probably more alkaline) than those at Ngozumpa Glacier. Ca⁺⁺ to NO₃^- ratios are also much greater at Mt. Geladaindong . However, there is no significant difference between NO₃^- concentrations and SO₄⁼ to NO₃^- ratios at the two sites (Table 3), suggesting that HNO3 deposition has not influenced NO₃^- concentrations. Furthermore, all aerosol samples discussed in this paper were collected using "low volume" systems (i.e. flow rates of 1 to 2 scm per hour). Under these flow conditions Appel et al. (1979) found teflon filters minimized artifact nitrate formation.

The aerosol NO₃^- levels from Xixabangma Peak are surprisingly low compared to Ngozumpa Glacier and Mt. Geladaindong. Our aerosol samples could have been collected from an air mass that was well aged and from which most of the particulate NO₃^- had already been removed; however if this was the case we would also expect to see lower sulfate concentrations. Interestingly, the samples at Xixabangma Peak were the only ones collected on 90 mm filters; the high SO₄⁼ to NO₃^- ratios at Xixabangma Peak may be indicative of NO₃^- loss from the 90 mm filters.

The dominant cation at Ngozumpa Glacier, Xixabangma Peak and Mt. Geladaindong is NH₄⁺. It has been suggested that acidic aerosols can be contaminated by trace amounts of ammonia during processing of filters in the laboratory (Hayes et al., 1980). Hayes et al. performed their investigation using artificial and stratospheric aerosol that was composed entirely of sulfuric acid. Their results are therefore not directly applicable to the more pH balanced, multi-species composition of tropospheric aerosols in our samples. Furthermore, several potential sources of NH3 exist in the region (e.g., biomass burning, livestock, and emission from soils (Warneck, 1988)). Snow samples collected in the mountains of central Asia which were returned to our laboratory in a frozen state and melted just prior to analysis also show significant levels of NH₄⁺. However, NH₄⁺ concentrations were very high in two samples from Ngozumpa Glacier; these samples were treated as spurious and removed from the data set (Table 3). The remaining samples from Ngozumpa Glacier show NH₄⁺ levels greater than those at Xixabangma Peak and Mt. Geladaindong. With the limited data set we are unable to determine if the higher NH₄⁺ at Ngozumpa Glacier is a result of contamination or if they represent real values reflecting local sources of NH₄⁺ which could originate from the agricultural and pastoral based lifestyles of the local population on the southern slopes of the Himalayas, such as urine from livestock, use of wood and animal excrement for fuel, and emissions from agricultural soils.

In contrast to the more southerly sites, the composition and concentration of aerosols at Meikuang Glacier is dominated by local sources. The very high concentrations of Ca⁺⁺, SO₄⁼, Cl^- and Mg⁺⁺ reflect the influx of surface material derived from the Qaidam Basin and/or Taklamakan Desert. This mineral aerosol, consisting of evaporite minerals and desert dust, can be transported to Meikuang Glacier with persistent northerly surface winds which predominate during the spring and summer (Luo and Yanai, 1983). The large distance from the ocean, the composition (i.e., Ca⁺⁺ > SO₄⁼ > Cl^- ), and very high concentrations of salt related species preclude the ocean as a primary source of salt related species at Meikuang Glacier. Similar daily variations in Ca⁺⁺, SO₄⁼, Cl^- and Mg⁺⁺ concentrations result from inputs of surface material from the Qaidam Basin. While Meikuang Glacier shows slightly elevated concentrations of NO₃^- compared to the more southerly sites, the more uniform concentrations of NO₃^- and NH₄⁺ in the four aerosol samples from Meikuang Glacier indicate that locally derived dust is not a major source for nitrogen species. The higher NO₃^- at Meikuang Glacier could possible be a result of HNO3 depostion on the alkaline aerosol which characterize this site (i.e. high Ca⁺⁺ and Mg⁺⁺). However, this seems unlikely as NO₃^- concentrations do not vary with concentrations of Ca⁺⁺ and Mg⁺⁺ (Table 3).

The lack of sychronicity in the collection of aerosol samples and snow deposition precludes a detailed discussion of air-snow fractionation processes. Rather, we include the fresh and surface snow chemistry data for comparison only. The overall similarity in the composition and spatial variation of major ions in aerosol and in snow samples is encouraging and indicates that, from a regional perspective, snow chemistry reflects the spatial pattern in tropospheric aerosol chemistry in central Asia.

