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Mercury is toxic to wildlife and humans, and forests are thought to be a globally important sink for gaseous elemental mercury (GEM) deposition from the atmosphere. Yet there are currently no annual GEM deposition measurements over rural forests. Here we present measurements of ecosystem-atmosphere GEM exchange using tower-based micrometeorological methods in a midlatitude hardwood forest. We measured an annual GEM deposition of 25.1 µg ⋅ m -2 (95% CI: 23.2 to 26.7 1 µg ⋅ m -2 ), which is five times larger than wet deposition of mercury from the atmosphere. Our observed annual GEM deposition accounts for 76% of total atmospheric mercury deposition and also is three times greater than litterfall mercury deposition, which has previously been used as a proxy measure for GEM deposition in forests. Plant GEM uptake is the dominant driver for ecosystem GEM deposition based on seasonal and diel dynamics that show the forest GEM sink to be largest during active vegetation growing periods and middays, analogous to photosynthetic carbon dioxide assimilation. Soils and litter on the forest floor are additional GEM sinks throughout the year. Our study suggests that mercury loading to this forest was underestimated by a factor of about two and that global forests may constitute a much larger global GEM sink than currently proposed. The larger than anticipated forest GEM sink may explain the high mercury loads observed in soils across rural forests, which impair water quality and aquatic biota via watershed Hg export., Competing Interests: The authors declare no competing interest., (Copyright © 2021 the Author(s). Published by PNAS.)

Measurements of land-air mercury (Hg) exchanges over vegetated surfaces are needed to further constrain Hg fluxes over vegetated terrestrial surfaces. Yet, knowledge of land-air Hg dynamics in alpine grasslands remains poor. Hg fluxes over an alpine meadow were measured throughout a full vegetation period in the central Tibetan Plateau (TP). This TP grassland served as a small source of atmospheric total gaseous Hg (TGM) during vegetation period (0.92 μg m -2 ). Hg fluxes decreased logarithmically during plant growing season, resulting from the influence of vegetation by light shading and plant Hg uptake, although the latter might be minor due to low biomass at this site. Temporal patterns of TGM indicated the importance of land-air dynamics in regulating TGM levels. During the plant emergence, diel pattern of TGM covaried with Hg emission fluxes resulting in lower concentrations at night and higher concentrations in afternoon. During all other vegetation stages, TGM showed minima before dawn and "morning peak" shortly after sunrise, in conjunction with corresponding Hg fluxes showing sink before dawn and source after sunrise. Moreover, TGM concentrations showed a decreasing trend with plant growing, further indicating the role of vegetation in driving seasonal TGM variations by regulating land-air Hg dynamics.

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 δ 202 Hg and Δ 199 Hg 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 206 Pb/ 207 Pb and 208 Pb/ 207 Pb 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.

The concentration and isotopic composition of mercury (Hg) were studied in frozen soils along a southwest-northeast transect over the Himalaya-Tibet. Soil total Hg (Hg T ) concentrations were significantly higher in the southern slopes (72 ± 54 ng g -1 , 2SD, n = 21) than those in the northern slopes (43 ± 26 ng g -1 , 2SD, n = 10) of Himalaya-Tibet. No significant relationship was observed between Hg T concentrations and soil organic carbon (SOC), indicating that the Hg T variation was not governed by SOC. Soil from the southern slopes showed significantly negative mean δ 202 Hg (-0.53 ± 0.50‰, 2SD, n = 21) relative to those from the northern slopes (-0.12 ± 0.40‰, 2SD, n = 10). The δ 202 Hg values of the southern slopes are more similar to South Asian anthropogenic Hg emissions. A significant correlation between 1/Hg T and δ 202 Hg was observed in all the soil samples, further suggesting a mixing of Hg from South Asian anthropogenic emissions and natural geochemical background. Large ranges of Δ 199 Hg (-0.45 and 0.24‰) were observed in frozen soils. Most of soil samples displayed negative Δ 199 Hg values, implying they mainly received Hg from gaseous Hg(0) deposition. A few samples had slightly positive odd-MIF, indicating precipitation-sourced Hg was more prevalent than gaseous Hg(0) in certain areas. The spatial distribution patterns of Hg T concentrations and Hg isotopes indicated that Himalaya-Tibet, even its northern part, may have been influenced by transboundary atmospheric Hg pollution from South Asia., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

