Browsing by Author "Xin Wan"

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Aerosol Properties Over Tibetan Plateau From a Decade of AERONET Measurements: Baseline, Types, and Influencing Factors
(Blackwell Publishing Ltd, 2019) Manisha Pokharel; Jie Guang; Bin Liu; Shichang Kang; Yaoming Ma; Brent N. Holben; Xiang'ao Xia; Jinyuan Xin; Kirpa Ram; Dipesh Rupakheti; Xin Wan; Guangming Wu; Hemraj Bhattarai; Chuanfeng Zhao; Zhiyuan Cong
In this study, a decade long measurement of aerosol optical properties at two AERONET stations (Nam Co during 2006–2016 and QOMS during 2009–2017) in the Tibetan Plateau (TP), a region sensitive to climate change and human perturbation, is presented. The baseline value of aerosol optical depth (AOD) was 0.029 and 0.027 at Nam Co and QOMS, respectively, which are comparable to or even lower than those at some Arctic and remote ocean locations. The seasonality of AOD values were the order of spring > summer > winter > autumn. Based on AOD and Ångström exponent (α), major aerosol types over the TP were further identified as continental background, biomass burning, and dust. Although continental background aerosol was the main feature in remote areas of TP, biomass burning plumes frequently occurred, especially during spring (March–April). In one of such biomass burning event in April 2014, MODIS observations demonstrated that intensive open fires occurred in South Asia, covering the foothills of Himalayas and Indo-Gangetic Plain. The air mass back trajectories and CALIOP observations further revealed that biomass burning plume could be uplifted to higher altitudes and reach the Himalayas. Moreover, an occasional dust event detected in April 2008 over the TP demonstrated that the dust from Taklamakan Desert may impact the main body of TP episodically, although the local dust from the inside of TP cannot be excluded and warrants further study. ©2019. American Geophysical Union. All Rights Reserved.
Aromatic acids as biomass-burning tracers in atmospheric aerosols and ice cores: A review
(Elsevier Ltd, 2019) Xin Wan; Kimitaka Kawamura; Kirpa Ram; Shichang Kang; Mark Loewen; Shaopeng Gao; Guangming Wu; Pingqing Fu; Yanlin Zhang; Hemraj Bhattarai; Zhiyuan Cong
Biomass burning (BB) is one of the largest sources of carbonaceous aerosols with adverse impacts on air quality, visibility, health and climate. BB emits a few specific aromatic acids (p-hydroxybenzoic, vanillic, syringic and dehydroabietic acids) which have been widely used as key indicators for source identification of BB-derived carbonaceous aerosols in various environmental matrices. In addition, measurement of p-hydroxybenzoic and vanillic acids in snow and ice cores have revealed the historical records of the fire emissions. Despite their uniqueness and importance as tracers, our current understanding of analytical methods, concentrations, diagnostic ratios and degradation processes are rather limited and scattered in literature. In this review paper, firstly we have summarized the most established methods and protocols for the measurement of these aromatic acids in aerosols and ice cores. Secondly, we have highlighted the geographical variability in the abundances of these acids, their diagnostic ratios and degradation processes in the environments. The review of the existing data indicates that the concentrations of aromatic acids in aerosols vary greatly with locations worldwide, typically more abundant in urban atmosphere where biomass fuels are commonly used for residential heating and/or cooking purposes. In contrast, their concentrations are lowest in the polar regions which are avoid of localized emissions and largely influenced by long-range transport. The diagnostic ratios among aromatic acids can be used as good indicators for the relative amounts and types of biomass (e.g. hardwood, softwood and herbaceous plants) as well as photochemical oxidation processes. Although studies suggest that the degradation processes of the aromatic acids may be controlled by light, pH and hygroscopicity, a more careful investigation, including closed chamber studies, is highly appreciated. © 2019 Elsevier Ltd; Current research trends on aromatic acids as biomass burning tracers were comprehensively reviewed. © 2019 Elsevier Ltd
Atmospheric phosphorus and its geochemical cycling: Fundamentals, progress, and perspectives
