Concentration, seasonality, and sources of trace elements in atmospheric aerosols from Godavari in the southern Himalayas.

Atmospheric pollution has detrimental effects on human health and ecosystems. The southern region of the Himalayas, undergoing rapid urbanization and intense human activities, faces poor air quality marked by high aerosol loadings. In this study, we conducted a two-year PM10 sampling in the suburban area (Godavari) of Kathmandu, a representative metropolis situated in the southern part of the central Himalayas. The trace elements were measured to depict aerosol-bound element loadings, seasonality, and potential sources. The mean concentrations of trace elements varied considerably, ranging from 0.27 ± 0.19 ng m-3 for Tl to 1252.78 ng m-3 for Zn. The average concentration of Co and Ni was 1.2 and 22.4 times higher, respectively, than those in Lhasa city in Tibet in the northern Himalayas. The concentration of Pb was 38 times lower than that in Lahore, Pakistan, and 9 times lower than urban sites in India. For the seasonality, the trace element concentrations displayed remarkable variation, with higher concentrations during the non-monsoon seasons and lower concentrations during the monsoon season. This trend was primarily influenced by anthropogenic activities such as low-grade fuel combustion in vehicles, coal combustion in brick kilns, and biomass burning, along with seasonal rainfall that induced aerosol washout. The enrichment factors (EFs) analysis revealed that Cd, Zn, Sb, Ni, Cu, Cr, and Pb had higher EFs, indicating their significant contributions from anthropogenic sources. In contrast, elements like Tl, Co, V, Cs, U, Ba, Th, and Sr, characterized by lower EFs, were mainly associated with natural sources. The Pb isotopic ratio profiles exhibited the Pb in PM10 are derived major contribution from legacy lead. Biomass burning contributed to the Pb source in winter. These findings provide policymakers with valuable insights to develop guidelines and strategies aimed at improving air quality and mitigating the impact of aerosol pollution on human health in the Himalayan region.

Isotopic Constraints on Sources and Transformations of Nitrate in the Mount Everest Proglacial Water.

Glacier melting exports a large amount of nitrate to downstream aquatic ecosystems. Glacial lakes and glacier-fed rivers in proglacial environments serve as primary recipients and distributors of glacier-derived nitrate (NO3-), yet little is known regarding the sources and cycling of nitrate in these water bodies. To address this knowledge gap, we conducted a comprehensive analysis of nitrate isotopes (δ15NNO3, δ18ONO3, and Δ17ONO3) in waters from the glacial lake and river of the Rongbuk Glacier-fed Basin (RGB) in the mountain Everest region. The concentrations of NO3- were low (0.43 ± 0.10 mg/L), similar to or even lower than those observed in glacial lakes and glacier-fed rivers in other high mountain regions, suggesting minimal anthropogenic influence. The NO3- concentration decreases upon entering the glacial lake due to sedimentation, and it increases gradually from upstream to downstream in the river as a soil source is introduced. The analysis of Δ17ONO3 revealed a substantial contribution of unprocessed atmospheric nitrate, ranging from 34.29 to 56.43%. Denitrification and nitrification processes were found to be insignificant in the proglacial water of RGB. Our study highlights the critical role of glacial lakes in capturing and redistributing glacier-derived NO3- and emphasizes the need for further investigations on NO3- transformation in the fast-changing proglacial environment over the Tibetan Plateau and other high mountain regions.

Insight into the relationships between total suspended particles and mercury in meltwater in a typical glacierized basin in the inland Tibetan Plateau.

Mercury (Hg) released by melting glaciers is likely to bind to suspended particles in meltwater runoff, posing potential risks to downstream ecosystems. The rapidly receding glaciers on the Tibetan Plateau promote the export of total suspended particles (TSP), increasing the uncertainty of Hg export released by glacier melting. To investigate the relationships between TSP and Hg, a multimedia sampling campaign was conducted in July 2020 in the Kuoqionggangri glacier region of the Lhasa River Valley No. 1 glacierized basin located in the inland Tibetan Plateau. Samples from glacier snow/ice, supraglacial rivers, subglacial rivers, proglacial lakes, and meltwater runoff were obtained, and the relationships between TSP and Hg and their transport in glacier meltwater runoff in the context of glacier retreat were explored. The average TSP concentration of different environmental samples ranged from 9.51 mg/L to 399. 27 mg/L, showing significant differences. The average total Hg (THg) concentrations ranged from 0.52 ng/L to 58.81 ng/L and decreased in the order of snow/ice >runoff> subglacial river > proglacial lake > supraglacial river. Both TSP mass concentration and number concentration have an impact on the diurnal variation in meltwater runoff Hg, and the influence of TSP number concentration is stronger than that of concentration. Sites with high TSP concentrations and quantities tended to have higher Hg concentrations, while TSP particle size had no significant effect on Hg concentration or spatial distribution. Our study further divided the glacier recharge basin into the glacier cover zone, the periglacial zone, and the downstream zone and discussed the potential impact of TSP on Hg transport in each zone. Our analysis highlights that the periglacial zone will expand and activate the resuspension process of river sediments in the warming future, which may increase the export of TSP and Hg downstream.

