Browsing by Author "Hemraj Bhattarai"

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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
Chemical components and distributions of aerosols in the third pole
(Elsevier, 2019) Kirpa Ram; Hemraj Bhattarai; Zhiyuan Cong
The crustal and anthropogenic emission sources play an important role in moderating not only aerosol composition but also have a profound impact on rainwater composition and neutralization processes over the Third Pole (TP). This chapter reports on key findings on spatiotemporal variations in chemical composition of aerosols and rainwater based on ground measurement and satellite retrievals, cryoconites of glaciers and ice-cores over the TP. Carbonaceous species [elemental carbon (EC), organic carbon (OC), and water-soluble organic carbon (WSOC)] are ubiquitously present in the entire TP with pronounced spatial and temporal variability in their concentrations. The western and southern parts of the TP are influenced by the transport of aerosols from the source regions in the Indo-Gangetic plain, Nepal, and neighboring areas. The elevated OC/EC ratios, together with K+ and levoglucosan markers, reflect biomass burning emissions as a major source of primary carbonaceous aerosols. In addition, a significant positive gradient observed in WSOC/OC ratios from the Gangetic plains toward high-altitude sites indicates a contribution from secondary organic aerosols (SOAs). In contrast, the eastern and southeastern TP is mostly influenced by emissions from both China and Southeast Asia. The ground-based measurements, as well as satellite retrievals, unveil a clear seasonal pattern over TP with relatively lower concentrations of chemical species and aerosol optical depth (AOD) during monsoon. In contrast, higher AOD values, along with lower fine-mode aerosol fraction, during spring and summer seasons, are attributed to an increase in the contribution of mineral aerosols from desert regions in the Middle East, and Taklamakan and Gobi, as well as the Thar, deserts. Studies on rainwater composition indicate its alkaline nature and the presence of high concentrations of neutralizing species further ascertaining that crustal aerosols have a profound impact on rainwater composition and neutralization processes over the region. © 2020 Elsevier Inc. All rights reserved.
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.
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.
Nitrogenous and carbonaceous aerosols in PM2.5 and TSP during pre-monsoon: Characteristics and sources in the highly polluted mountain valley
(Chinese Academy of Sciences, 2022) Hemraj Bhattarai; Lekhendra Tripathee; Shichang Kang; Pengfei Chen; Chhatra Mani Sharma; Kirpa Ram; Junming Guo; Maheswar Rupakheti
This study reports for the first time a comprehensive analysis of nitrogenous and carbonaceous aerosols in simultaneously collected PM2.5 and TSP during pre-monsoon (March–May 2018) from a highly polluted urban Kathmandu Valley (KV) of the Himalayan foothills. The mean mass concentration of PM2.5 (129.8 µg/m3) was only ~25% of TSP mass (558.7 µg/ m3) indicating the dominance of coarser mode aerosols. However, the mean concentration as well as fractional contributions of water-soluble total nitrogen (WSTN) and carbonaceous species reveal their predominance in find-mode aerosols. The mean mass concentration of WSTN was 17.43±4.70 µg/m3 (14%) in PM2.5 and 24.64±8.07 µg/m3 (5%) in TSP. Moreover, the fractional contribution of total carbonaceous aerosols (TCA) is much higher in PM2.5 (~34%) than that in TSP (~20%). The relatively low OC/EC ratio in PM2.5 (3.03 ± 1.47) and TSP (4.64 ± 1.73) suggests fossil fuel combustion as the major sources of carbonaceous aerosols with contributions from secondary organic aerosols. Five-day air mass back trajectories simulated with the HYSPLIT model, together with MODIS fire counts indicate the influence of local emissions as well as transported pollutants from the Indo-Gangetic Plain region to the south of the Himalayan foothills. Principal component analysis (PCA) also suggests a mixed contribution from other local anthropogenic, biomass burning, and crustal sources. Our results highlight that it is necessary to control local emissions as well as regional transport while designing mitigation measures to reduce the KV's air pollution. © 2021
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.