Article pubs.acs.org/est
Dissolved Organic Matter and Inorganic Ions in a Central Himalayan GlacierInsights into Chemical Composition and Atmospheric Sources Jianzhong Xu,*,†,‡ Qi Zhang,‡ Xiangying Li,† Xinlei Ge,‡ Cunde Xiao,† Jiawen Ren,† and Dahe Qin† †
State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions Environmental and Engineering Research Institute, CAS, Lanzhou 730000, China ‡ Department of Environmental Toxicology, University of California, Davis, California 95616, United States S Supporting Information *
ABSTRACT: Melting of Himalayan glaciers can be accelerated by the deposition of airborne black carbon and mineral dust as it leads to significant reductions of the surface albedo of snow and ice. Whereas South Asia has been shown a primary source region to these particles, detailed sources of these aerosol pollutants remain poorly understood. In this study, the chemical compositions of snow pit samples collected from Jima Yangzong glacier in the central Himalayas were analyzed to obtain information of atmospheric aerosols deposited from summer 2009 to spring 2010. Especially, an Aerodyne high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) was used for the first time to chemically characterize the dissolved organic and inorganic matter (DOM and DIM) in snow samples. The concentrations of these species varied seasonally, with high levels observed during the winter−spring period and low levels during the summer monsoon period. On average, the dissolved substances was dominated by organics (58%) with important contributions from inorganic species, NO3− (12.5%), Ca2+ (9.1%), NH4+ (8.7%), and SO42− (8.1%). DOM was found more oxidized with an average (±1σ) atomic oxygen-to-carbon ratio (nO/nC) of 0.64 (±0.14) and organic mass-to-carbon ratio (OM/OC) of 2.01 (±0.19) during the winter−spring periods compared to the summer season (nO/nC = 0.31 ± 0.09 and OM/OC = 1.58 ± 0.12). In addition, biomass burning particles were found significantly enhanced in snow during the winter−spring periods, consistent with HYSPLIT back trajectory analysis of air mass history, which indicates prevailing atmospheric transport from northwest India and Nepal.
1. INTRODUCTION The rapid urbanization and industrialization in South Asia over past decades have led to a substantial increase in the atmospheric abundance of aerosols, which can significantly affect climate and environment in this region,1 causing issues such as slow down of the monsoon circulation and reduction in monsoon rainfall.2 Moreover, when air pollutants emitted from this region are transported and deposited over the Himalayas, they can decrease the surface albedo and accelerate the melting of snow/glacier.3 Many studies have demonstrated a close connection between the chemical compositions of snow/ice and atmospheric aerosols. For example, the mass concentrations of major soluble ionic species (e.g., Ca2+, Mg2+, Na+, K+, and SO42−) were found to display similar temporal patterns in airborne particles and snow waters sampled simultaneously at a Himalayan site.4 The chemical composition of snow/ice at this remote region can be used to estimate atmospheric aerosol content on a regional scale,5 although some physical and chemical transformation processes could occur in snow/ice after deposition. In addition, analyses of trace metal elements,6,7 inorganic ions,8 and black carbon9 in ice cores drilled from the Himalayas have greatly extended our understanding of aerosol © XXXX American Chemical Society
loading and composition in the past, for up to hundreds of years ago. Ambient aerosols over the Himalayan region are usually composed of mineral dust and carbonaceous species10 due to the widely spread arid and semiarid areas in western India and highly polluted areas in south Asia, which together account for 50−80% of the total suspended particle mass.11,12 A clear seasonality of aerosol loading had been observed in this predominantly high mountain region, showing peaks in winter−spring (premonsoon period) and troughs during summer months (monsoon period).11,13 Previous studies have shown that aerosols from densely populated cities in India and arid areas in northwest India are frequently transported over the Himalayas region because of favorable atmospheric circulation and weather patterns.14 The emission sources may include biomass and fossil fuel burning,12,15 biological activities,16 and mineral dust.11 However, the specific Received: March 5, 2013 Revised: May 13, 2013 Accepted: May 13, 2013
