Qianggong Zhang

Sources of black carbon to the Himalayan–Tibetan Plateau glaciers

Combustion-derived black carbon (BC) aerosols accelerate glacier melting in the Himalayas and in Tibet (the Third Pole (TP)), thereby limiting the sustainable freshwater supplies for billions of people. However, the sources of BC reaching the TP remain uncertain, hindering both process understanding and efficient mitigation. Here we present the source-diagnostic Δ14C/δ13C compositions of BC isolated from aerosol and snowpit samples in the TP. For the Himalayas, we found equal contributions from fossil fuel (46±11%) and biomass (54±11%) combustion, consistent with BC source fingerprints from the Indo-Gangetic Plain, whereas BC in the remote northern TP predominantly derives from fossil fuel combustion (66±16%), consistent with Chinese sources. The fossil fuel contributions to BC in the snowpits of the inner TP are lower (30±10%), implying contributions from internal Tibetan sources (for example, yak dung combustion). Constraints on BC sources facilitate improved modelling of climatic patterns, hydrological effects and provide guidance for effective mitigation actions.

Mercury distribution and deposition in glacier snow over western China.

Western China is home to the largest aggregate of glaciers outside the polar regions, yet little is known about how the glaciers in this area affect the transport and cycling of mercury (Hg) regionally and globally. From 2005 to 2010, extensive glacier snow sampling campaigns were carried out in 14 snowpits from 9 glaciers over western China, and the vertical distribution profiles of Hg were obtained. The Total Hg (THg) concentrations in the glacier snow ranged from <1 to 43.6 ng L(-1), and exhibited clear seasonal variations with lower values in summer than in winter. Spatially, higher THg concentrations were typically observed in glacier snows from the northern region where atmospheric particulate loading is comparably high. Glacier snowpit Hg was largely dependent on particulate matters and was associated with particulate Hg, which is less prone to postdepositional changes, thus providing a valuable record of atmospheric Hg deposition. Estimated atmospheric Hg depositional fluxes ranged from 0.74 to 7.89 μg m(-2) yr(-1), agreeing very well with the global natural values, but are one to two orders of magnitude lower than that of the neighboring East Asia. Elevated Hg concentrations were observed in refrozen ice layers in several snowpits subjected to intense melt, indicating that Hg can be potentially released to meltwater.

Wet deposition of mercury at Lhasa, the capital city of Tibet

Quantifying the contribution of mercury (Hg) in wet deposition is important for understanding Hg biogeochemical cycling and anthropogenic sources, and verifying atmospheric models. Mercury in wet deposition was measured over the year 2010, in Lhasa the capital and largest city in Tibet. Precipitation samples were analyzed for total Hg (HgT), particulate-bound Hg (HgP), and reactive Hg (HgR). The volume-weighted mean (VWM) concentrations and wet deposition fluxes of HgT, HgP and HgR in precipitation were 24.8 ng L-1 and 8.2 μg m-2 yr-1, 19.9 ng L-1 and 7.1 μg m-2 yr-1, and 0.5 ng L-1 and 0.19 μg m-2 yr-1, respectively. Concentrations of HgT and HgP were statistically higher during the non-monsoon season than during the monsoon season, while HgR concentrations were statistically higher during the monsoon season than during the non-monsoon season. Most HgT, HgP and HgR wet deposition occurred during the monsoon season. Concentrations of HgP and HgR were 77% and 5% of the HgT on average (VWM), respectively. Concentrations of HgT and HgP were weakly negatively correlated with precipitation amount (r2 = 0.09 and 0.10; p < 0.05), indicating that below-cloud scavenging of Hg from the local atmosphere was an important mechanism contributing Hg to precipitation. High HgP%, as well as a significant positive correlation between HgT and HgP, confirms that Hg wet deposition at Lhasa was primarily occurring in the form of atmospheric HgP below-cloud scavenging. Moreover, the HgT concentration, rather than the precipitation amount, was found to be the governing factor affecting HgT wet deposition flux. A comparison among modeled wet and dry deposition fluxes, and measurements suggested that estimates of both wet and dry Hg deposition fluxes by the GEOS-Chem model were 2 to 3 times higher than the measured annual wet flux.

