ALBERT

Description: Atmospheric processes (air–surface exchange, and atmospheric deposition and degradation) are crucial for understanding the global cycling and fate of organic pollutants (OPs). However, such assessments over the Tibetan Plateau (TP) remain uncertain. More than 50 % of Chinese lakes are located on the TP, which exerts a remarkable influence on the regional water, energy, and chemical cycling. In this study, air and water samples were simultaneously collected in Nam Co, a large lake on the TP, to test whether the lake is a secondary source or sink of OPs. Lower concentrations of organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs) were observed in the atmosphere and lake water of Nam Co, while the levels of polycyclic aromatic hydrocarbons (PAHs) were relatively higher. Results of fugacity ratios and chiral signatures both suggest that the lake acted as the net sink of atmospheric hexachlorocyclohexanes (HCHs), following their long-range transport driven by the Indian monsoon. Different behaviours were observed in the PAHs, which primarily originated from local biomass burning. Acenaphthylene, acenaphthene, and fluorene showed volatilization from the lake to the atmosphere, while other PAHs were deposited into the lake due to the integrated deposition process (wet/dry and air–water gas deposition) and limited atmospheric degradation. As the dominant PAH compound, phenanthrene exhibited a seasonal reversal of air–water gas exchange, which was likely related to the melting of the lake ice in May. The annual input of HCHs from the air to the entire lake area (2015 km2) was estimated as 1.9 kg yr−1, while input estimated for  ∑ 15PAHs can potentially reach up to 550 kg yr−1. This study highlights the significance of PAH deposition on the regional carbon cycling in the oligotrophic lakes of the TP.

Description: The study of persistent organic pollutants (POPs) in low-latitude tropical and subtropical urban cities is necessary to assess their local and global impacts on ecosystems and human health. Despite studies on levels of POPs in water, soils, and sediments, analysis of the distribution patterns, seasonality, and sources of POPs in urban regions of Nepal remain limited. Polyurethane foam (PUF)-based passive air samplers were deployed in three major cities in Nepal: Kathmandu (the capital city), Pokhara, and Hetauda (agricultural cities). Dichlorodiphenyltrichloroethane (DDT) and hexachlorocyclohexane (HCH) were the dominant organochlorine pesticides in the atmosphere at all sites. The average concentrations of POPs were  ∑ DDTs, 8.7–1.0  ×  103 pg m−3;  ∑ HCHs, 5.3–3.3  ×  103 pg m−3; HCB, 5.8–3.4  ×  102 pg m−3;  ∑ endosulfan, BDL–51 pg m−3; and  ∑ 6PCBs, 1.4–47 pg m−3. Isomer and metabolite ratio analyses suggested that the concentrations present were from both new and historical applications of the POPs. Vegetable production sites and their market places appeared to be the major DDT and HCH source areas. Higher atmospheric concentrations of DDT and HCH occurred during the pre-monsoon and monsoon seasons, and winter, respectively, closely associated with their local application for soil preparation and vegetable spraying. The estimated travel distances of the POPs (HCB, α-HCH, γ-HCH, and p, p′-DDT) under the Nepalese tropical climate were all above 1000 km, suggesting that high precipitation levels in the tropical climate were not enough to scavenge the POPs and that Nepal could be an important source region for POPs. Due to their close proximity and cold trapping (driven by low temperatures), the high Himalayas and the Tibetan Plateau are likely the key receptors of POPs emitted in Nepal. These results add to the information available on POPs from tropical developing countries.

Description: The Tibetan Plateau (TP) has been contaminated by persistent organic pollutants (POPs), including legacy organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs) through atmospheric transport. The exact source regions, transport pathways and time trends of POPs to the TP are not well understood. Here XAD-based passive air samplers (PAS) were deployed at 16 Tibetan background sites from 2007 to 2012 to gain further insight into spatial patterns and temporal trends of OCPs and PCBs. The southeastern TP was characterized by dichlorodiphenyltrichloroethane (DDT) -related chemicals delivered by Indian Monsoon air masses. The northern and northwestern TP displayed the greatest absolute concentration and relative abundance of hexachlorobenzene (HCB) in the atmosphere, caused by the westerly-driven European air masses. The interactions between the DDT polluted Indian monsoon air and the clean westerly winds formed a transition zone in central Tibet where both DDT and HCB were the dominant chemicals. Based on 5-year of continuous sampling, our data indicated declining concentrations of HCB and hexachlorocyclohexanes (HCHs) across the Tibetan region. Inter-annual trends of DDT class chemicals, however, showed less variation during this 5-year sampling period, which may be due to the on-going usage of DDT in India. This paper demonstrates the possibility of using POPs fingerprints to investigate the climate interactions and the validity of using PAS to derive inter-annual atmospheric POPs time trends.