The unique geographic location of Mt. Yulong in the Tibetan Plateau (TP) makes it a favorable site for mercury (Hg) study. Various snow samples, such as surface snow, snow pit, and snowmelt water were collected from Mt. Yulong in the southeastern TP. The average concentration of Hg was found to be 37 ± 26 ng L -1 (mean ± SD), comparable to Hg concentration from other parts of TP in the same year, though it was comparatively higher than those from previous years, suggesting a possible increase of Hg concentration over the TP. The concentration of Hg was higher in the lower elevation of the glaciers possibly due to the surface melting concentration of particulates. Higher concentration of Hg was observed in the fresh snow, suggesting the possibility of long-range transportation. The average concentration of Hg from the snow pit was 1.49 ± 0.78 ng L -1 , and the concentration of Hg in the vertical profile of the snow pit co-varied with calcium ion (Ca 2+ ) supporting the fact that the portion of Hg is from the crustal origin. In addition, the principal component analysis (PCA) confirmed that the source of Hg is from the crustal origin; however, the presence of anthropogenic source in the Mt. Yulong was also observed. In surface water around Mt. Yulong, the concentration of Hg T was found in the order of Lashihai Lake > Reservoirs > Rivers > Swamps > Luguhu Lake. In lake water, the concentration of Hg T showed an increasing trend with depth. Overall, the increased concentration of Hg in recent years from the TP can be of concern and may have an adverse impact on the downstream ecosystem, wildlife, and human health.

Glacierized mountain environments can preserve and release mercury (Hg) and play an important role in regional Hg cycling. In the Tibetan Plateau (TP), most glaciers have been retreating at unprecedented rate in recent decades, acting as one of the most active factors in regional hydrological cycling. In this mini-review, we summarized recent studies on Hg distribution, transport, and accumulation in Tibetan glacierized environments. We highlight that melting glacier may represent a stimulator that exports Hg to glacier-fed ecosystems. We identified major knowledge gaps and proposed future research needs with several emphases, including quantifying Hg in glacier ablation zone, depicting Hg transport and transformation in glacial rivers during spring melt season, and better constraining glacier-export Hg and its environmental risks to the downstream. Besides, Hg isotopic technical, passive sampling and hydrological transport model should be utilized to improve the understanding of Hg cycling in high mountain regions in the TP.

The Tibetan Plateau (TP) is recognized as "Water Tower of Asia". Yet our understanding of mechanisms influencing incorporation of mercury (Hg) into freshwater in mountain glaciers on the TP remains quite limited. Extensive sampling of environmental matrices (e.g., snow/ice) were conducted on the East Rongbuk glacier on Mt. Everest and Zhadang glacier on Mt. Nyainqentanglha for Hg speciation analysis. Speciated Hg behaved quite different during snowmelt: a preferential early release of DHg (dissolved Hg) was observed at the onset of snowmelt, whereas PHg (particulate-bound Hg) and THg (total Hg) become relatively enriched in snow and released later. Small fraction of Hg in snow was lost during a snowmelt day (18.9%-34.7%) with a large proportion (58.1%-87.3%) contributed by PHg decrease, indicating that the deposited Hg is most likely retained in glacier snow/ice. Furthermore, THg were positively correlated with PHg and crustal major ions (e.g., Ca 2+ , Mg 2+ ) during snowmelt, indicating that Hg is mainly migrated with particulates. The main pathway of Hg loss during snowmelt was most probably associated with release of PHg with meltwater, which was greatly influenced by ablation intensity of snow/ice. This should be paid particular concern as Hg preserved in mountain glaciers will mostly enter aquatic ecosystem as climate warms, impacting on downstream ecosystems adversely. Obvious decrease of THg during the downstream transport from glacier was observed with a large proportion contributed by PHg decrease. The main removal mechanism of Hg was associated with sedimentation of PHg during the transport process., (Copyright © 2018. Published by Elsevier B.V.)

This study quantified concentrations of mercury (Hg) and its trophic transfer along the fish community in the Indrawati River, Nepal. Stable isotope ratios of nitrogen (δ 15 N) and carbon (δ 13 C), complemented by stomach contents data were used to assess the food web structure and trophic transfer of Hg in 54 fishes; 43 Shizothorax richardsonii and 11 Barilius spp. [B. bendelisis (1), B. vagra (3) and B. barila (7)]. Sixty-one muscle samples (including six replicates) were used for the analysis of total mercury (THg) and stable isotopes. Mean THg concentrations in B. spp. and the more common species S. richardsonii was observed to be 218.23 (ng/g, ww) and 90.82 (ng/g, ww), respectively. THg versus total length in both S. richardsonii and B. spp. showed a decreasing tendency with an increase in age. Regression of logTHg versus δ 15 N among the fish species showed a significant positive correlation only in S. richardsonii indicating biomagnification along the trophic level in this species.