Dramatic Carbon Loss in a Permafrost Thaw Slump in the Tibetan Plateau is Dominated by the Loss of Microbial Necromass Carbon.

Thaw slumps can lead to considerable carbon loss in permafrost regions, while the loss of components from two major origins, i.e., microbial and plant-derived carbon, during this process remains poorly understood. Here, we provide direct evidence that microbial necromass carbon is a major component of lost carbon in a retrogressive permafrost thaw slump by analyzing soil organic carbon (SOC), biomarkers (amino sugars and lignin phenols), and soil environmental variables in a typical permafrost thaw slump in the Tibetan Plateau. The retrogressive thaw slump led to a ∼61% decrease in SOC and a ∼25% SOC stock loss. As evident in the levels of amino sugars (average of 55.92 ± 18.79 mg g-1 of organic carbon, OC) and lignin phenols (average of 15.00 ± 8.05 mg g-1 OC), microbial-derived carbon (microbial necromass carbon) was the major component of the SOC loss, accounting for ∼54% of the SOC loss in the permafrost thaw slump. The variation of amino sugars was mainly related to the changes in soil moisture, pH, and plant input, while changes in lignin phenols were mainly related to the changes in soil moisture and soil bulk density.

Isotopic Fractionation Characteristics of Speciated Mercury from Local Biomass Combustion in the Tibetan Plateau.

As the Third Pole of the world, the Tibetan Plateau (TP) is sensitive to anthropogenic influences. Biomass combustion is one of the most important anthropogenic sources of mercury (Hg) emissions in the TP. However, due to the lack of knowledge about Hg emission characteristics and activity levels in the plateau, atmospheric Hg emissions from biomass combustion in the TP are under large uncertainties. Here, based on pilot-scale experiments, we found that particle-bound mercury (PBM; mean of 83.1-87.7 ng/m3) occupied 17.93-49.31% of the total emitted Hg and the PBM δ202Hg values (average -1.65‰ to -0.77‰) were significantly higher than those of the corresponding feeding biomass. The Δ200Hg values of total gaseous mercury and PBM were more negative (-0.08‰ to -0.05‰) than other anthropogenic emissions, providing unique isotopic fingerprints for this sector. Together with the investigated local activity levels, Hg emissions from biomass combustion reached 402 ± 74 kg/a, which were dozens of times higher than previous estimates. The emissions were characterized by conspicuous spatial heterogeneity, concentrated in the northern and central TP. Specialized Hg emissions and the Hg isotope fingerprint of local biomass combustion can aid in evaluating the influence of this sector on the fragile ecosystems of the TP.

Local temperature increases reduce soil microbial residues and carbon stocks.

Warming is known to reduce soil carbon (C) stocks by promoting microbial respiration, which is associated with the decomposition of microbial residue carbon (MRC). However, the relative contribution of MRC to soil organic carbon (SOC) across temperature gradients is poorly understood. Here, we investigated the contribution of MRC to SOC along two independent elevation gradients of our model system (i.e., the Tibetan Plateau and Shennongjia Mountain in China). Our results showed that local temperature increases were negatively correlated with MRC and SOC. Further analyses revealed that rising temperature reduced SOC via decreasing MRC, which helps to explain future reductions in SOC under climate warming. Our findings demonstrate that climate warming has the potential to reduce C sequestration by increasing the decomposition of MRC, exacerbating the positive feedback between rising temperature and CO2 efflux. Our study also considered the influence of multiple environmental factors such as soil pH and moisture, which were more important in controlling SOC than microbial traits such as microbial life-style strategies and metabolic efficiency. Together, our work suggests an important mechanism underlying long-term soil C sequestration, which has important implications for the microbial-mediated C process in the face of global 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.