(Elsevier B.V., 2023) Xing Diao; David Widory; Kirpa Ram; Lekhendra Tripathee; Srinivas Bikkina; Kimitaka Kawamura; Shaopeng Gao; Xin Wan; Guangming Wu; Qiaomin Pei; Xiaoping Wang; Zhiyuan Cong
Phosphorus (P) is an essential macronutrient for all organisms that can be redistributed between terrestrial and oceanic systems via atmospheric emission, transport, transformation, and deposition. Moreover, since natural P mobilization from the lithosphere to ecosystems is a relatively slow process, the role of atmospheric P seems to play an important role in its cycling. This paper provides a comprehensive review of the analytical methods used for characterizing atmospheric P species and the methods used for identifying P sources (e.g., oxygen stable isotope compositions of phosphate, δ18OP) discussing their respective suitability, advantages, and limitations. While at a regional scale δ18OP of atmospheric P are generally source-specific, at a more global scale these isotope compositions tend to overlap between sources, rendering their tracer potential more difficult. Furthermore, various sources of atmospheric P and their fluxes are compiled, and the potential uncertainties in the estimates of their respective contributions are reviewed, which suggest that more model inter-comparations, parameter optimizations, and field observations are still needed. Moreover, we summarize the long-range transport process controlling P atmospheric dispersion at various scales (focusing on dust and biomass burning). In addition, the transformation mechanism, especially acid dissolution, that modifies the P cycle during its residence time in the atmosphere is depicted. Finally, we propose that land cover may be a potential key control to the atmospheric P deposition rate based on the critical analysis of previously published rates. This review allows us to ultimately propose key recommendations for fostering future research on P geochemical cycling. © 2023
Attributing Atmospheric Phosphorus in the Himalayas: Biomass Burning vs Mineral Dust
(American Chemical Society, 2024) Xing Diao; David Widory; Kirpa Ram; Enzai Du; Xin Wan; Shaopeng Gao; Qiaomin Pei; Guangming Wu; Shichang Kang; Zhong Wang; Xiaoping Wang; Zhiyuan Cong
Atmospheric phosphorus is a vital nutrient for ecosystems whose sources and fate are still debated in the fragile Himalayan region, hindering our comprehension of its local ecological impact. This study provides novel insights into atmospheric phosphorus based on the study of total suspended particulate matter at the Qomolangma station. Contrary to the prevailing assumptions, we show that biomass burning (BB), not mineral dust, dominates total dissolved phosphorus (TDP, bioavailable) deposition in this arid region, especially during spring. While total phosphorus is mainly derived from dust (77% annually), TDP is largely affected by the transport of regional biomass-burning plumes from South Asia. During BB pollution episodes, TDP causing springtime TDP fluxes alone accounts for 43% of the annual budget. This suggests that BB outweighs dust in supplying bioavailable phosphorus, a critical nutrient, required to sustain Himalayas’ ecological functions. Overall, this first-hand field evidence refines the regional and global phosphorus budget by demonstrating that BB emission, while still unrecognized, is a significant source of P, even in the remote mountains of the Himalayas. It also reveals the heterogeneity of atmospheric phosphorus deposition in that region, which will help predict changes in the impacted ecosystems as the deposition patterns vary. © 2023 American Chemical Society.
Fluorescence characteristics of water-soluble organic carbon in atmospheric aerosol☆
(Elsevier Ltd, 2021) Guangming Wu; Pingqing Fu; Kirpa Ram; Jianzhong Song; Qingcai Chen; Kimitaka Kawamura; Xin Wan; Shichang Kang; Xiaoping Wang; Alexander Laskin; Zhiyuan Cong
