Attributing Atmospheric Phosphorus in the Himalayas: Biomass Burning vs Mineral Dust.

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.

Atmospheric Microplastics: Perspectives on Origin, Abundances, Ecological and Health Risks.

Microplastic (MP) pollution has aroused a tremendous amount of public and scientific interest worldwide. MPs are found widely ranging from terrestrial to aquatic ecosystems primarily due to the over-exploitation of plastic products and unscientific disposal of plastic waste. There is a large availability of scientific literature on MP pollution in the terrestrial and aquatic ecosystems, especially the marine environments; however, only recently has greater scientific attention been focused on the presence of MPs in the air and its retrospective health implications. Besides, atmospheric transport has been reported to be an important pathway of transport of MPs to the pristine regions of the world. From a health perspective, existing studies suggest that airborne MPs are priority pollutant vectors, that may penetrate deep into the body through inhalation leading to adverse health impacts such as neurotoxicity, cancer, respiratory problems, cytotoxicity, and many more. However, their effects on indoor and outdoor air quality, and on human health are not yet clearly understood due to the lack of enough research studies on that and the non-availability of established scientific protocols for their characterization. This scientific review entails important information concerning the abundance of atmospheric MPs worldwide within the existing literature. A thorough comparison of existing sampling and analytical protocols has been presented. Besides, this review has unveiled the areas of scientific concern especially air quality monitoring which requires immediate attention, with the information gaps to be filled have been addressed.

Wildfire-Derived Nitrogen Aerosols Threaten the Fragile Ecosystem in Himalayas and Tibetan Plateau.

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.

Anthropogenic and natural drivers of seesaw-like spatial patterns in precipitation mercury over western China.

Investigation of mercury (Hg) from atmospheric precipitation is important for evaluating its ecological impacts and developing mitigation strategies. Western China, which includes the Tibetan Plateau and the Xinjiang Uyghur Autonomous Region, is one of the most remote region in the world and is understudied in regards to Hg precipitation. Here we report seesaw-like patterns in spatial variations of precipitation Hg in Western China, based on Hg speciation measurements at nine stations over this remote region. The Hg fraction analyzed included total Hg (HgT), particulate-bound Hg (HgP) and methylmercury (MeHg). Spatially, HgT concentrations and percentage of HgP in precipitation were markedly greater in the westerlies domain than those in the monsoon domain, but the higher wet HgT flux, MeHg concentration and percentage of MeHg in precipitation mainly occurred in the monsoon domain. Similar spatial patterns of wet Hg deposition were also obtained from GEOS-Chem modeling. We show that the disparity of anthropogenic and natural drivers between the two domains are mainly responsible for this seesaw-like spatial patterns of precipitation Hg in Western China. Our study may provide a baseline for assessment of environmental Hg pollution in Western China, and subsequently assist in protecting this remote alpine ecosystem.

Integrating magnetic susceptibility, hydrogeochemical, and isotopic data to assess the seawater invasion in coastal aquifers of Digha, West Bengal, India.

Seawater intrusion in coastal aquifers is a major concern due to geogenic and anthropogenic activities leading to declining groundwater quality. The present study focuses on deciphering the sea water intruded zones and its extent in the Quaternary alluvial aquifer system in the coastal belt of Digha, West Bengal, India. In this study, 36 groundwater samples were collected during pre-monsoon (2020). Subsequently, an integrated approach of hydrogeological, hydrogeochemistry, bulk magnetic susceptibility, isotopic, multivariate statistical, and geochemical modeling is adopted. Spatial distribution maps of hydrological parameters (salinity, conductivity, TDS) and major ion concentration (Na+, K+, Ca2+, Mg2+, Cl-, SO42-, F-, and Br-) suggest that the northern, south-west, and eastern parts of the study area are largely affected by saltwater intrusion and are corroborated with seawater mixing index (SMI). Based on sodium adsorption ratio (SAR), sodium percentage (Na%), and Permeability index (PI) distribution maps, the same locations are identified under critical condition for the suitability of groundwater for irrigation. The order of concentration of cations and anions in the water samples are Na+ > Ca2+ > Mg2+ > K+ and HCO3- > SO4- > Cl- > Br- > F- respectively. Piper diagram shows three principal hydrochemical water types with water composition changes from fresh (86%) to saline water mix (14%). The hydrochemical facies evolution diagram depicts 81% of water samples are in the freshening phase, and 19% are in the intrusion phase. The various bivariate plots revealed that ion exchange, reverse ion exchange, silicate weathering, seawater mixing, and anthropogenic inputs are the governing factors that control groundwater evolution. R-mode factor analysis, principal component analysis (PCA), and agglomerative hierarchical cluster (AHC) also indicate the influence on groundwater from seawater mixing and/or seawater intrusion. The superlativeness of bulk magnetic susceptibility (χ) analysis of water samples in delineating seawater intruded zones is elaborated. Saturation index (SI) shows that groundwater is saturated (> 0) with calcite, dolomite, and aragonite, plausibly due to seawater ingression. Stable isotopic analysis of δ2H (- 53.979 to - 16.9578‰) and δ18O (- 7.00183 to - 1.37 ‰) suggests precipitation recharge/paleo-water at some locations and evaporation enrichment of groundwater. It is recommended to increase groundwater recharge, reduce groundwater extraction at critically affected locations, and have regular monitoring and management to control seawater intrusion.