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spectrometer. Detailed procedure of HR-ToF-AMS analysis of aqueous samples is given in Sun et al.28,29 The HR-ToF-AMS was operated under the “V” and “W” ion optical modes alternatively, spending 2.5 min in each mode, during this study. Under V-mode operation, the HR-ToF-AMS cycled through the mass spectrum (MS) mode and the particle time-of-flight (PToF) mode every 15 s, spending 6 and 9 s, respectively, in each mode. Whereas, under W-mode operation, the HR-ToFAMS only cycled on MS mode spending 6 s in each cycle. Between every two samples, purified waters (>18.2 MΩ cm, Millipore, USA) were analyzed in the same way to generate analytical blanks. Every sample was measured twice to ensure the reproducibility of the analysis. Elemental analysis was performed on the high-resolution mass spectra (HRMS) up to m/z 100 to determine the atomic ratios (i.e., nO/nC, nH/nC, and nN/nC) and the mass ratio for organic mass-to-carbon (OM/ OC) of DOM.22 The element contributions of C, O, H, and N reported in this study are mass-based. The concentration of DOM is obtained by TOC multiplied by the OM/OC ratio. The signals of H2O+ for organics were not directly measured but scaled to that of CO2+ based on the fragmentation patterns proposed for ambient aerosols:22 H2O+ = 0.225 × CO2+. The signal of CO+ is fitted instead of scaling to that of CO2+ due to the argon used to atomize samples. The PAH concentrations are determined by fitting the characteristic ions of PAHs from m/z 202 to 300 in the high resolution mass spectrum (Wmode) of DOM following the method of Dzepina et al.27 Additional information about the sample collection, preparation, and analysis procedures and data processing are provided in the Supporting Information. All the measured data are shown in Table S1.
sources of Himalayan aerosols remain unknown, especially with respect to the organic fraction.17 Organic species have been found to be a major component of airborne particles globally, in both urban and remote regions.18 Dry and wet deposition of organic aerosols may contribute significantly to the dissolved substances present in the Himalayan glacier. Detailed characterization of the composition of organic compounds in snow samples will help to unravel the sources of atmospheric particles and their influence on glacial melting. An Aerodyne aerosol mass spectrometer (AMS) has been used for characterizing the composition and size distribution of ambient aerosol worldwide.19 The AMS uses thermo-vaporization and a 70 eV electron-impact mass spectrometric technique to analyze particles in the submicrometer size range. For each analyte, it generates an ensemble mass spectrum that represents a linear combination of the mass spectra of individual species weighted by their concentrations.20 Advanced data analyses of the ensemble mass spectra may reveal valuable information on the mass loading, processing history, and sources of the particulate organics.21 The AMS equipped with a high resolution time-of-flight mass spectrometer (HR-ToF-AMS) can further measure the elemental ratios of oxygen (O), carbon (C), hydrogen (H), nitrogen (N), and sulfur (S) in organics.22 In addition, the AMS spectra allow the identification of several signature ions, including C4H7+ (m/z 57)a spectral tracer for traffic-influenced particles,23 CO2+ (m/z 44)a tracer ion indicative of the abundance of organic acids,24 C2H4O2+ (m/z 60) and C3H5O2+ (m/z 73)tracer ions for organics emitted from biomass burning,25 CH2SO2+ and CH3SO2+ions mainly associated with methane sulfonic acid (MSA) which is a tracer for marine influence,26 and ions (e.g., C16H10+, C17H12+, and C18H10+) indicative of the presence of polycyclic aromatic hydrocarbons (PAHs).27 In this paper, we report the chemical compositions of snow samples collected from Jima Yangzong (JMYZ) glacier located in the central Himalayas based on analyses using a HR-ToFAMS, two ion chromatographs (IC), and a total carbon analyzer. In particular, the bulk chemical characteristics of DOM were studied for the first time in detail via HR-ToF-AMS measurements. Finally, based on these result, we discuss the sources and processes of aerosols transported over the Himalayas and the implications for snow/glacier chemistry and physics in this region.