Fluorescence spectroscopy is a commonly used technique to analyze dissolved organic matter in aquatic environments. Given the high sensitivity and non-destructive analysis, fluorescence has recently been used to study water-soluble organic carbon (WSOC) in atmospheric aerosols, which have substantial abundance, various sources and play an important role in climate change. Yet, current research on WSOC characterization is rather sparse and limited to a few isolated sites, making it challenging to draw fundamental and mechanistic conclusions. Here we presented a review of the fluorescence properties of atmospheric WSOC reported in various field and laboratory studies, to discuss the current advances and limitations of fluorescence applications. We highlighted that photochemical reactions and relevant aging processes have profound impacts on fluorescence properties of atmospheric WSOC, which were previously unnoticed for organic matter in aquatic environments. Furthermore, we discussed the differences in sources and chemical compositions of fluorescent components between the atmosphere and hydrosphere. We concluded that the commonly used fluorescence characteristics derived from aquatic environments may not be applicable as references for atmospheric WSOC. We emphasized that there is a need for more systematic studies on the fluorescence properties of atmospheric WSOC and to establish a more robust reference and dataset for fluorescence studies in atmosphere based on extensive source-specific experiments. © 2020 Elsevier Ltd
Humic-Like Substances (HULIS) in Aerosols of Central Tibetan Plateau (Nam Co, 4730 m asl): Abundance, Light Absorption Properties, and Sources
(American Chemical Society, 2018) Guangming Wu; Xin Wan; Shaopeng Gao; Pingqing Fu; Yongguang Yin; Gang Li; Guoshuai Zhang; Shichang Kang; Kirpa Ram; Zhiyuan Cong
Humic-like substances (HULIS) are major components of light-absorbing brown carbon that play an important role in Earth's radiative balance. However, their concentration, optical properties, and sources are least understood over Tibetan Plateau (TP). In this study, the analysis of total suspended particulate (TSP) samples from central of TP (i.e., Nam Co) reveal that atmospheric HULIS are more abundant in summer than that in winter without obvious diurnal variations. The light absorption ability of HULIS in winter is 2-3 times higher than that in summer. In winter, HULIS are mainly derived from biomass burning emissions in South Asia by long-range transport. In contrast, the oxidation of anthropogenic and biogenic precursors from northeast part of India and southeast of TP are major sources of HULIS in summer. Copyright © 2018 American Chemical Society.
Levoglucosan as a tracer of biomass burning: Recent progress and perspectives
(Elsevier Ltd, 2019) Hemraj Bhattarai; Eri Saikawa; Xin Wan; Hongxia Zhu; Kirpa Ram; Shaopeng Gao; Shichang Kang; Qianggong Zhang; Yulan Zhang; Guangming Wu; Xiaoping Wang; Kimitaka Kawamura; Pingqing Fu; Zhiyuan Cong
Biomass burning (BB) is a major source of air pollution from local to global scale, having variable effects on air quality, human health, and climate system. Therefore, the source identification and characterization of BB-derived aerosols and tracer gases in the ambient environment is crucial. This review provides recent updates on the applicability of levoglucosan as a BB tracer in different environmental matrices such as aerosols, marine, snow and ice-cores etc. Among several tracer of BB emissions, levoglucosan has recently received widespread attention due to its unique origin solely from the pyrolysis of cellulose and hemicellulose, making it as a robust marker for characterization and quantification of BB throughout the world. This review first summarizes the established and emerging analytical methods, and their advantages and disadvantages for measurement of levoglucosan. Second, we discuss the formation mechanism, lifetime and its stability in different environmental conditions. In addition, we also try to deliberate on the application of ratios of levoglucosan with different organic components such as mannosan (M) and organic carbon (OC) for better identification of emission sources. Spatial distributions of levoglucosan in different locations (e.g., urban, rural, forest, marine, poles and higher altitude) are discussed scrupulously and meticulously on a global scale. We also reviewed the distributions of levoglucosan in snow, ice core and sediments to understand its applicability to construct paleofire records. Finally, we propose some key recommendations for future work in different ambient environmental conditions by utilizing the ratios of levoglucosan with other compounds (not limited only to M and OC) and the use of levoglucosan to reconstruct the paleo-historical records of fire-activity. © 2019 Elsevier B.V.