Description: The study of persistent organic pollutants (POPs) in low latitude tropical and subtropical urban cities is necessary to assess their local and global impacts on ecosystems and human health. Despite studies on levels of POPs in water, soils, and sediments, analysis of the distribution patterns, seasonality and sources of POPs in urban regions of Nepal remain limited. Polyurethane foam (PUF)-based passive air samplers were deployed in three major cities in Nepal: Kathmandu (the capital city), Pokhara and Hetauda (agricultural cities). Dichlorodiphenyltrichloroethane (DDT) and hexachlorocyclohexane (HCH) were the dominant organochlorine pesticides in the atmosphere at all sites. The average concentrations of POPs were: ∑DDTs, 11.0–885.2 pg/m3; ∑HCHs, 4.0–2623.8 pg/m3; HCB, 2.7–186.2 pg/m3; ∑endosulfan, 1.4–34.8 pg/m3; and ∑6PCBs, 1.4–29.2 pg/m3. Isomer and metabolite ratio analyses suggested that the concentrations present were from both new and historical applications of the POPs. Vegetable production sites and their market places appeared to be the major DDT and HCH source areas. Higher atmospheric concentrations of DDT and HCH occurred during the pre-monsoon and monsoon seasons, and winter, respectively, closely associated with their local application for soil preparation and vegetable spraying. The estimated travel distances of the POPs (HCB, α-HCH, γ-HCH, and p,p'-DDT) under the Nepalese tropical climate were all above 1000 km, suggesting that high precipitation levels in the tropical climate were not enough to scavenge the POPs and that Nepal could be an important source region for POPs. Due to their close proximity and cold trapping (driven by low temperatures), the high Himalayas and the Tibetan Plateau are likely the key receptors of POPs emitted in Nepal. These results add to the information available on POPs from tropical developing countries.

Description: Atmospheric processes (air-surface exchange, and atmospheric deposition and degradation) are crucial for understanding the global cycling and fate of persistent organic pollutants (POPs). However, such assessment over the Tibetan Plateau (TP) remains uncertain. More than 50 % of the Chinese lakes are located on the TP, which exerts a remarkable influence on the regional water, energy, and chemical cycling. In this study, air and water samples were simultaneously collected in Nam Co, a large lake on the TP, to test whether the lake is a "secondary source" or "sink" of POPs. Lower concentrations of organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs) were observed in the atmosphere and lake water of Nam Co, while the levels of polycyclic aromatic hydrocarbons (PAHs) were relatively higher. Results of fugacity ratios and chiral signatures both suggest that the lake acted as the net sink of atmospheric hexachlorocyclohexanes (HCHs), following their long-range transport driven by the Indian Monsoon. Different behaviors were observed in the PAHs, which primarily originated from local biomass burning. Acenaphthylene, acenaphthene, and fluorene showed volatilization from the lake to the atmosphere; while other PAHs were deposited into the lake due to the integrated deposition process (wet/dry and air-water gas deposition) and limited atmospheric degradation. As the dominant PAH compound, phenanthrene exhibited a seasonal reversal of air-water gas exchange, which was likely related to the melting of the lake ice in May. The annual input of HCHs from air to the entire lake area (2015 km2) was estimated as 1.9 kg year−1, while those estimated for PAHs can potentially reach up to 550 kg year−1. This study highlights the significance of PAH deposition on the regional carbon cycling in the oligotrophic lakes of the TP.

Description: The mid-latitude westerlies and South Asian Summer Monsoon (SASM) are two major atmospheric circulation systems influencing the Tibetan Plateau (TP). We report a seven-year (2007/2008–2013/2014) dataset of δ18O in precipitation (δ18Op) collected at three stations. Taxkorgan (TX) and Bulunkou (BLK) are located on the northwestern TP where westerly winds dominate while Lulang (LL) is situated on the southeastern TP where the SASM dominates. δ18O in precipitation (δ18Op) in northwestern TP varies with surface temperature (T) throughout the study period, and is depleted in 18O in precipitation during June to September when the monsoonal circulation enters the TP. Integration with model outputs suggests that large-scale atmospheric circulation plays a major role in isotopic seasonality in both regions. A teleconnection between precipitation on the northwestern TP and the El Niño-Southern Oscillation (ENSO) warm phase is suggested by changes in the relationship between δ18O and δD (e.g., reduced slope and weighted d-excess) in precipitation samples. These observations are indicative of a weakening of the mid-latitude westerly jet allowing local processes in the continental interior to become more dominant, thereby increasing the contribution of secondary evaporation from falling raindrops and kinetic fractionation. Under the conditions of a high Northern Annular Mode (NAM) the westerly jet is intensified over the southeastern TP which enhances local evaporation and continental recycling as revealed by a lower δD-δ18O slope and intercept, but higher d-excess average in contemporaneously collected precipitation samples. The significant correlation between T and δ18Op in the northwestern TP during various composite periods highlights a variation from 0.39 ‰ / ℃ (ENSO warm) to 0.77 ‰ / ℃ (high NAM), attributable to decreased (increased) water vapor availability over the northwestern TP during the ENSO warm (strong positive NAM) phase. ENSO cold and strong negative NAM phases show analogous effects on atmospheric circulation over both regions.