The Tibetan Plateau (TP) is home to the largest permafrost bodies at low- and mid-latitudes, yet little is known about the distribution and variation of mercury (Hg) in frozen soil of the permafrost regions. In this study, extensive soil sampling campaigns were carried out in 23 soil pits from 12 plots in a high-altitude permafrost region of the Shule River Basin, northeastern TP. Hg distribution, variation, and their dependences on soil properties were analyzed. The results have revealed that total Hg (THg) concentrations were low ranging from 6.3 to 29.1 ng g -1 . A near-surface peak of THg concentrations followed by a continuous decrease were observed on the vertical profiles of most soil pits. Significant positive relationships among THg concentrations, soil organic carbon (SOC) contents, and silty fractions were observed, indicating that SOC content and silty fraction are two dominant factors influencing the spatial distribution of THg. THg concentrations in soils showed a decreasing trend with altitude, which was probably attributed to a lower soil potential to Hg accumulation under the condition of lower SOC contents and silty fractions at high altitudes. Approximately, 130.6 t Hg in soils (0-60 cm) was estimated and a loss of 64.2% of Hg from the highly stable and stable permafrost (H-SP) region via permafrost degradation was expected in the upstream regions of the Shule River Basin, indicating that the large areas of permafrost regions may become an important source of global Hg emission as a result of the ongoing widespread permafrost degradation.

Glaciers, particularly alpine glaciers, have been receding globally at an accelerated rate in recent decades. The glacial melt-induced release of pollutants (e.g., mercury) and its potential impact on the atmosphere and glacier-fed ecosystems has drawn increasing concerns. During 15th-20th August, 2011, an intensive sampling campaign was conducted in Qugaqie Basin (QB), a typical high mountain glacierized catchment in the inland Tibetan Plateau, to investigate the export and transport of mercury from glacier to runoff. The total mercury (THg) level in Zhadang (ZD) glacier ranged from <1 to 20.8 ng L -1 , and was slightly higher than levels measured in glacier melt water and the glacier-fed river. Particulate Hg (PHg) was the predominant form of Hg in all sampled environmental matrices. Mercury concentration in Qugaqie River (QR) was characterized by a clear diurnal variation which is linked to glacier melt. The estimated annual Hg exports by ZD glacier, the upper river basin and the entire QB were 8.76, 7.3 and 157.85 g, respectively, with respective yields of 4.61, 0.99 and 2.74 μg m -2  yr -1 . Unique landforms and significant gradients from the glacier terminus to QB estuary might promote weathering and erosion, thereby controlling the transport of total suspended particulates (TSP) and PHg. In comparison with other glacier-fed rivers, QB has a small Hg export yet remarkably high Hg yield, underlining the significant impact of melting alpine glaciers on regional Hg biogeochemical cycles. Such impacts are expected to be enhanced in high altitude regions under the changing climate., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Monsoon circulation is an important process that affects long-range transboundary transport of anthropogenic contaminants such as mercury (Hg). During the Indian monsoon season of 2013, a total of 92 and 26 atmospheric water vapor samples were collected at Lhasa, the largest city of the Tibet, for Hg and major ions analysis, respectively. The relatively low pH/high electronic conductivity values, together with the fact that NH4(+) in atmospheric water vapor was even higher than that determined in precipitation of Lhasa, indicated the effects of anthropogenic perturbations through long-range transboundary atmospheric transport. Concentrations of Hg in atmospheric water vapor ranged from 2.5 to 73.7ngL(-1), with an average of 12.5ngL(-1). The elevated Hg and major ions concentrations, and electronic conductivity values were generally associated with weak acidic samples, and Hg mainly loaded with anthropogenic ions such as NH4(+). The results of principal component analysis and trajectory analysis suggested that anthropogenic emissions from the Indian subcontinent may have largely contributed to the determined Hg in atmospheric water vapor. Furthermore, our study reconfirmed that below-cloud scavenging contribution was significant for precipitation Hg in Lhasa, and evaluated that on average 74.1% of the Hg in precipitation could be accounted for by below-cloud scavenging., (Copyright © 2016 Elsevier B.V. All rights reserved.)