3. RESULTS AND DISCUSSION 3.1. Dating of Snow Pit Samples. The results of δ18O, concentrations of major ions, and pH values of the snow samples are shown in Figure 1. In the Himalayas, the value of δ18O has been found to be mainly governed by the “amount effect”,30 with a relatively low value in the summer and a high value during the winter and spring.31 The trend of δ18O in this study presents a “V” shape without cyclical fluctuations, suggesting only one year accumulation. The δ18O values vary from −3.4‰ to −20.7‰ with an average −15.4 ± 6.5‰, which is consistent with the result (−14‰ to −21‰) observed at the Naimona’Nyi Glacier32 about 100 km west of the JMYZ Glacier and the value of −14‰ (−7‰ to −26‰) at Shiquanhe about 300 km west of the JMYZ.31 On the basis of the results of δ18O, we date the top 30 cm samples to the period of winter 2009 to spring 2010, samples between 30 and 85 cm to the monsoon period of 2009, and the bottom 15 cm to the spring of 2009. However, the bottom snow samples were evidently affected by melting and percolation of water in the snow which usually involve biological activity and inorganic ion mass loss as illustrated by the abnormally high chemical concentration of TOC and low concentrations of SO42−, NO3−, and Ca2+ (Figure S1). The results of these samples (85−100 cm) are thus excluded from the following discussion. As shown in Figure 1, the concentration profiles of ions and TOC are consistent with previous observations33 that snow chemistry is usually characterized with high concentrations during the winter−spring period and low concentrations during the monsoon period. A similar seasonality is also shown by the variation of PAHs (Figure 1), which are known byproducts of incomplete combustion from sources like diesel and gasoline
2. SITE DESCRIPTION AND SAMPLE ANALYSIS In May 2010, a 100 cm snow pit, accumulated over 1-year period (see section 3), was excavated and sampled at 5-cm resolution vertically in the accumulation zone of JMYZ Glacier, northern slopes of the central Himalaya (30.21° N; 82.14° E; 5750 m a.s.l). The snow samples were filtered (0.45 μm Acrodisc syringe filters, Pall Science, USA) to remove insoluble substances and then analyzed for inorganic ions (DX-320 for Cation and ICS-1500 for Anion, Dionex, California, USA), oxygen isotope (δ18O; L1102-i water analyzer, Picarro, Sunnyvale, California, USA), pH (Thermo Scientific Orion Star A221, USA), and total organic carbon (TOC) (Shimadzu TOC-V CPH , Japan). To analyze with a HR-ToF-AMS (Aerodyne Inc., Billerica, MA, USA), the snow sample solutions were nebulized by argon and dehumidified via a diffusion dryer. The resulting aerosol particles were sampled into the HR-ToFAMS through an aerodynamic lens inlet, vaporized at ∼600 °C, ionized with 70 eV electron impact, and analyzed under the high resolution mode (mass resolution of ∼6000) of the mass B
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Figure 2. (a) Average mass contributions and (b) average mass fractional contributions of major species for JMYZ snow samples in different seasons as marked in Figure 1 (Win−Spr: winter 2009 to spring 2010 and Summ: summer 2009); (c) charge balance between cations and anions measured in JMYZ snow samples colored by depth. Figure 1. The variation profiles (0−85 cm) of oxygen isotopic ratio (δ18O), pH, inorganic ions (μg L−1), TOC (μg L−1), PAH (Arb. Unit), organic mass-to-carbon (OM/OC), and elemental ratios of nO/nC, nH/ nC, and nN/nC of the total dissolved organic matter (DOM) in JMYZ snow samples. The samples at a depth of 85−100 cm are not included due to the percolation process. The vertical dash lines indicate the dating of the snow pit.