Atmospheric Mercury Depositional Chronology Reconstructed from Lake Sediments and Ice Core in the Himalayas and Tibetan Plateau

Alpine lake sediments and glacier ice cores retrieved from high mountain regions can provide long-term records of atmospheric deposition of anthropogenic contaminants such as mercury (Hg). In this study, eight lake sediment cores and one glacier ice core were collected from high elevations across the Himalaya-Tibet region to investigate the chronology of atmospheric Hg deposition. Consistent with modeling results, the sediment core records showed higher Hg accumulation rates in the southern slopes of the Himalayas than those in the northern slopes in the recent decades (post-World War II). Despite much lower Hg accumulation rates obtained from the glacier ice core, the temporal trend in the Hg accumulation rates matched very well with that observed from the sediment cores. The combination of the lake sediments and glacier ice core allowed us to reconstruct the longest, high-resolution atmospheric Hg deposition chronology in High Asia. The chronology showed that the Hg deposition rate was low between the 1500s and early 1800, rising at the onset of the Industrial Revolution, followed by a dramatic increase after World War II. The increasing trend continues to the present-day in most of the records, reflecting the continuous increase in anthropogenic Hg emissions from South Asia.

Mercury in Wild Fish from High-Altitude Aquatic Ecosystems in the Tibetan Plateau

Our understanding of the biogeochemistry of mercury (Hg) in high-altitude aquatic environments remains limited. The Tibetan Plateau (TP) is one of the Earths most significant continental-scale high lands, yet much remains unknown about the Hg bioaccumulation and biomagnification in these pristine ecosystems. In this study, 166 wild fish samples of 13 species were collected from 13 rivers and lakes across the southern TP. Total Hg (THg) and methyl-Hg (MeHg) concentrations in the axial muscle of fish ranged from 25.1 to 1218 ng g(-1) of wet weight (median ± average deviation of 100.5 ± 149.2 ng g(-1)) and from 24.9 to 1196 ng g(-1) of wet weight (median ± average deviation of 90.7 ± 137.0 ng g(-1)), respectively. Hg concentrations varied greatly within and between species. The fish Hg concentrations were then linked to the limited available environmental Hg data and special geochemical characteristics in the region, such as Hg loading, pH, low temperature, and high ultraviolet (UV). The long lifespan and slow growth of the fish under the low-productivity environments may be the major biological factors that help to build up the fish Hg levels comparable to those observed in wild fish growing in human-impacted areas. δ(13)C signals suggested that pelagic fish had higher Hg concentrations, but no relationship was found between the Hg concentrations and the trophic levels. Zooplankton and benthic amphipods had typically higher percentages of MeHg compared to the previously reported values, suggesting the efficient transfer of MeHg from the base of the aquatic food web. This study sheds some light on the geochemical and biological controls of Hg bioaccumulation in fish and biomagnification in the aquatic food web in arid high-altitude environments.

Atmospheric deposition of trace elements recorded in snow from the Mt. Nyainqentanglha region, southern Tibetan Plateau