The Tibetan Plateau is home to the largest aggregate of glaciers outside the Polar Regions and is a source of fresh water to 1.4 billion people. Yet little is known about the transportation and cycling of Hg in high-elevation glacier basins on Tibetan Plateau. In this study, surface snow, glacier melting stream water and lake water samples were collected from the Qiangyong Glacier Basin. The spatiotemporal distribution and transportation of Hg from glacier to lake were investigated. Significant diurnal variations of dissolved Hg (DHg) concentrations were observed in the river water, with low concentrations in the morning (8:00am-14:00pm) and high concentrations in the afternoon (16:00pm-20:00pm). The DHg concentrations were exponentially correlated with runoff, which indicated that runoff was the dominant factor affecting DHg concentrations in the river water. Moreover, significant decreases of Hg were observed during transportation from glacier to lake. DHg adsorption onto particulates followed by the sedimentation of particulate-bound Hg (PHg) could be possible as an important Hg removal mechanism during the transportation process. Significant decreases in Hg concentrations were observed downstream of Xiao Qiangyong Lake, which indicated that the high-elevation lake system could significantly affect the distribution and transportation of Hg in the Qiangyong Glacier Basin., (Copyright © 2016. Published by Elsevier B.V.)

Accurate measurements of wet mercury (Hg) deposition are critically important for the assessment of ecological responses to pollutant loading. The Hg in wet deposition was measured over a 3-year period in the southeastern Tibetan Plateau. The volume-weighted mean (VWM) total Hg (HgT) concentration was somewhat lower than those reported in other regions of the Tibetan Plateau, but the VWM methyl-Hg concentration and deposition flux were among the highest globally reported values. The VWM HgT concentration was higher in non-monsoon season than in monsoon season, and wet HgT deposition was dominated by the precipitation amount rather than the scavenging of atmospheric Hg by precipitation. The dominant Hg species in precipitation was mainly in the form of dissolved Hg, which indicates the pivotal role of reactive gaseous Hg within-cloud scavenging to wet Hg deposition. Moreover, an increasing trend in precipitation Hg concentrations was synchronous with the recent economic development in South Asia., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

[Display omitted] • First comprehensive investigation and assessments of topsoil Hg in Central Asia. • Topsoil THg concentrations exhibited obvious differences among various landscapes. • High THg concentrations were found in the capital cities/urban clusters. • Topsoils in densely-populated areas were contaminated by Hg and had ecological risks. • Topsoil Hg posed no unacceptable health risk to both adults and children. Central Asia is one of the largest arid areas on earth, yet little is known about the concentration levels and risks of mercury (Hg) in the soils of this region. In this study, extensive sampling of topsoils (0–10 cm) from representative landscapes was carried out over Central Asia (i.e., Tajikistan, Uzbekistan, and Kyrgyzstan). The total mercury (THg) concentrations in topsoils varied widely from 1.6 to 908.0 ng/g, with high values observed in samples collected in the capital cities and urban areas. Topsoil THg concentrations among different landscapes showed a decreasing order of urban (79.8±184.0 ng/g) > woodland (27.3±28.9 ng/g) > grassland (20.6±15.9 ng/g) > farmland (18.3±9.5 ng/g) > desert (12.3±8.0 ng/g). High THg concentrations were found in the capital cities/urban clusters, followed by a gradual decrease towards the peripheries. THg concentrations were found to be negatively correlated with the distance from the sampling sites to their nearest cities, indicating that anthropogenic emissions significantly influenced the spatial distribution of topsoil Hg. A significant correlation between THg concentrations and topsoil total organic carbon (TOC) contents was also observed, suggesting that TOC played an essential role in the spatial distribution of topsoil Hg. The assessments of pollution and potential ecological risk suggested that topsoils in highly densely-populated areas were contaminated by Hg and had higher degrees of potential ecological risks. The health risk assessment results showed that the exposure risk of topsoil Hg to children was higher than that to adults. Fortunately, there was no unacceptable human health risk of topsoil Hg. This study clarified the spatial distribution and risks of Hg in the Central Asian topsoils, offering new insight into the risk prevention and control of soil Hg. [ABSTRACT FROM AUTHOR]

Mercury (Hg), a global contaminant, can sink into cryosphere and be released into runoff through meltwater. The Tibetan Plateau (TP) has been witnessing ongoing shrinkage of alpine glaciers. However, the export of Hg from melting glacier is still sparsely reported. From October 16, 2020 to October 15, 2021, we conducted daily observations to study the variation in total Hg concentrations and its export to the Mingyong River, a glacier-fed river in southeastern TP. Results showed that the Hg concentrations were high during the monsoon season but low during the non-monsoon period. The Hg in runoff correlated with the concentrations of total suspended particulates (TSP) and dissolved inorganic carbon (DIC) during both monsoon and non-monsoon seasons (p < 0.01), and the correlation of Hg with other parameters showed seasonal variations. The input from meltwater, precipitation, and groundwater to riverine Hg were 8.3 g, 264.4 g, and 71.0 g, respectively, and the total export was 211.0 g (yield: 4.3 g/km2/year) in the hydrological year, indicating that Mingyong catchment act as a sink for Hg. For the entire TP, the annual export of Hg from glacier runoff was estimated to be 947.7 kg/year. Our study highlights the necessity for further investigations on Hg dynamics to understand the changes in the Hg cycle within glaciated aquatic ecosystems. [Display omitted] • The daily variation of Hg concentration in a glacier-fed river was analyzed. • The monsoon period accounted for 90.8 % of Hg in the observed year. • The Hg concentrations in runoff was influenced by environmental and hydrological processes. • Mingyong catchment was a sink for Hg, with an input of 343.8 g and an export of 211.0 g. • The annual export of Hg from glacier runoff over TP was 947.7 kg/year. [ABSTRACT FROM AUTHOR]