The most striking aspect of our Asian aerosol data set is the low atmospheric concentrations of NO₃^- and SO₄⁼ at Ngozumpa Glacier, Xixabangma Peak, and Mt. Geladaindong, which are comparable to previously reported levels in the remote troposphere (Table 4). Included in Table 4 are data collected from aircraft based sampling programs in remote regions of North America (Gillette and Blifford, 1971; Huebert and Lazrus, 1980; and Talbot et al., 1992) and a land based sampling site in the Bolivian Andes (Adams et al., 1977). (It has recently been suggested that chemical data derived from aerosol samples collected using an airplane as a sampling platform underestimates ambient concentrations [Huebert et al., 1990]. It should be noted that if this is the case, the conclusions drawn here would only be strengthened.) Aerosol chemistry data from Shemaya and Midway islands in the north Pacific are also used for comparison. Aerosol chemistry at these islands is strongly influenced by the transport of mineral aerosol from Asia (Umetsu et al., 1983; Prospero and Savoie, 1985; Merrill et al., 1989).

Sulfate concentrations at Ngozumpa Glacier, Xixabangma Peak, and Mt. Geladaindong are consistently lower than those measured in the free troposphere in remote regions over North America. Anthropogenic emissions from eastern China, Europe and North America account for 80% of the total sulfur emissions in the northern hemisphere (Spiro et al., 1992). The lower levels of SO₄⁼ measured at our sites indicate that the sulfur cycle in the remote troposphere over central Asia is not affected to the same extent by human activities as it is in North America (Table 4), Europe (e.g., Reiter et al., 1981) or eastern China (e.g., Galloway et al., 1987). Sulfate levels in central Asia are also 25-50% lower than those in the north Pacific. The sampling sites in the north Pacific lie within the marine boundary layer and are strongly influenced by emission of reduced sulfur gases by biological activity in the oceans (Savoie and Prospero, 1989).

Nitrate levels from Ngozumpa Glacier and Mt. Geladaindong are greater than those measured over North America. High NO₃^- levels measured in remote tropospheric aerosol are associated with elevated levels of mineral aerosol derived from the Sahara (Talbot et al., 1986), central Asia (Prospero and Savoie, 1989), and eastern Africa (Savoie at el., 1987). It has been shown that the formation of nitrate on sea-salt and aerosol particles is an effective sink for and HNO3 (Mamane and Gottlieb, 1992). A survey of NO₃^- in snow from remote regions of the globe found that aerosols derived from continental interiors in temperate regions tend to be rich in NO₃^- (Lyons et al., 1990). Nitrate levels in snow (Wake et al., 1990) and aerosol samples from central Asia show a regional trend, with higher concentrations from sites adjacent to the large arid regions in central Asia. Heterogeneous reactions of HNO3 with dust particles derived from the arid regions of Asia are likely responsible for higher levels of NO₃^- compared to remote sites in North America.

Nitrate levels from Ngozumpa Glacier and Mt. Geladaindong are similar to oceanic 'background' levels measured in the Pacific during low dust periods (Prospero and Savoie, 1989). (While the NO₃^- concentrations presented in Table 4 suggest that levels in the north Pacific are greater, the variability in the NO₃^- concentrations from Ngozumpa Glacier and Mt. Geladaindong due to blanks and analytical precision [=25%] and day to day variations [30-60%] is sufficiently large to eliminate any significant difference. In addition, potential artifact problems associated with deposition of HNO3 on Whatman 41 filters could produce elevated levels of aerosol NO₃^- at the north Pacific sites [Appel et al., 1984; Prospero et al., 1985]). Air lofted from the polluted boundary layer over Europe and eastern North America is the dominant source of NOx for the remote troposphere in the northern hemisphere (Logan 1983; Ehhalt et al., 1992). While dust from Asia influences aerosol NO₃^- levels in the region (even during low dust periods) the source of reactive nitrogen species is likely dominated by anthropogenic emissions in Europe and North America.