Nitrogenous and carbonaceous aerosols in PM2.5 and TSP during pre-monsoon: Characteristics and sources in the highly polluted mountain valley.

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.

Why airborne transmission hasn't been conclusive in case of COVID-19? An atmospheric science perspective.

Airborne transmission is one of the routes for the spread of COVID-19 which is caused by inhalation of smaller droplets1 containing SARS-CoV-2 (i.e., either virus-laden particulate matter: PM and/or droplet nuclei) in an indoor environment. Notably, a significant fraction of the small droplets, along with respiratory droplets, is produced by both symptomatic and asymptomatic individuals during expiratory events such as breathing, sneezing, coughing and speaking. When these small droplets are exposed to the ambient environment, they may interact with PM and may remain suspended in the atmosphere even for several hours. Therefore, it is important to know the fate of these droplets and processes (e.g., physical and chemical) in the atmosphere to better understand airborne transmission. Therefore, we reviewed existing literature focussed on the transmission of SARS-CoV-2 in the spread of COVID-19 and present an environmental perspective on why airborne transmission hasn't been very conclusive so far. In addition, we discuss various environmental factors (e.g., temperature, humidity, etc.) and sampling difficulties, which affect the conclusions of the studies focussed on airborne transmission. One of the reasons for reduced emphasis on airborne transmission could be that the smaller droplets have less number of viruses as compared to larger droplets. Further, smaller droplets can evaporate faster, exposing SARS-CoV-2 within the small droplets to the environment, whose viability may further reduce. For example, these small droplets containing SARS-CoV-2 might also physically combine with or attach to pre-existing PM so that their behaviour and fate may be governed by PM composition. Thus, the measurement of their infectivity and viability is highly uncertain due to a lack of robust sampling system to separately collect virions in the atmosphere. We believe that the present review will help to minimize the gap in our understanding of the current pandemic and develop a robust epidemiological method for mortality assessment.

Fluorescence characteristics of water-soluble organic carbon in atmospheric aerosol☆.

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.

Desert dust as a significant carrier of atmospheric mercury.

The atmospheric circulation plays a critical role in the global transport and deposition of atmospheric pollutants such as mercury (Hg). Desert dust emissions contribute to nearly 60-95% of the global dust budget and thus, desert dust may facilitate atmospheric Hg transport and deposition to the downwind regions worldwide. The role of desert dust in biogeochemical cycling of Hg, however, has not been well recognized by the Hg research community. In this study, we measured the concentration of particulate bound Hg (HgP) in total suspended particulate (TSP) collected from China's largest desert, Taklimakan Desert, between 2013 and 2017. The results show that HgP concentrations over the Taklimakan Desert atmosphere are remarkably higher than those observed from background sites in China and are even comparable to those measured in most of the Chinese metropolitan cities. Moreover, HgP concentrations in the Taklimakan Desert exhibit a distinct seasonal pattern peaking during dust storm outbreak periods in spring and summer (March to August). A preliminary estimation demonstrates that export of total Hg associated with atmospheric dust from the Taklimakan Desert could be 59.7 ± 60.3 (1SD) Mg yr-1. The unexpectedly high HgP concentrations during duststorms, together with consistent seasonal pattern of Hg revealed from the snow/ice, clearly demonstrate that Asian desert dust could act as a significant carrier of atmospheric Hg to the cryosphere of Western China and even can have further global reach.

Garbage Burning in South Asia: How Important Is It to Regional Air Quality?