Melting Himalayas and mercury export: Results of continuous observations from the Rongbuk Glacier on Mt. Everest and future insights.

Glaciers in the Himalayan region have been receding rapidly in recent decades, drawing increasing concerns about the release of legacy pollutants (e.g., mercury (Hg)). To investigate the distribution, transport and controlling factors of Hg in glacier-fed runoff, from June 2019 to July 2020, a continuous monitoring and an intensive sampling campaign were conducted in the Rongbuk Glacier-fed basin (RGB) on the north slope of Mt. Everest in the middle Himalayas. The total Hg (THg) and methyl Hg (MeHg) concentrations were 1.56 ± 0.85 and 0.057 ± 0.025 ng/L, respectively, which were comparable to the global background levels and were mainly affected by the total suspended particulate matter (TSP). In addition, THg and MeHg showed significant diurnal variations, with peak values appearing at approximately 17:00 (upstream) and 19:00 (downstream). Based on the annual runoff and average Hg concentration, the annual export fluxes of THg and MeHg were estimated to be 441 g and 16 g, respectively. The yields of THg and MeHg in the RGB were 1.6 and 0.06 µg/m2/year, respectively. Currently, the annual Hg export of meltwater runoff in the Himalayan region is approximately 337 kg/year. When flowing through the proglacial lake, the THg concentrations decreased by 32% and 15% in the proglacial lake water and in the outlet, respectively, indicating that proglacial lakes had a sedimentation effect on the Hg transport. The Hg export from meltwater runoff in the Himalayas will likely increase considering the meltwater runoff has been projected to increase in the future. Nonetheless, emerging proglacial lakes may exert ambiguous effects on the glacier exported Hg under changing climate. Proglacial lakes could lower the levels and amounts of Hg in the glacier runoff, whereas the outburst of proglacial lakes could lead to an instantaneous release of Hg stored in lake waters and sediments. Our analysis shed light on the environmental impact of glacier retreat in the Himalayas and highlighted the need for integrated monitoring and study of Hg in glacier runoff and glacial lakes.

14C characteristics of organic carbon in the atmosphere and at glacier region of the Tibetan Plateau.

As an important component of carbonaceous aerosols (CA), organic carbon (OC) exerts a strong, yet insufficiently constrained perturbation of the climate. In this study, we reported sources of OC based on its natural abundance radiocarbon (14C) fingerprinting in aerosols and water-insoluble organic carbon (WIOC) in snowpits across the Tibetan Plateau (TP) - one of the remote regions in the world and a freshwater reservoir for billions of people. Overall, the proportions from 14C-based non-fossil fuel contribution (fnon-fossil) for OC in aerosols was 74 ± 10%, while for WIOC in snowpits was 81 ± 10%, both of which were significantly higher than that of elemental carbon (EC). These indicated sources of OC (WIOC) and EC were different at remote TP. Spatially, high fnon-fossil of WIOC of snowpit samples appeared at the inner part of the TP, indicating the important contribution of local non-fossil sources. Therefore, local non-fossil sources rather than long-range transportation OC dominants its total amount of the TP. In addition, the contribution of local non-fossil sourced WIOC increased during the monsoon period because heavy precipitation removed a high ratio of long-range transportation WIOC. The results of this study showed that not only OC and EC but also their different fuel sources should be treated separately in models to investigate their sources and atmospheric transportation.

Microbial assemblies associated with temperature sensitivity of soil respiration along an altitudinal gradient.

Identifying the drivers of the response of soil microbial respiration to warming is integral to accurately forecasting the carbon-climate feedbacks in terrestrial ecosystems. Microorganisms are the fundamental drivers of soil microbial respiration and its response to warming; however, the specific microbial communities and properties involved in the process remain largely undetermined. Here, we identified the associations between microbial community and temperature sensitivity (Q10) of soil microbial respiration in alpine forests along an altitudinal gradient (from 2974 to 3558 m) from the climate-sensitive Tibetan Plateau. Our results showed that changes in microbial community composition accounted for more variations of Q10 values than a wide range of other factors, including soil pH, moisture, substrate quantity and quality, microbial biomass, diversity and enzyme activities. Specifically, co-occurring microbial assemblies (i.e., ecological clusters or modules) targeting labile carbon consumption were negatively correlated with Q10 of soil microbial respiration, whereas microbial assemblies associated with recalcitrant carbon decomposition were positively correlated with Q10 of soil microbial respiration. Furthermore, there were progressive shifts of microbial assemblies from labile to recalcitrant carbon consumption along the altitudinal gradient, supporting relatively high Q10 values in high-altitude regions. Our results provide new insights into the link between changes in major microbial assemblies with different trophic strategies and Q10 of soil microbial respiration along an altitudinal gradient, highlighting that warming could have stronger effects on microbially-mediated soil organic matter decomposition in high-altitude regions than previously thought.