big anion charge deficit is observed in most samples (the average ratio of cation to anion equivalents = 2.77; Figure 2c), likely due to the lack of detection of CO32− and HCO3− in this study. Indeed, carbonates (e.g., CaCO3 and MgCO3) are often high in mineral dust. The concentrations of inorganic ions in the snow samples of this study are lower than those in precipitation and fogwater in urban sites of India by 1−2 orders of magnitude35 but are very similar to previous measurements in the Himalayas,5,8,32,36 suggesting a regional characteristic which is mainly due to its long distance from air pollution centers and its high altitude. The relatively high fraction of NH4+ and NO3− in the JMYZ snow samples suggests that an important component of atmospheric particles is NH4NO3, which may be contributed by various biogenic and agricultural activities in the region.8,36 The TOC levels vary from 0.3 to 2.1 mg L−1 with an average of 0.73 mg L−1, substantially higher than the values observed in snow/ice at the polar regions ( 0.001) are shown in bold.
as proved by the inverse relationship between Ca2+ and NO3−/ SO42− (r2 = 0.24), while the enrichment of NO3− during the summer period could reflect snow scavenging of gas-phase HNO3.42 Furthermore, the relatively lower correlation between NO3− and Ca2+ (r2 = 0.66) compared to that of SO42− vs Ca2+ also supports the multiprocessing for NO3−. The high enrichment of NH4+ relative to SO42− (average equivalence ratio = 3.77 ± 1.58) suggests that only a small fraction of NH4+ in snow is neutralized by SO42−, and a large fraction of NH4+ is balanced by NO3− in snow. It is interesting that the equivalence ratios of NH4+/(SO42− + NO3−) varied from 0.77 to 3.83 (avg: 1.49 ± 0.75) with relatively high values in the winter−spring period (Figure 3), revealing an excess of ammonium. The excess of ammonium may be related to the dry deposition of ammonia which has the highest mixing ratio of ammonium to dust over the Indian subcontinent, especially during premonsoon.43 TOC is correlated well with all measured inorganic ions (Table 1) with the highest correlation coefficient with K+ and NH4+ (0.97 and 0.96), which are likely related to biomass burning (see section 3.3). 3.3. DOM Composition and Mass Spectral Features. Figure 4 shows the average mass spectra of DOM based on the contributions of elements and ion categories during the winter−spring and summer periods, respectively. The elemental composition of DOM during winter−spring is dominated by C (55%) and O (36%) with minor contributions from H (7%) and N (2%) (Figure 4a), while the element contributions during summer are C (67%), O (22%), H (9%),
Figure 3. The equivalence ratios between different inorganic ions in JMYZ snow samples. The vertical dash line is the same as in Figure 1.
species (H2SO4, HNO3). The equivalence ratios of Cl−/Na+ (avg = 0.52 ± 0.24; Figure 3) are significantly lower in the snow samples than that in seawater (1.56), indicating a dominant contribution of NaCl salt from regional dust events. The equivalence ratios of NO3−/ SO42− in the snow samples vary from 0.81 to 3.47 (avg: 1.70 ± 0.84) with the low values mostly at the winter−spring period and high values during the summer period (Figure 3). These low values of NO3−/ SO42− in the snow samples could attribute to the important input of gypsum
Figure 4. Chemical characteristics of dissolved organic matter (DOM) in JMYZ snow pit. Average mass spectra of DOM colored by mass contributions of elements (C, O, H, N) during (a) winter 2009 to spring 2010 (N = 6) and (c) summer 2009 (N = 11). Average spectra colored by six ion categories (CxHy+, CxHyO1+, CxHyO2+, CxHyNp+, CxHyOzNp+, HyO1+) and elemental ratios of nO/nC, nH/nC, and nN/nC of the total dissolved organic matter (DOM) during (b) winter 2009 to spring 2010 (N = 6) and (d) summer 2009 (N = 11). D
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Figure 5. (a) Average values and 1σ (error bars) of f60 (fraction of total DOM spectral signal at m/z 60), f 73 (fraction of total DOM spectral signal at m/z 73), and K+ in different depths of the snow pit (Win-Spr: 0−30 cm, Summ: 30−85 cm). Scatter plots that compare (b) m/z 60 vs m/z 73, (c) (m/z 60 + m/z 73) vs K+, and (d) seven-day backward air-mass trajectories for JMYZ sampling site during July 2009, January 2010, and April 2010 which represent the seasons of summer 2009, winter 2009, and spring 2010, respectively. The trajectories are calculated every 4 h at 500 m above sampling site using the NOAA HYSPLIT model.