Increasing air pollution in South Asia has serious consequences for air quality and human/ecosystem health within the region. South Asia, including India and Nepal, suffers from severe air pollution, including high concentrations of aerosols, as well as gaseous pollutants. One of the often-neglected sources contributing to the regional air pollution is garbage burning. It is mostly related to numerous yet small, open, uncontrolled fires burning diverse fuels, making it difficult to quantify activity and emissions. In this study, we attempted to quantify the total emissions due to garbage burning and its contribution to regional air quality, using new observational data, a new inventory, and a regional chemical transport model. We implemented the newly available emission factors (EFs) from a recent field campaign, Nepal Ambient Monitoring and Source Testing Experiment (NAMaSTE), which took place in April 2015. Using a chemical transport model-Weather Research and Forecasting model coupled with Chemistry version 3.5 (WRF-Chem)-and three emission scenarios, we assessed the impact of open garbage burning emissions on regional air quality. Our results show that garbage burning emissions could increase PM2.5 concentrations by nearly 30% in India and Nepal, and result in ∼300 000 premature deaths from chronic obstructive pulmonary disease in the two countries.

Decoupling Natural and Anthropogenic Mercury and Lead Transport from South Asia to the Himalayas.

Mercury (Hg) and lead (Pb) accumulation since the Industrial Revolution has been generally observed to increase concurrently in lake sedimentary records around the world. Located downwind during the monsoon season from the rapidly developing South Asia, the Himalayas and the Tibetan Plateau are expected to receive direct anthropogenic Hg and Pb loadings, yet the source, pathway, and effects of such transport remain poorly known due to logistic challenges in accessing this region. When studying the sediment record from Lake Gokyo (4750 m above sea level (a.s.l.)) in the Himalayas, we find remarkably different Hg and Pb accumulation trends over the past 260 years. Although Hg accumulation has continued to increase since the Industrial Revolution, Pb accumulation peaked during that time and has been decreasing since then. Stable isotope analysis reveals that the decoupling trends between these two elements are due to different sources and pathways of Hg and Pb in the region. Both δ202Hg and Δ199Hg have been increasing since the Industrial Revolution, suggesting that anthropogenic Hg emissions from South Asia have been continuously increasing and that the Indian monsoon-driven wet deposition of atmospheric Hg is the dominant pathway for Hg accumulation in the sediments. In contrast, analysis of 206Pb/207Pb and 208Pb/207Pb ratios suggests that Pb accumulation in the sediments originates primarily from natural sources and that the decreasing trend of Pb accumulation is most likely due to a weakening input of atmospheric mineral dust by the westerlies. These decoupling trends highlight the ongoing issue of transboundary Hg transport to the Himalayas and the Tibetan Plateau that are source waters for major freshwater systems in Asia and calls for regional and international collaborations on Hg emission controls in South Asia.

Light absorption properties of elemental carbon (EC) and water-soluble brown carbon (WS-BrC) in the Kathmandu Valley, Nepal: A 5-year study.

This study presents a comprehensive analysis of organic carbon (OC), elemental carbon (EC), and particularly the light absorption characteristics of EC and water-soluble brown carbon (WS-BrC) in total suspended particles in the Kathmandu Valley from April 2013 to January 2018. The mean OC, EC, and water-soluble organic carbon (WSOC) concentrations were 34.8 ± 27.1, 9.9 ± 5.8, and 17.4 ± 12.5 µg m-3, respectively. A clear seasonal variation was observed for all carbonaceous components with higher concentrations occurring during colder months and lower concentrations in the monsoon season. The relatively low OC/EC ratio (3.6 ± 2.0) indicates fossil fuel combustion as the primary source of carbonaceous components. The optical attenuation (ATN) at 632 nm was significantly connected with EC loading (ECS) below 15 µg cm-2 but ceased as ECS increased, reflecting the increased influence of the shadowing effect. The derived average mass absorption cross-section of EC (MACEC) (7.0 ± 4.2 m2 g-1) is comparable to that of freshly emitted EC particles, further attesting that EC was mainly produced from local sources with minimal atmospheric aging processes. Relatively intensive coating with organic aerosols and/or salts (e.g., sulfate, nitrate) was probably the reason for the slightly higher MACEC during the monsoon season, whereas increased biomass burning was a major factor leading to lower MACEC in other seasons. The average MACWS-BrC at 365 nm was 1.4 ± 0.3 m2 g-1 with minimal seasonal variations. In contrast to MACEC, biomass burning was the main reason for a higher MACWS-BrC in the non-monsoon season. The relative light absorption contribution of WS-BrC to EC was 9.9% over the 300-700 nm wavelength range, with a slightly higher ratio (13.6%) in the pre-monsoon season. Therefore, both EC and WS-BrC should be considered in the study of optical properties and radiative forcing of carbonaceous aerosols in this region.