Light absorption, fluorescence properties and sources of brown carbon aerosols in the Southeast Tibetan Plateau
(Elsevier Ltd, 2020) Guangming Wu; Xin Wan; Kirpa Ram; Peilin Li; Bin Liu; Yongguang Yin; Pingqing Fu; Mark Loewen; Shaopeng Gao; Shichang Kang; Kimitaka Kawamura; Yongjie Wang; Zhiyuan Cong
Brown carbon (BrC) has been proposed as an important driving factor in climate change due to its light absorption properties. However, our understanding of BrC's chemical and optical properties are inadequate, particularly at remote regions. This study conducts a comprehensive investigation of BrC aerosols in summer (Aug. 2013) and winter (Jan. 2014) at Southeast Tibetan Plateau, which is ecologically fragile and sensitive to global warming. The concentrations of methanol-soluble BrC (MeS-BrC) are approximately twice of water-soluble BrC (WS-BrC), demonstrating the environmental importance of water-insoluble BrC are previously underestimated with only WS-BrC considered. The mass absorption efficiency of WS-BrC (0.27–0.86 m2 g−1) is lower than those in heavily polluted South Asia, indicating a distinct contrast between the two sides of Himalayas. Fluorescence reveals that the absorption of BrC is mainly attributed to humic-like and protein-like substances, which broaden the current knowledge of BrC's chromophores. Combining organic tracer, satellite MODIS data and air-mass backward trajectory analysis, this study finds BrC is mainly derived from bioaerosols and secondary formation in summer, while long-range transport of biomass burning emissions in winter. Our study provides new insights into the optical and chemical properties of BrC, which may have implications for environmental effect and sources of organic aerosols. Optical properties and organic tracers demonstrate that BrC is derived from bioaerosols in summer, while biomass burning emissions in winter. © 2019 Elsevier Ltd
Nitrogen aerosols in New Delhi, India: Speciation, formation, and sources
(Elsevier Ltd, 2024) Qiaomin Pei; Xin Wan; David Widory; Kirpa Ram; Bhupesh Adhikary; Guangming Wu; Xing Diao; Hemraj Bhattarai; Yan-Lin Zhang; Mark Loewen; Zhiyuan Cong
Delhi, the capital city of India, experiences severe air pollution and suffers from its adverse effects on human health and ecosystems. This pollution is characterized by high levels of pollutants, including atmospheric nitrogen in both the gaseous and particulate phases. However, there is a lack of simultaneous measurement of chemical composition, tracers and 15N data in aerosols to understand the influence of different sources on N aerosols over Delhi. Here, we measured total nitrogen (TN), water-soluble total nitrogen (WSTN), water-soluble inorganic nitrogen (WSIN), and N stable isotope compositions (δ15N) in PM2.5 samples covering the post-monsoon, winter, and summer periods of the year 2018–19. NH4+-N was the major N species, accounting for an average 58% of TN and 68% of WSIN. The temporal variations of TN, WSTN, NH4+-N, NO3−-N, and WSON showed peaks in the post-monsoon and winter seasons, exhibiting seasonality similar to PM2.5 and levoglucosan (a biomass-burning tracer) indicating their co-genetic sources. Based on the correlation analysis between δ15N and N-species, we identified two distinct secondary chemical processes: i) in an NH4+-poor atmosphere, the gas-to-particle (NH3 → NH4+) conversion and subsequent formation of NH4HSO4 was the main process controlling the 15N and nitrogen enrichments in PM2.5; whereas ii) under NH4+-rich conditions, the formation and dissociation of NH4NO3 dominated. The coupled HYSPLIT and PSCF analyses highlighted the transport and contributions of open biomass burning emissions under a northwesterly atmospheric flow during post-monsoon as well as from local biomass combustion (from cooking and heating) during winter in the city and its vicinity. Our results suggested that i) both NH4+-N and NO3−-N were mainly impacted by biomass combustion during post-monsoon and winter seasons, and ii) NO3−-N resulted of dust transport from the Thar Desert in the summer season, but not NH4+-N. Finally, we recommend that future research focuses on the study of the seasonality of atmospheric nitrogen composition using 15N data from their different sources to design tailor-made measures and policies regarding the different potential sources, combining them within a comprehensive framework to ultimately improve air quality and the living environment in Delhi. © 2023
Nitrogen Speciation and Isotopic Composition of Aerosols Collected at Himalayan Forest (3326 m a.s.l.): Seasonality, Sources, and Implications
(American Chemical Society, 2019) Hemraj Bhattarai; Yan-Lin Zhang; Chandra Mouli Pavuluri; Xin Wan; Guangming Wu; Peilin Li; Fang Cao; Wenqi Zhang; Yongjie Wang; Shichang Kang; Kirpa Ram; Kimitaka Kawamura; Zhenming Ji; David Widory; Zhiyuan Cong
Nitrogenous aerosols are ubiquitous in the environment and thus play a vital role in the nutrient balance as well as the Earth's climate system. However, their abundance, sources, and deposition are poorly understood, particularly in the fragile and ecosensitive Himalayan and Tibetan Plateau (HTP) region. Here, we report concentrations of nitrogen species and isotopic composition (δ15N) in aerosol samples collected from a forest site in the HTP (i.e., Southeast Tibet). Our results revealed that both organic and inorganic nitrogen contribute almost equally with high abundance of ammonium nitrogen (NH4 +-N) and water-insoluble organic nitrogen (WION), contributing ∼40% each to aerosol total nitrogen (TN). The concentrations and δ15N exhibit a significant seasonality with ∼2 times higher in winter than in summer with no significant diurnal variations for any species. Moreover, winter aerosols mainly originated from biomass burning emissions from North India and East Pakistan and reached the HTP through a long-range atmospheric transport. The TN dry deposition and total deposition fluxes were 2.04 kg ha-1 yr-1 and 6.12 kg ha-1 yr-1 respectively. Our results demonstrate that the air contamination from South Asia reach the HTP and is most likely impacting the high altitude ecosystems in an accepted scenario of increasing emissions over South Asia. Copyright © 2019 American Chemical Society.