Long-term monitoring of global pollutant such as Mercury (Hg) in the cryosphere is very essential for understanding its bio-geochemical cycling and impacts in the pristine environment with limited emission sources. Therefore, from May 2015 to Oct 2015, surface snow and snow-pits from Xiao Dongkemadi Glacier and glacier melt water were sampled along an elevation transect from 5410 to 5678 m a.s.l. in the central Tibetan Plateau (TP). The concentration of Hg in surface snow was observed to be higher than that from other parts of the TP. Unlike the southern parts of the TP, no clear altitudinal variation was observed in the central TP. The peak Total Hg (Hg T ) concentration over the vertical profile on the snow pits corresponded with a distinct yellowish-brown dust layer supporting the fact that most of the Hg was associated with particulate matter. It was observed that only 34% of Hg in snow was lost when the surface snow was exposed to sunlight indicating that the surface snow is less influenced by the post-depositional process. Significant diurnal variation of Hg T concentration was observed in the river water, with highest concentration observed at 7 pm when the discharge was highest and lowest concentration during 7–8 am when the discharge was lowest. Such results suggest that the rate of discharge was influential in the concentration of Hg T in the glacier fed rivers of the TP. The estimated export of Hg T from Dongkemadi river basin is 747.43 g yr − 1 , which is quite high compared to other glaciers in the TP. Therefore, the export of global contaminant Hg might play enhanced role in the Alpine regions as these glaciers are retreating at an alarming rate under global warming which may have adverse impact on the ecosystem and the human health of the region. [ABSTRACT FROM AUTHOR]

In an effort to understand the biogeochemical cycling and seasonal characteristics of atmospheric Hg, a total of 80 daily sampled total suspended particulates were collected at Lhasa, the largest city of Tibet, from April 2013 to August 2014 for particulate-bound Hg (Hg P ) analysis. Daily concentrations of atmospheric Hg P ranged from 61.2 to 831 pg m −3 with an average of 224 pg m −3 , which were unexpectedly comparable to those measured in most of the Chinese metropolises. Both the daily/monthly average Hg P concentrations were slightly but not significantly higher during the non-monsoon season than during the monsoon season. Together with the fact that there was lack of significant relationship between Hg P concentration and most meteorological parameters, no significant and distinct pattern for the seasonal characteristics of atmospheric Hg P could be mainly attributed to the almost equal emission strength of two principal anthropogenic Hg sources (i.e., industrial emission sources during the non-monsoon season, and vehicular traffic and religious sources during the monsoon season). Moreover, the Hg P dry deposition rate was estimated to be 35.3 μg m −2 yr −1 by using a theoretical model, which was significantly higher than those Hg wet fluxes. The elevated deposition rate implied that dry deposition may play an important role in the biogeochemical Hg cycling over the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

• The permafrost regions on the Tibetan Plateau store about 21.65 Gg (interquartile: 18.43–24.24) Hg in 0–3 m soils. • The active layers on the Tibetan Plateau store 16.58 Gg (Interquartile: 14.55–18.48 Gg) Hg. • The mercury in the Tibetan Plateau is sensitive to permafrost degradation. Permafrost on the Qinghai-Tibetan Plateau (QTP) has been degrading in the past decades. While the degradation may mobilize previously protected material from the permafrost profile, little is known about the stocks and stability of mercury (Hg) in the QTP permafrost. Here we measured total soil Hg in 265 samples from 15 permafrost cores ranging from 3 to 18 m depth, and 45 active layer (AL) soil samples from different land cover types on the QTP. Approximately 21.7 Gg of Hg was stored in surficial permafrost (0–3 m), with 16.58 Gg of Hg was stored in the active layer. Results from six permafrost collapse areas showed that much of the thawed Hg is mobile, with decreases in total Hg mass of 17.6–30.9% for the AL (top 30 cm) in comparison with non-thermokarst surfaces. We conclude that the QTP permafrost region has a large mercury pool, and the stored mercury is sensitive to permafrost degradation. [ABSTRACT FROM AUTHOR]