These data represent the first, albeit limited, regional survey of aerosol chemistry in central Asia. During low dust periods in central Asia, the concentrations of NO₃^- and SO₄⁼ in the Himalayas and southern/central regions of the eastern Tibetan Plateau are surprisingly low, and are similar to concentrations measured in remote regions of North America. Aerosols in the Himalayas and the southern/central regions of the eastern Tibetan Plateau are dominated by NH₄⁺, SO₄⁼, and NO₃^-. Ca⁺⁺ becomes more important in central Tibet due to inputs of dust from the north and west. Aerosol in the north-eastern region of the Tibetan Plateau shows very high levels Ca⁺⁺, SO₄⁼, Cl^- and Mg⁺⁺. Aerosol composition in this region is dominated by surface material derived from nearby arid and semi-arid regions.

Our results confirm that high elevation mountain sites in the Himalaya and southern/central regions of the eastern Tibetan Plateau (i.e., distant from the vast desert lands on the northern and western margins of the Tibetan Plateau) provide isolated platforms above the planetary boundary layer from which to investigate the composition of the remote continental troposphere.

ACKNOWLEDGMENTS

We thank Sallie Whitlow and Robert Talbot for assistance with chemical analyses. Permission to work in Nepal was kindly provided by Tribhuvan University. The research in China was supported by the U.S. NSF office of Atmospheric Chemistry (ATM-9014768) and Academia Sinica, P. R. China. The work in Nepal was supported by the General Electric Company (England) Young Employees Nepal 1990 Expedition.

Adams F., Dams R., Guzman L., and Winchester J. W. (1977) Background aerosol composition on Chacaltaya Mountain, Bolivia. Atmos. Environ.11, 629-634.

Alekseyev B. A., and 13 others (1988) Geographic Belts and Zonal Types of Landscapes of the World. Scale 1: 15,000,000. School of Geography, Moscow State University, Moscow. (Translated edition first presented at the 86th Annual Meeting of the Ass. of American Geographers, April 19-22, Toronto)

Appel B. R., Wall S. M., Tokiwa Y., and Haik M. (1979) Interference effects in sampling particulate nitrate in ambient air. Atmos. Environ.13, 319-325.

Appel B. R., Wall S. M., Tokiwa Y., and Haik M. (1980) Simultaneous nitric acid, particulate nitrate and acidity measurements in ambient air. Atmos. Environ. 14, 594-554.

Appel B. R. and Tokiwa Y. (1981) Atmospheric particulate nitrate sampling errors due to reactions with particulate and gaseous acids. Atmos. Environ.15, 1087-1089.

Appel B. R., Tokiwa Y., Haik M., and Kothny E. L. (1984) Artifact particulate sulfate and nitrate formation on filter media. Atmos. Environ. 18, 409-416.

Barry R. G. (1981) Mountain Weather and Climate. Methuen, New York.

Barry R. G. and Chorely R. J. (1982) Atmosphere, Weather & Climate - 4th ed. Methuen, New York.

Blank M., Leinen M., and Prospero J. M. (1985) Major Asian aeolian inputs indicated by the mineralogy of aerosols and sediments in the western Pacific. Nature 314, 84-86.

Chen K. and Bowler J. M. (1986) Late Pleistocene evolution of salt lakes in the Qaidam Basin, Qinghai province, China. Paleogeog. Paleoclim. Paleoecol. 54, 87-104.

Davidson C. I., Lin S., Osborn J. F., Pandey M. R., Rasmussen R. A., and Khalil M. A. K. (1986) Indoor and outdoor air pollution in the Himalayas. Environ. Sci. Technol. 20, 561-567.

Dregne H. E. (1968) Surface materials of desert environments. In Deserts of the World (edited by W. G. McGinnies, W. G. Goldman, & P. Paylore), pp. 286-377. University of Arizona Press, Tuscon.

Ehhalt D. H., Rohrer F., and Wahner A. (1992) Sources and distribution of NOx in the upper troposphere at northern mid-latitudes. J. Geophys. Res 97, 3725-3738.

Galloway J. N., Zhou D., Xiong J., and Likens G. E. (1987) Acid Rain: China, United States, and a remote Area. Science 236, 1559-1562.

Gao Y., Arimoto R., Duce R. A., Lee D. S., and Zhou M. Y. (1992a) Input of atmospheric trace elements and mineral matter to the Yellow Sea during the spring of a low-dust year. J. Geophys. Res. 97, 3767-3777.