winter−spring is close to the estimated OM/OC ratio for nonurban ambient OA (2.1 ± 0.2)48 and the value (2.0) determined for fine particles sampled at a rural site in Nepal.16 A higher OM/OC ratio suggests more important contribution from SOA than POA. Indeed, a recent study at a rural site of Nepal has shown evident SOA contribution during the fall months (∼23%).16 The seasonal variations of OM/OC and nO/ nC ratios are similar to those of the inorganic ions (Figure 1), which could serve as indexes for the dating of precipitation in this region. The nH/nC ratio shows an opposite trend compared to that of nO/nC, with an average value of 1.51, and the ratios are 1.52 and 1.65 for the winter−spring and summer samples, respectively (Figure 4). The nN/nC ratio exhibits relatively weak seasonal variations with an average value of ∼0.02, yet they are slightly higher (∼0.03) in the winter−spring samples (Figure 4). The slightly higher nN/nC ratio is mainly attributed to two notable nitrogen-containing ions, i.e. CN+ and CHN+, in the mass spectrum of DOM in winter−spring samples, which are likely generated from biomass burning.49,50 The mass spectrum of DOM in summer samples presents a prominent peak at m/z 44 (primarily CO2+), as well as many significant peaks of the chemically reduced CxHy+ ions such as m/z’s 39, 41, 55, and 57 (Figure 4d). On the other hand, the spectrum of winter−spring samples shows prominent peaks at m/z 28, 29, 43, and 44 which are primarily oxygenated ions of CO+, CHO+, C2H3O+, and CO2+, respectively. Compared to the AMS mass spectra of ambient OA factors retrieved from Positive Matrix Factorization (PMF), the mass spectrum of winter−spring samples shows the highest correlation (r2 = 0.82) with the reference mass spectrum of low-volatility oxygenated OA (LV-OOA) among all types of OA (Table S2). And it is also correlated with semivolatility oxygenated OA (SV_OOA; r2 = 0.64) and biomass burning OA (BBOA; r2 = 0.37) but not with hydrocarbon-like OA (HOA) (r2 < 0.1). In contrast, the average mass spectrum of summer samples correlates less well with LV_OOA (r2 = 0.45) and SV_OOA (r2 = 0.44), but better with HOA (r2 = 0.37). These correlations also support the notion that the OA in the winter−spring period was more oxidized and OA in the summer period was less oxidized. In
and N (2%) (Figure 4c). Higher oxygen fractions in winter− spring snow samples indicate more oxidized organic matter deposited during these periods. Correspondingly, the average (±1σ) nO/nC ratio of DOM during the winter−spring period is 0.64 (±0.14), which falls within the higher end of the nO/nC values (0.3−1.0) observed in ambient organic aerosols (OA), generally representative of more oxidized secondary OA (SOA) in the atmosphere.21,22,44 The mass spectrum is dominated by CxHyO1+ (36%) and CxHyO2+ (22%) (x ≥1; y ≥ 0)) with less contribution from CxHy+ (x ≥1; y ≥ 0) (32%) ions (Figure 4b). Organic species in snow samples corresponding to the summer monsoon season, on the other hand, are much less oxidized with an average nO/nC of only 0.31(±0.09) and the dominance of CxHy+ ions (57%) and minor contributions from oxygencontaining ions (34% in total: CxHyO1+ is 17% and CxHyO2+ is 17%) in their mass spectrum (Figure 4d). This indicates the presence of a significant fraction of hydrocarbon-like organic species in the summer samples, likely originated from primary OA (POA) emission sources, which commonly release fresh particles with low nO/nC ratios and high nH/nC ratios.22 The lower oxidation of OA during the summer period is mostly likely due to the scavenging of aerosols by precipitation, which preferentially removes more aged species which are usually also more hygroscopic due to high degree of oxidation. Also, recent studies have indicated that rain could more effectively scavenge oxygenated OA than the less oxygenated OA.26 The highly oxidized DOM in snow samples during the winter−spring period indicates that the deposited ambient aerosol was highly aged with a long residence time in the atmosphere due to the rarity of precipitation. Furthermore, the high solar radiation during premonsoon period 45 is also favorable for the atmospheric photochemical reaction on aerosol.46 In addition, the aqueous process in fog/cloudwater is also possible and will be another important pathway to form oxidized organic species.47 The average OM/OC ratio for the samples during these two periods is 1.89 (±0.26), and they are 2.01 (±0.19) and 1.58 (±0.12), respectively, for the winter−spring and the summer samples (Figure 1). The high OM/OC ratio during the E
