The lockdowns implemented during the coronavirus disease 2019 (COVID-19) pandemic provide a unique opportunity to investigate the impact of emission sources and meteorological conditions on the trans-boundary transportation of black carbon (BC) aerosols to the Tibetan Plateau (TP). In this study, we conducted an integrative analysis, including in-situ observational data, reanalysis datasets, and numerical simulations, and found a significant reduction in the trans-boundary transport of BC to the TP during the 2020 pre-monsoon season as a result of the lockdowns and restrictive measures. Specifically, we observed a decrease of 0.0211 mu g m- 3 in surface BC concentration over the TP compared to the 2016 pre-monsoon period. Of this reduction, approximately 6.04 % can be attributed to the decrease in emissions during the COVID-19 pandemic, surpassing the 4.47 % decrease caused by changes in meteorological conditions. Additionally, the emission reductions have weakened the transboundary transport of South Asia BC to the TP by 0.0179 mu g m � 2s 1; indicating that the recurring spring atmospheric pollution from South Asia to the TP will be alleviated through the reduction of anthropogenic emissions. Moreover, it is important to note that BC deposition on glaciers contributes significantly to glacier melting due to its enrichment, posing a threat to the water sustainability of the TP. Therefore, urgent measures are needed to reduce emissions from adjacent regions to preserve the TP as the Asian Water Tower.

Transport of exogenous anthropogenic mercury (Hg) is an important source of Hg pollution in the Tibetan Plateau (TP) and its downstream water ecosystems, but the origins and contributions of Hg sources remain uncertain. Here, we investigate the concentrations and isotopic compositions of gaseous elemental mercury (GEM) at four rural sites in the TP and three urban sites surrounding the TP to quantify the sources of GEM in the TP. GEM concentrations in the surrounding cities (site-specific means: 2.36-9.12 ng m-3) were highly elevated mainly due to strong local anthropogenic emissions as indicated by their negative delta 202Hg and near zero Delta 199Hg and Delta 200Hg signatures. GEM isotopes indicate that GEM pollution in the TP, typically observed during the summer monsoon and the pre-monsoon, were mainly caused by trans-boundary transport of anthropogenic Hg from surroundings. Using an Hg isotope mixing model, we estimate that exogenous anthropogenic emissions on average contributed 26 +/- 5% (1sd) to the GEM in the TP. Further analysis of the transport of anthropogenic Hg emissions based on the backward trajectory and gridded anthropogenic Hg emissions suggests that 16 +/- 9% and 6 +/- 13% of the GEM in the TP were derived from anthropogenic sources in South Asia and China, respectively. Our study suggests that anthropogenic Hg emissions in South Asia could be effectively transported to the TP across the Himalayan range. Future studies are needed to better assess the role of rapidly increasing anthropogenic Hg emissions in South Asia on the regional to global scale atmospheric Hg cycling.

Winter-specific characteristics of airborne particulates over middle Indo-Gangetic Plain (IGP) were evaluated in terms of aerosol chemical and micro-physical properties under three-dimensional domain. Emphases were made for the first time to identify intra-seasonal variations of aerosols sources, horizontal and vertical transport, effects of regional meteorology and estimating composite aerosol short-wave radiative forcing over an urban region (25 degrees 10'-25 degrees 19'N; 82 degrees 54'-83 degrees 4'E) at middle-IGP. Space-borne passive (Aqua and Terra MODIS, Aura OMI) and active sensor (CALIPSO-CALIOP) based observations were concurrently used with ground based aerosol mass measurement for entire winter and pre-summer months (December, 1, 2014 to March, 31, 2015). Exceptionally high aerosol mass loading was recorded for both PK (267.6 +/- 107.0 mu g m(-3)) and PM2.5 (150.2 +/- 89.4 mu g m(-3)) typically exceeding national standard. Aerosol type was mostly dominated by fine particulates (particulate ratio: 0.61) during pre to mid-winter episodes before being converted to mixed aerosol types (ratio: 0.41-0.53). Time series analysis of aerosols mass typically identified three dissimilar aerosol loading episodes with varying attributes, well resemble to that of previous year's observation representing its persisting nature. Black carbon (9.4 +/- 3.7 mu g m(-3)) was found to constitute significant proportion of fine particulates (2-27%) with a strong diurnal profile. Secondary inorganic ions also accounted a fraction of particulates (PM2.5: 22.5%; PM10: 26.9%) having SO4-2, NO3- and NH4+ constituting major proportion. Satellite retrieved MODIS-AOD (0.01-2.30) and fine mode fractions (FMF: 0.01-1.00) identified intra-seasonal variation with transport of aerosols from upper to middle-IGP through continental westerly. Varying statistical association of columnar and surface aerosol loading both in terms of fine (r; PM2.5: MODIS-AOD: 0.51) and coarse particulates (PM10: MODIS-AOD: 0.53) was found influenced by local meteorology (boundary layer and humidity) and aerosol vertical profile. A gradual increase in aerosol vertical profile (surface to 4.9 km) was evident with dominance of polluted continental, polluted dust and smoke at lower altitude. Presence of mineral dusts in higher altitude during later phase was linked with its transboundary transport, originating from western dry regions. Conclusively, winter-specific short-wave aerosol radiative forcing revealed an ATM warming effect (31-47 W m(-2)) while cooling both at TOA (-20 to -32 W m(-2)) and SUF (-51 to -80 W m(-2)) with significant level of intra-seasonal variations in heating rates (0.86-132 K day(-1)). (C) 2016 Elsevier B.V. All rights reserved.