Tracing the Transboundary Transport of Mercury to the Tibetan Plateau Using Atmospheric Mercury Isotopes.

Deposition of atmospheric mercury (Hg) is the most important Hg source on the high-altitude Himalayas and Tibetan Plateau. Herein, total gaseous Hg (TGM) at an urban and a forest site on the Tibetan Plateau was collected respectively from May 2017 to October 2018, and isotopic compositions were measured to clarify the influences of landforms and monsoons on the transboundary transport of atmospheric Hg to the Tibetan Plateau. The transboundary transported anthropogenic emissions mainly originated over Indo-Gangetic Plain and carried over the Himalayas by convective storms and mid-tropospheric circulation, contributing over 50% to the TGM at the Lhasa urban site, based on the binary mixing model of isotopes. In contrast, during the transport of TGM from South Asia with low altitude, the uptake by evergreen forest in Yarlung Zangbo Grand Canyon largely decreased the TGM level and shifted isotopic compositions in TGM at the Nyingchi forest site, which are located at the high-altitude end of the canyon. Our results provided direct evidence from Hg isotopes to reveal the distinct patterns of transboundary transport to the Tibetan Plateau shaped by landforms and climates, which is critical to fully understand the biogeochemical cycling of Hg in the high-altitude regions.

Contrasting changes in long-term wet mercury deposition and socioeconomic development in the largest city of Tibet.

Information about the long-term trends of wet mercury (Hg) deposition is important for assessing the impact of atmospheric pollution on environmental health. As the most populated and capital city of Tibet, Lhasa is isolated far away from the heavily-polluted urban clusters in China. In this study, a 10-year observation was conducted in Lhasa to establish the long-term trend of wet Hg deposition and investigate the possible causes of this variation trend. Our study showed no significant increase in wet Hg deposition while Lhasa has achieved rapid population and economic growth during the study period. The contrasting changes in long-term wet Hg deposition and socioeconomic development (e.g., GDP growth) could be greatly attributed to the efforts in preventing and controlling air pollution at regional and local levels. This trend in Lhasa differs greatly from those observed by a rapid increase of Hg trend in the remote areas of the Tibetan Plateau. Our findings indicate that the remote cryospheric areas over the Tibetan Plateau are prone to be affected by transboundary Hg pollution, and more attention should be paid to its environmental and health effects for future study.

Observation-Based Mercury Export from Rivers to Coastal Oceans in East Asia.

Globally, the consumption of coastal fish is the predominant source of human exposure to methylmercury, a potent neurotoxicant that poses health risks to humans. However, the relative importance of riverine inputs and atmospheric deposition of mercury into coastal oceans remains uncertain owing to a lack of riverine mercury observations. Here, we present comprehensive seasonal observations of riverine mercury and methylmercury loads, including dissolved and particulate phases, to East Asia's coastal oceans, which supply nearly half of the world's seafood products. We found that East Asia's rivers annually exported 95 ± 29 megagrams of mercury to adjacent seas, 3-fold greater than the corresponding atmospheric deposition. Three rivers alone accounted for 71% of East Asia's riverine mercury exports, namely: Yangtze, Yellow, and Pearl rivers. We further conducted a metadata analysis to discuss the mercury burden on seawater and found that riverine export, combined with atmospheric deposition and terrestrial nutrients, quantitatively elevated the levels of total, methylated, and dissolved gaseous mercury in seawater by an order of magnitude. Our observations support that massive amounts of riverine mercury are exported to coastal oceans on a continental scale, intensifying their spread from coastal seawater to the atmosphere, marine sediments, and open oceans. We suggest that the impact of mercury transport along the land-ocean aquatic continuum should be considered in human exposure risk assessments.

Soil microbial trait-based strategies drive metabolic efficiency along an altitude gradient.