Gao Y., Arimoto R., Zhou M. Y., Merrill J. T., and Duce R. A. (1992b) Relationships between the dust concentration over eastern Asia and the remote north Pacific. J. Geophys. Res. 97, 9867-9872.

Gillette D. A. and Blifford H. (1971) Composition of tropospheric aerosols as a function of altitude. J. Atmos. Sci. 28, 1199-1210.

Gillette D.A. and Dobrowski J.P. 1991 Measurements of depostion of dust at Shaartuz, Tadzhik, S.S.R., and comparison with sedimentary records. EOS Trans, AGU 72, 105.

Hastenrath S. (1985) Climate and Circulation in the Tropics. D. Reidel, Boston.

Hayes D., Snetsinger K., Ferry G., Oberbeck V., and Farlow N. (1980) Reactivity of stratospheric aerosols to small amounts of ammonia in the laboratory environment. Geophys. Res. Lett. 7, 974-976.

Huebert B. J. and Lazrus A. L. (1980) Bulk composition of aerosols in the remote troposphere. J. Geophys. Res. 85, 7337-7344.

Huebert B. J., Lee G., and Warren W. L. (1990) Airborne aerosol inlet passing efficiency measurement. J. Geophys. Res. 95, 16,369-16,381.

Ikegami K., Inoue J., Higuchi K., and Ono A. (1978) Atmospheric aerosol particles observed in the high altitude Himalayas. Seppyo 40, 50-55.

Junge C. (1979) The importance of mineral dust as an atmospheric constituent. In SCOPE Report 14; Saharan Dust: Mobilization, Transport, Deposition (edited by C. Morales), pp. 49-60. John Wiley & Sons, New York.

Kapoor R. K. and Paul S. K. (1980) A study of the chemical components of aerosols and snow in the Kashmir region. Tellus 32, 33-41.

Legrand M. and Delmas R. (1988) Formation of HCl in the Antarctic Atmosphere. J. Geophys. Res. 93, 7153-7168.

Liu B. Y. H., Pui D. Y. H., and Rubow K. L. (1984) Characteristics of air sampling filter media. In Aerosols in the Mining and Industrial Work Environments, Vol. 3, Instrumentation (edited by V. A. Marple & B. Y. U. Liu), pp. Ch. 70, 989-1037. Ann Arbor Science,

Liu T. S., Gu X. F., An Z. S., and Fan Y. X. (1981) The dust fall in Beijing, China on April 18, 1980. In Desert Dust: Origin, Characteristics, and Effect on Man (edited by T. L. Péwé), pp. 149-157. Geological Society of America Special Paper 186, Boulder.

Logan J. A. (1983) Nitrogen Oxides in the Troposphere: Global and Regional Budgets. J. Geophys. Res. 88, 10785-10807.

Luo H. and Yanai M. (1983) The large-scale circulation and heat sources over the Tibetan Plateau and surrounding areas during the early summer of 1979. Part I: Precipitation and Kinematic Analyses. Mon. Weath. Rev.111, 922-944.

Lyons W. B., Mayewski P. A., Spencer M. J., and Twickler M. S. (1990) Nitrate concentrations in snow from remote areas: implications for the global NOx flux. Biogeochem. 9, 211-222.

Mamane Y. and Gottlieb J. (1992) Nitrate formation on sea-salt and mineral particles-a single particle approach. Atmos. Environ. 26A, 1763-1769.

Mayewski P. A., Lyons W. B., Ahmad N., Smith G., and Pourchet M. (1984) Interpretation of the chemical and physical time-series retrieved from Sentik Glacier, Ladakh Himalaya, India. J. Glaciol. 30, 66-76.

Merrill J. T., Uematsu M., and Bleck R. (1989) Meteorological analysis of long range transport of mineral aerosol over the north Pacific. J. Geophys. Res. 94, 8584-8598.

Mészáros E. (1978) Concentration of sulfur compounds in remote continental and oceanic areas. Atmos. Environ. 12, 699-705.

Miller J. C. and Miller J. N. (1988) Statistics for Analytical Chemistry. Ellis Horwood, New York.

Murakami T. (1987) Effects of the Tibetan Plateau. In Monsoon Meteorology (edited by C. P. Chang & T. N. Krishnamurti), pp. 235-270. Oxford University Press, New York.