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addition, the mass spectra of winter−spring and summer samples are also correlated well (r2 = 0.74). Another significant feature in the mass spectra of the winter− spring samples is the enhanced relative signal intensities at m/z 60 (94% of which is C2H4O2+) and 73 (85% of which is C3H5O2+), which are usually used as tracer ions for biomass burning in the AMS spectra because of their strong association with levoglucosan and wood tissue related sugar species that are abundant in wood burning emissions.25 The fractions of total organic signals at m/z 60 and 73 (f60 and f 73, respectively) and the concentration of K+another tracer for biomass burning51change seasonally with higher values occurring during the winter and spring periods and lower values in the summer period (Figure 5a). Actually, the f60 and f 73 are more enhanced in the bottom 15 cm samples (Figure S3), although these samples have undergone a percolation process. The close relationship between m/z 60 and m/z 73 (r2 = 0.99; Figure 5b) indicates the same source or transport route for these two ions (Figure 5b). The fact that both m/z 60 and m/z 73 correlate well with the concentration of K+ (r2 = 0.82; Figure 5c) further proves the strong influence of biomass burning emissions on snow composition during the winter and spring seasons. Previous studies reported the observations of biomass burning influences at high altitude stations in the Himalayan regions52,53 and suggested that the sources probably include wood and dung-cake burning, which are commonly used methods for domestic heating and cooking in north India and Nepal.15,54 Indeed, air mass back trajectory analysis using the NOAA HYSPLIT (Hybrid Single Particle Lagrangian Integrated Trajectories) model demonstrates that air masses originated from northwestern India and Nepal are frequently transported over the Himalayas during the winter and spring seasons (Figure 5d). 3.4. Implications. Our results show that organics dominate the dissolved solute mass with important contributions from inorganic ions such as nitrate, sulfate, ammonium, and calcium. These results may shed some lights on the future needs for snow/glacier studies in this region. First, the identification of biomass burning related species in snow pit samples of Himalayas indicates the need to further address the effects of biomass burning on air quality, atmospheric circulation, and snow/glacier albedo in this region. Second, the dominance of highly oxidized DOM in the snow/glacier of the Himalayas may have important impacts on the snow/glacier surface albedo because some dissolved organic species can significantly absorb sunlight (such as brown carbon). 55 The recent studies on snow samples in the Arctic area have detected notable light absorption due to brown carbon; 56 however, the effect of light absorbing DOM on Himalayan glacial melting has not been considered before. Finally, the oxidation states of DOM may need to be considered for the interpretation of snow or ice core chemical records in the future as well as for the inference of the chemical processing of aerosols during atmospheric transport and deposition.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by grants from Natural Science Foundation of China (NSFC, 40901043), the Scientific Research Foundation of the Key Laboratory of Croyspheric Sciences (SKLCS-ZZ-2010-01), and the U.S. Department of Energy Office of Science (BER) (Grant No. DE-FG0211ER65293). We are also grateful to many scientists, technicians, graduate students, and porters for their hard work expertly carried out in the field. We also thank three anonymous reviewers for their constructive comments and suggestions that helped to improve this manuscript.
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ASSOCIATED CONTENT
S Supporting Information *
Additional detail regarding the description of snow samples collection and analysis, the data set of snow samples, the correlation coefficients between mass spectra of snow samples and those from PMF results of two ambient data sets, and average mass spectrum for bottom samples. This information is available free of charge via the Internet at http://pubs.acs.org/. F
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