Trait-based approaches provide a candidate framework for linking soil microbial community to ecosystem processes, yet how the trade-offs in different microbial traits regulate the community-level metabolic efficiency remains unknown. Herein we assessed the roles of the microbial taxa with particular trait strategies in mediating soil microbial metabolic efficiency along an altitude gradient on the Tibetan Plateau. Results showed that soil microbial metabolic efficiency declined with increasing altitude, as indicated by the increasing metabolic quotient (microbial respiration per unit biomass, qCO2) and decreasing carbon use efficiency (CUE). Both qCO2 and CUE were predominantly predicted by microbial physiological and taxonomic attributes after considering key environmental factors including soil pH, substrate quantity and quality. Specifically, the reduced metabolic efficiency was associated with higher investment into nutrient (particularly for phosphorus) acquisitions via enzymes. Furthermore, we identified key microbial assemblies selected by harsh environments (low substrate quality and temperature) as important predictors of metabolic efficiency. These results suggest that particular microbial assemblies adapted to nutrient limited and cold habitats, but at the expense of lower metabolic efficient at higher altitude. Our findings provide a candidate mechanism underlying community-level metabolic efficiency, which has important implications for microbial-mediated processes such as carbon dynamics under global climate changes.

Spatio-temporal characteristics of air pollutants over Xinjiang, northwestern China.

To understand the characteristics of particulate matter (PM) and other air pollutants in Xinjiang, a region with one of the largest sand-shifting deserts in the world and significant natural dust emissions, the concentrations of six air pollutants monitored in 16 cities were analyzed for the period January 2013-June 2019. The annual mean PM2.5, PM10, SO2, NO2, CO, and O3 concentrations ranged from 51.44 to 59.54 µg m-3, 128.43-155.28 µg m-3, 10.99-17.99 µg m-3, 26.27-31.71 µg m-3, 1.04-1.32 mg m-3, and 55.27-65.26 µg m-3, respectively. The highest PM concentrations were recorded in cities surrounding the Taklimakan Desert during the spring season and caused by higher amounts of wind-blown dust from the desert. Coarse PM (PM10-2.5) was predominant, particularly during the spring and summer seasons. The highest PM2.5/PM10 ratio was recorded in most cities during the winter months, indicating the influence of anthropogenic emissions in winters. The annual mean PM2.5 (PM10) concentrations in the study area exceeded the annual mean guidelines recommended by the World Health Organization (WHO) by a factor of ca. ∼5-6 (∼7-8). Very high ambient PM concentrations were recorded during March 19-22, 2019, that gradually influenced the air quality across four different cities, with daily mean PM2.5 (PM10) concentrations ∼8-54 (∼26-115) times higher than the WHO guidelines for daily mean concentrations, and the daily mean coarse PM concentration reaching 4.4 mg m-3. Such high PM2.5 and PM10 concentrations pose a significant risk to public health. These findings call for the formulation of various policies and action plans, including controlling the land degradation and desertification and reducing the concentrations of PM and other air pollutants in the region.

The impact of the Three Gorges Dam on the fate of metal contaminants across the river-ocean continuum.

The Three Gorges Dam (TGD) is the world's largest hydropower construction. It can significantly impact contaminant transport in the Yangtze River-East China Sea Continuum (YR-ECSC). In addition to evaluating the impact of the TGD on the deposition of contaminants in the reservoir, we also address their cycles in the river below the dam and in the coastal East China Sea. A comprehensive study of metal contaminant transport along the YR-ECSC has not been previously attempted. We quantified the fates of mercury (Hg), arsenic (As), lead (Pb), cadmium (Cd) and chromium (Cr) within the YR-ECSC, and the impacts of the TGD, by sampling water and suspended particles along the Yangtze River during spring, summer, fall, and winter and by modeling. We found that the Yangtze River transports substantial amounts of heavy metals into the coastal ocean. In 2016, riverine transport amounted to 48, 5900, 11,000, 230, and 15,000 megagrams (Mg) for Hg, As, Pb, Cd, and Cr, respectively, while other terrestrial contributions were negligible. Metal flux into the coastal ocean was primarily derived from the downstream portion of the river (84-97%), while metals transported from upstream were largely trapped in the Three Gorges Reservoir (TGR, 72%-96%). For example, 34 Mg of Hg accumulated in the TGR owing to river damming, large-scale soil erosion, and anthropogenic point source releases, while 21 Mg of Hg was depleted from the riverbed downstream owing to the altered river hydrology caused by the TGD. Overall the construction of TGD resulted in a 6.9% net decrease in the Hg burden of the East China Sea, compared to the pre-TGD period.

Seasonal Variation of Mercury and Its Isotopes in Atmospheric Particles at the Coastal Zhongshan Station, Eastern Antarctica.