Norton R. B., Carroll M. A., Montzka D. D., Hübler G., Huebert B. J., Lee G., Warren W. W., Ridley B. A., and Walega J. G. (1992) Measurements of nitric acid and aerosol nitrate at the Mauna Loa Observatory during the Mauna Loa Observatory Photochemistry Experiment 1988. J. Geophys. Res. 97, 10,415-10,425.

Parrington J. R., Zoller W. H., and Aras N. K. (1983) Asian Dust: Seasonal transport to the Hawaiian Islands. Science 220, 195-197.

Prospero J. M., Charlson R. J., Mohnen V., Jaenicke R., Delany A. C., Moyers J., Zoller W., and Rahn K. (1983) The atmospheric aerosol system: An overview. J. Geophys. Res. 21, 1607-1629.

Prospero J. M., Savoie D. L., Nees R. T., Duce R. A., and Merrill J. (1985) Particulate sulfate and nitrate in the boundary layer over the north Pacific ocean. J. Geophys. Res. 90, 10,586-10,596.

Prospero J. M. and Savoie D. L. (1989) Effect of continental sources on nitrate concentrations over the Pacific Ocean. Nature 339, 687-689.

Rahn K., Borys R. D., and Shaw G. E. (1977) The Asian source of Arctic haze bands. Nature 268, 713-715.

Ramage C. S. (1971) Monsoon Meteorology. Academic Press, New York.

Reiter R., Sládkovic R., and Pötzl K. (1976) Chemical components of aerosol particles in the lower troposphere above central Europe under pure-air conditions. Atmos. Environ. 10, 841-853.

Savoie D. L., Prospero J. M., and Nees R. T. (1987) Nitrate, non-sea-salt sulfate, and mineral aerosol over the northwestern Indian Ocean. J. Geophys. Res. 92, 933-942.

Savoie D. L. and Prospero J. M. (1989) Comparison of oceanic and continental sources of non-sea-salt sulphate over the Pacific Ocean. Nature339, 685-687.

Spiro P. A., Jacob D. J., and Logan J. A. (1992) Global inventory of sulfur emissions with 1ox1o resolution. J. Geophys. Res. 97, 6023-6036.

Talbot R. W., Harriss R. C., Browell E. V., Gregory G. L., Sebacher D. I., and Beck S. M. (1986) Distribution and geochemistry of aerosols in the tropical north Atlantic: Relationship to Saharan dust. J. Geophys. Res 91, 5173-5182.

Talbot R. W., Vijgen A. S., and Harriss R. C. (1992) Soluble species in the Arctic summer troposphere: Acidic gases, aerosols, and precipitation. J. Geophys. Res. 97, 16,531-16,543.

Uematsu M., Duce R. A., Prospero J. M., Chen L., Merrill J. T., and McDonald R. (1983) Transport of mineral aerosol from Asia over the north Pacific. J. Geophys. Res. 88, 5343-5352.

Wake C. P. (1989) Glaciochemical investigations as a tool for determining the spatial and seasonal variation of snow accumulation in the central Karakoram, northern Pakistan. Annal. Glaciol. 13, 279-284.

Wake C. P., Mayewski P. A., and Spencer M. J. (1990) A review of central Asian glaciochemical data. Annal. Glaciol. 14, 301-306.

Wake C. P., Mayewski P. A., Wang P., Yang Q., Han J., and Xie Z. (1992) Anthropogenic sulfate and Asian dust signals in snow from Tien Shan, northwest China. Annal Glaciol. 16, 45-52.

Wake C. P., Mayewski P. A., Xie Z., Wang P., and Li Z. (1993) Regional variation of monsoon and desert dust signals recorded in Asian glaciers. Geophys. Res. Lett. 20, 1411-1414.

Warneck P. (1988) Chemistry of the Natural Atmosphere. Academic Press, New York.

Welch H. E., Muir D. C. G., Billeck B. N., Lockhart W. L., Brunskill G. J., Kling H. J., Olson M. P., and Lemoine R. M. (1991) Brown Snow: A long-range transport event in the Canadian Arctic. Environ. Sci. Technol.25, 280-286.

Winchester J. W., Lü W., Ren L., Wang M., and Maehaut W. (1981) Fine and coarse aerosol composition from a rural area in north China. Atmos. Environ. 15, 933-937.