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 mu g.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. (C) 2020 Elsevier B.V. All rights reserved.

Aufeis is a common phenomenon in cold regions of the Northern Hemisphere that develops during winter by successive water overflow and freezing on ice-covered surfaces. Most studies on aufeis occurrence focus on regions in North America and Siberia, while research in High Mountain Asia (HMA) is still in an exploratory phase. This study investigates the extent and dynamics of icing processes and aufeis in the Tso Moriri basin, eastern Ladakh, India. Based on a combination of 235 Landsat 5 TM/8 OLI and Sentinel-2 imagery from 2008 to 2021 the occurrence of icing and aufeis was classified using a random forest classifier. A total of 27 frequently occurring aufeis fields with an average maximum extent of 9 km(2) were identified, located at a mean elevation of 4,700 m a.s.l. Temporal patterns show a distinct accumulation phase (icing) between November and April, and a melting phase lasting from May until July. Icing is characterized by high seasonal and inter-annual variability. Successive water overflow mainly occurs between January and March and seems to be related to diurnal freeze-thaw-cycles, whereas higher daytime temperatures result in larger icing areas. Aufeis feeding sources are often located within or in close vicinity to wetland areas, while vegetation is largely absent on surfaces with frequent aufeis formation. These interactions require more attention in future research. In addition, this study shows the high potential of a machine learning approach to monitor icing processes and aufeis, which can be transferred to other regions.

Analysis of the climatology of aerosol properties is performed over Hanle (4500 m) and Merak (4310 m), two remote-background sites in the western trans-Himalayas, based on eleven years (2008-2018) of sun/sky radiometer (POM-01, Prede) measurements. The two sites present very similar atmospheric conditions and aerosol properties allowing us to examine them as continuous single-data series. The annual average aerosol optical depth at 500 nm (AOD(500)) is 0.04 +/- 0.03, associated with an Angstrom exponent (AE(440-870)) of 0.58 +/- 0.35 and a single scattering albedo (SSA(500)) of 0.95 +/- 0.05. AOD(500) exhibits higher values in May (similar to 0.07) and lower in winter (similar to 0.03), while AE(400-870) minimizes in spring, indicating influence by coarse-mode dust aerosols, either emitted regionally or long-range transported. The de-convolution of AOD(500) into fine and coarse modes justifies the aerosol seasonality and sources, while the marginal diurnal variation in all aerosol properties reveals a weak influence from local sources, except for some few aerosol episodes. The aerosol-volume size distribution presents a mode value at similar to 10 mu m with secondary peaks at accumulation (similar to 2 mu m) and fine modes (similar to 0.03 mu m) and low variability between the seasons. A classification of the aerosol types based on the fine-mode fraction (FMF) vs. SSA(500) relationship reveals the dominance of aerosols in the FMF range of 0.4-0.6, characterized as mixed (39%), followed by fine aerosols with high scattering efficiency (26%), while particles related to dust contribute similar to 21%, with low fractions of fine-absorbing aerosols (similar to 13%). The aerosol radiative forcing (ARF) estimates reveal a small cooling effect at the top of the atmosphere (-1.3 Wm(-2)), while at the surface, the ARF ranges from -2 Wm(-2) to -6 Wm(-2) on monthly basis. The monthly-mean atmospheric radiative forcing (similar to 1 to 4 Wm(-2)) leads to heating rates of 0.04 to 0.13 K day(-1). These ARF values are higher than the global averages and may cause climate implications over the trans-Himalayan region. (C) 2020 Elsevier B.V. All rights reserved.