Water-Soluble Brown Carbon in Atmospheric Aerosols from Godavari (Nepal), a Regional Representative of South Asia
(American Chemical Society, 2019) Guangming Wu; Kirpa Ram; Pingqing Fu; Wan Wang; Yanlin Zhang; Xiaoyan Liu; Elizabeth A. Stone; Bidya Banmali Pradhan; Pradeep Man Dangol; Arnico K. Panday; Xin Wan; Zhipeng Bai; Shichang Kang; Qianggong Zhang; Zhiyuan Cong
Brown carbon (BrC) has recently emerged as an important light-absorbing aerosol. This study provides interannual and seasonal variations in light absorption properties, chemical composition, and sources of water-soluble BrC (WS-BrC) based on PM10 samples collected in Godavari, Nepal, from April 2012 to May 2014. The mass absorption efficiency of WS-BrC at 365 nm (MAE365) shows a clear seasonal variability, with the highest MAE365 of 1.05 ± 0.21 m2 g-1 in premonsoon season and the lowest in monsoon season (0.59 ± 0.16 m2 g-1). The higher MAE365 values in nonmonsoon seasons are associated with fresh biomass burning emissions. This is further substantiated by a strong correlation (r = 0.79, P < 0.01) between Abs365 (light absorption coefficient at 365 nm) and levoglucosan. We found, using fluorescence techniques, that humic-like and protein-like substances are the main chromophores in WS-BrC and responsible for 80.2 ± 4.1% and 19.8 ± 4.1% of the total fluorescence intensity, respectively. BrC contributes to 8.78 ± 3.74% of total light absorption over the 300-700 nm wavelength range. Considering the dominant contribution of biomass burning to BrC over Godavari, this study suggests that reduction in biomass burning emission may be a practical method for climate change mitigation in South Asia. Copyright © 2019 American Chemical Society.
Wildfire-Derived Nitrogen Aerosols Threaten the Fragile Ecosystem in Himalayas and Tibetan Plateau
(American Chemical Society, 2023) Hemraj Bhattarai; Guangming Wu; Xiaoyan Zheng; Hongxia Zhu; Shaopeng Gao; Yan-Lin Zhang; David Widory; Kirpa Ram; Xintong Chen; Xin Wan; Qiaomin Pei; Yuepeng Pan; Shichang Kang; Zhiyuan Cong
Himalayas and Tibetan Plateau (HTP) is important for global biodiversity and regional sustainable development. While numerous studies have revealed that the ecosystem in this unique and pristine region is changing, their exact causes are still poorly understood. Here, we present a year-round (23 March 2017 to 19 March 2018) ground- and satellite-based atmospheric observation at the Qomolangma monitoring station (QOMS, 4276 m a.s.l.). Based on a comprehensive chemical and stable isotope (15N) analysis of nitrogen compounds and satellite observations, we provide unequivocal evidence that wildfire emissions in South Asia can come across the Himalayas and threaten the HTP’s ecosystem. Such wildfire episodes, mostly occurring in spring (March-April), not only substantially enhanced the aerosol nitrogen concentration but also altered its composition (i.e., rendering it more bioavailable). We estimated a nitrogen deposition flux at QOMS of ∼10 kg N ha-1 yr-1, which is approximately twice the lower value of the critical load range reported for the Alpine ecosystem. Such adverse impact is particularly concerning, given the anticipated increase of wildfire activities in the future under climate change. © 2023 American Chemical Society.