Description: In northwestern Tibet (34.0°N, 82.2°E) near lake Aru Co, the entire ablation area of two glaciers (Aru-1 and Aru-2) suddenly collapsed on 17 July 2016 and 21 September 2016, respectively, and transformed into 68 and 83 106m3 mass flows that ran out up to 7km, killing nine people. The only similar event currently documented is the 2002 Kolka Glacier mass flow (Caucasus Mountains). Using climatic reanalysis, remote sensing and 3D thermo-mechanical modeling, we reconstructed in detail the glaciers' thermal regimes, thicknesses, velocities, basal shear stresses and ice damage prior to the collapse. We show that frictional change leading to the collapses occurred in the temperate areas of polythermal glacier structures and are not linked to thaw of cold based ice. The two glaciers experienced a similar stress transfer from predominant basal drag towards predominant lateral shearing in the later detachment areas and during the 5–6 years before the collapses, though with a high friction patch on Aru-2 tongue which is inexistent on Aru-1. The latter led to distinctly disparate behaviour making the development of the instability more visible for the Aru-1 glacier compared to Aru-2 through enhanced crevassing over a longer period and terminus advance. Field investigations reveal that those two glaciers are flowing on a soft, highly erodible, and fine-grained sedimentary lithology. We propose that specific bedrock lithology played a key role in the two Tibet, and also in the Caucasus gigantic glacier collapses documented to date by producing low bed roughness and large amount of till rich in clay/silt with low friction angle. The twin Aru collapses would have been driven by a failing substrate linked to increasing water pore pressure in the subglacial drainage system in response to recent increases of surface melting and rain.

Description: In north-western Tibet (34.0∘ N, 82.2∘ E) near lake Aru Co, the entire ablation areas of two glaciers (Aru-1 and Aru-2) suddenly collapsed on 17 July and 21 September 2016. The masses transformed into ice avalanches with volumes of 68 and 83×106 m3 and ran out up to 7 km in horizontal distance, killing nine people. The only similar event currently documented is the 130×106 m3 Kolka Glacier rock and ice avalanche of 2002 (Caucasus Mountains). Using climatic reanalysis, remote sensing, and three-dimensional thermo-mechanical modelling, we reconstructed the Aru glaciers' thermal regimes, thicknesses, velocities, basal shear stresses, and ice damage prior to the collapse in detail. Thereby, we highlight the potential of using emergence velocities to constrain basal friction in mountain glacier models. We show that the frictional change leading to the Aru collapses occurred in the temperate areas of the polythermal glaciers and is not related to a rapid thawing of cold-based ice. The two glaciers experienced a similar stress transfer from predominant basal drag towards predominant lateral shearing in the detachment areas and during the 5–6 years before the collapses. A high-friction patch is found under the Aru-2 glacier tongue, but not under the Aru-1 glacier. This difference led to disparate behaviour of both glaciers, making the development of the instability more visible for the Aru-1 glacier through enhanced crevassing and terminus advance over a longer period. In comparison, these signs were observable only over a few days to weeks (crevasses) or were absent (advance) for the Aru-2 glacier. Field investigations reveal that those two glaciers were underlain by soft, highly erodible, and fine-grained sedimentary lithologies. We propose that the specific bedrock lithology played a key role in the two Tibet and the Caucasus Mountains giant glacier collapses documented to date by producing low bed roughness and large amounts of till, rich in clay and silt with a low friction angle. The twin 2016 Aru collapses would thus have been driven by a failing basal substrate linked to increasing pore water pressure in the subglacial drainage system in response to increases in surface melting and rain during the 5–6 years preceding the collapse dates.

Description: The Tibetan Plateau (TP) has been contaminated by persistent organic pollutants (POPs), including legacy organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs) through atmospheric transport. The exact source regions, transport pathways and time trends of POPs to the TP are not well understood. Here polystyrene–divinylbenzene copolymer resin (XAD)-based passive air samplers (PASs) were deployed at 16 Tibetan background sites from 2007 to 2012 to gain further insight into spatial patterns and temporal trends of OCPs and PCBs. The southeastern TP was characterized by dichlorodiphenyltrichloroethane (DDT)-related chemicals delivered by Indian monsoon air masses. The northern and northwestern TP displayed the greatest absolute concentration and relative abundance of hexachlorobenzene (HCB) in the atmosphere, caused by the westerly-driven European air masses. The interactions between the DDT polluted Indian monsoon air and the clean westerly winds formed a transition zone in central Tibet, where both DDT and HCB were the dominant chemicals. Based on 5 years of continuous sampling, our data indicated declining concentrations of HCB and hexachlorocyclohexanes (HCHs) across the Tibetan region. Inter-annual trends of DDT class chemicals, however, showed less variation during this 5-year sampling period, which may be due to the ongoing usage of DDT in India. This paper demonstrates the possibility of using POP fingerprints to investigate the climate interactions and the validity of using PAS to derive inter-annual atmospheric POP time trends.