Light absorption, fluorescence properties and sources of brown carbon aerosols in the Southeast Tibetan Plateau.

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.

Nitrogen Speciation and Isotopic Composition of Aerosols Collected at Himalayan Forest (3326 m a.s.l.): Seasonality, Sources, and Implications.

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.

Accumulation of Atmospheric Mercury in Glacier Cryoconite over Western China.

Cryoconite is a granular aggregate, comprised of both mineral and biological material, and known to accumulate atmospheric contaminants. In this study, cryoconite was sampled from seven high-elevation glaciers in Western China to investigate the spatial and altitudinal patterns of atmospheric mercury (Hg) accumulation in the cryoconite. The results show that total Hg (HgT) concentrations in cryoconite were significant with relatively higher Hg accumulation in the southern glaciers (66.0 ± 29.3 ng g-1), monsoon-influenced regions, than those in the northern glaciers (42.5 ± 20.7 ng g-1), westerlies-influenced regions. The altitudinal profile indicates that HgT concentrations in the northern glaciers decrease significantly with altitude, while those in the southern glaciers generally increase toward higher elevations. Unexpectedly high accumulation of methyl-Hg (MeHg) with an average of 1.0 ± 0.4 ng g-1 was also detected in the cryoconite samples, revealing the surface of cryoconite could act as a potential site for Hg methylation in alpine environments. Our preliminary estimate suggests a storage of ∼34.3 ± 17.4 and 0.65 ± 0.28 kg of HgT and MeHg from a single year of formation process in the glacier cryoconite. Therefore, glacier cryoconite could play an important role in Hg storage and transformation, which may result in downstream effects on glacier-fed ecosystems under climate warming scenario.

Historical Black Carbon Reconstruction from the Lake Sediments of the Himalayan-Tibetan Plateau.

Black carbon (BC) is one of the major drivers of climate change, and its measurement in different environment is crucial for the better understanding of long-term trends in the Himalayan-Tibetan Plateau (HTP) as climate warming has intensified in the region. We present the measurement of BC concentration from six lake sediments in the HTP to reconstruct historical BC deposition since the pre-industrial era. Our results show an increasing trend of BC concurrent with increased anthropogenic emission patterns after the commencement of the industrialization era during the 1950s. Also, sedimentation rates and glacier melt strengthening influenced the total input of BC into the lake. Source identification, based on the char and soot composition of BC, suggests biomass-burning emissions as a major contributor to BC, which is further corroborated by open-fire occurrence events in the region. The increasing BC trend continues to recent years, indicating increasing BC emissions, mainly from South Asia.

Water-Soluble Brown Carbon in Atmospheric Aerosols from Godavari (Nepal), a Regional Representative of South Asia.

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.

Aromatic acids as biomass-burning tracers in atmospheric aerosols and ice cores: A review.

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.

Microbial mercury methylation in the cryosphere: Progress and prospects.

Mercury (Hg) is one of the most toxic heavy metals, and its cycle is mainly controlled by oxidation-reduction reactions carried out by photochemical or microbial process under suitable conditions. The deposition and accumulation of methylmercury (MeHg) in various ecosystems, including the cryospheric components such as snow, meltwater, glaciers, and ice sheet, and subsequently in the food chain pose serious health concerns for living beings. Unlike the abundance of knowledge about the processes of MeHg production over land and oceans, little is known about the sources and production/degradation rate of MeHg in cryosphere systems. In addition, processes controlling the concentration of Hg and MeHg in the cryosphere remains poorly understood, and filling this scientific gap has been challenging. Therefore, it is essential to study and review the deposition and accumulation by biological, physical, and chemical mechanisms involved in Hg methylation in the cryosphere. This review attempts to address knowledge gaps in understanding processes, especially biotic and abiotic, applicable for Hg methylation in the cryosphere. First, we focus on the variability in Hg concentration and mechanisms of Hg methylation, including physical, chemical, microbial, and biological processes, and transportation in the cryosphere. Then, we elaborate on the mechanism of redox reactions and biotic and abiotic factors controlling Hg methylation and biogeochemistry of Hg in the cryosphere. We also present possible mechanisms of Hg methylation with an emphasis on microbial transformation and molecular function to understand variability in Hg concentration in the cryosphere. Recent advancements in the genetic and physicochemical mechanisms of Hg methylation are also presented. Finally, we summarize and propose a method to study the unsolved issues of Hg methylation in the cryosphere.

Humic-Like Substances (HULIS) in Aerosols of Central Tibetan Plateau (Nam Co, 4730 m asl): Abundance, Light Absorption Properties, and Sources.

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.