Mercury (Hg) is a globally spread trace metal due to its long atmospheric residence time. Yet, our understanding of atmospheric processes (e.g., redox reactions and deposition) driving Hg cycling is still limited, especially in polar regions. The Antarctic continent, by virtue of its remoteness, is the perfect location to investigate Hg atmospheric processes in the absence of significant local anthropogenic impact. Here, we present the first 2 year record (2016-2017) of total suspended particulate mercury (PHg) concentrations along with a year-round determination of an Hg stable isotopic composition in particles collected at Zhongshan Station (ZSS), eastern Antarctic coast. The mean PHg concentration is 21.8 ± 32.1 pg/m3, ranging from 0.9 to 195.6 pg/m3, and peaks in spring and summer. The negative mass-independent fractionation of odd Hg isotopes (odd-MIF, average -0.38 ± 0.12‰ for Δ199Hg) and the slope of Δ199Hg/Δ201Hg with 0.91 ± 0.12 suggest that the springtime isotope variation of PHg is likely caused by in situ photo-oxidation and reduction reactions. On the other hand, the increase of PHg concentrations and the observed odd-MIF values in summer are attributed to the transport by katabatic winds of divalent species derived from the oxidation of elemental Hg in the inland Antarctic Plateau.

Mercury variation and export in trans-Himalayan rivers: Insights from field observations in the Koshi River.

Strengthening the research of riverine mercury (Hg) export is of great significance for understanding the regional and global Hg cycle, especially for the data lacking trans-Himalayan rivers. In this study, three systematic sampling campaigns were conducted in the Koshi River Basin (KRB) during the post-monsoon, pre-monsoon and monsoon seasons. Hg speciation and distribution of river water were analyzed among the different seasons for a total of 88 water samples. The total Hg (THg) concentration of surface water in the KRB ranged from 0.64 to 32.96 ng·L-1 with an average of 5.83 ± 6.19 ng·L-1 and decreased in the order of post-monsoon (8.79 ± 7.32 ng·L-1) > monsoon (6.68 ± 6.12 ng·L-1) > pre-monsoon (2.18 ± 1.29 ng·L-1). Particulate Hg (PHg) accounted for 63% of THg on average and had a positive correlation with THg among all the three sampling seasons, indicating that the differences in PHg concentration were likely one of the main factors leading to the seasonal and spatial variations in THg in the KRB surface water. The annual Hg exports and fluxes were estimated to be 339.04 kg and 3.88 µg·m-2·yr-1, respectively. Furthermore, Hg export from the KRB had significant seasonal variation and decreased in the order of monsoon (259.47 kg) > post-monsoon (61.18 kg) > winter (9.31 kg) > pre-monsoon (9.08 kg), and this pattern was mainly related to seasonal changes in river runoff. The annual Hg export is projected to increase in the future, especially in the post-monsoon season. Therefore, more attention should be paid to river runoff observations and riverine Hg research for water resources management in the Himalaya.

Latest observations of total gaseous mercury in a megacity (Lanzhou) in northwest China.

One year of online total gaseous mercury (TGM) measurements were carried out for the first time in Lanzhou, a city in northwest China that was once seriously polluted. Measurements were made from October 2016 to October 2017 using the Tekran 2537B instrument, and the annual mean concentration of TGM in Lanzhou was 4.48 ± 2.32 ng m-3 (mean ± standard deviation). TGM concentrations decreased during the measurement period, with autumn 2017 average concentrations 2.87 ng m-3 lower than autumn 2016 average concentrations. Similar diurnal variations of TGM were obtained in different seasons with low concentrations observed in the afternoon and high concentrations at night. The principal component analysis and conditional probability function results revealed that the sources of mercury were similar to the other atmospheric pollutants such as SO2, CO, NO2 and PM2.5, and were mainly from industrial combustion plants in urban districts. Concentration weighted trajectory analysis using backward trajectories demonstrated that higher mercury concentrations were related to air masses from adjacent regions, indicating the importance of influences from local-to-regional scale sources. A synthesis of multi-decadal atmospheric mercury measurements in Lanzhou and other Chinese megacities revealed that atmospheric mercury concentrations were either generally stable or experienced a slight decrease, during a time when China implemented control measures on atmospheric pollution. Long-term atmospheric mercury observations in urban and background sites in China are warranted to assess mercury pollution and the effectiveness of China's mercury control policies.