In May 2009, snowpit samples were collected from a high-elevation glacier in the Mt. Nyainqêntanglha region on the southern Tibetan Plateau. A set of elements (Al, V, Cr, Mn, Co, Ni, Cu, Zn, Cd, Hg and Pb) was analyzed to investigate the concentrations, deposition fluxes of trace elements, and the relative contributions from anthropogenic and natural sources deposited on the southern Tibetan Plateau. Concentrations of most of the trace elements in snowpit samples from the Zhadang glacier are significantly lower than those examined from central Asia (e.g., eastern Tien Shan), with higher concentrations during the non-monsoon season than during the monsoon season. The elements of Al, V, Cr, Mn, Co, and Ni display low crustal enrichment factors (EFs), while Cu, Zn, Cd, Hg, and Pb show high EF values in the snow samples, suggesting anthropogenic inputs are potentially important for these elements in the remote, high-elevation atmosphere on the southern Tibetan Plateau. Together with the fact that the concentration levels of such elements in the Mt. Nyainqêntanglha region are significantly higher than those observed on the south edge of the Tibetan Plateau, our results suggest that the high-elevation atmosphere on the southern Tibetan Plateau may be more sensitive to variations in the anthropogenic emissions of atmospheric trace elements than that in the central Himalayas. Moreover, the major difference between deposition fluxes estimated in our snow samples and those recently measured at Nam Co Station for elements such as Cr and Cu may suggest that atmospheric deposition of some of trace elements reconstructed from snowpits and ice cores could be grossly underestimated on the Tibetan Plateau.

Seasonal variations, speciation and possible sources of mercury in the snowpack of Zhadang glacier, Mt. Nyainqentanglha, southern Tibetan Plateau

Ten snowpits were sampled at the Zhadang glacier during 2008 and 2011 to investigate the seasonal variations, speciation, and sources of mercury (Hg) in the southern Tibetan Plateau. In the 2008 snowpit, total Hg (Hg(T)), particulate matter, most of major ions were found in higher concentrations during the non-monsoon season than in the monsoon season. Analysis of Hg speciation indicated that Hg(T) in the 2011 snowpits was dominated by particulate-bound Hg (Hg(P)). Most of particulate matter in the 2008 snowpit was dominated by fine particulates, indicating that the influx of particulate matter and Hg(P) was probably occurring by long-range transportation via general atmospheric circulation. Analysis of dominant ion Ca²⁺ and alkaline pH values has suggested that the long-range transported Hg(P), originating from dust storm activities, may be the most important source for Hg in the Zhadang glacier snowpit during the non-monsoon season. Backward-trajectory analysis indicates the majority of the air masses arriving at the Zhandang glacier originated from the arid regions of northwestern India (e.g., Thar Desert), confirming that arid regions in central and southern Asia are likely the main sources of Hg being deposited in the Zhadang glacier snowpit. This study also suggests that ice core records from the Tibetan Plateau may be useful tools for interpreting long-term historical records of atmospheric Hg deposition, and reconstructing Hg biogeochemical cycling.

Characterizations of wet mercury deposition on a remote high-elevation site in the southeastern Tibetan Plateau.

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.

Spatial distribution, sources and risk assessment of potentially toxic trace elements and rare earth elements in soils of the Langtang Himalaya, Nepal

Soils in the fragile Himalayan region could be affected by the transport and deposition of potentially toxic trace metals (PTEs) from urban and industrialized areas of South Asia. The transported pollutants could pose a serious threat to the soil quality in the pristine regions at high elevations having minimal direct human influence. Therefore, it is important to understand the geochemical and physical characteristics of soils in this region and determine the extent of their chemical pollution. In order to achieve these objectives, soil samples were collected from different elevation transects of the Langtang Himalaya in Nepal. The samples were analyzed for PTEs and rare earth elements for the purpose of identifying their possible sources and to evaluate their environmental risk in the region. The PTEs and REEs concentrations were measured by ICP-MS (X-7; Thermo-elemental, USA) and total organic carbon (TOC) by TOC analyzer. The results of this study were comparable to those of the world average background soil as well as the Tibetan plateau surface soil. TOC revealed a decreasing trend with increasing elevation. Correlation analysis and principle component analysis (PCA) indicated that most of the elements were highly associated with major crustal elements, suggesting that their primary sources were of natural origin. Furthermore, the geo-accumulation index (Igeo), enrichment factor (EF) and pollution index (PI) analyses indicated that the Himalayan soils represent minimal pollution and the data from this study may be used as background values for the Himalayan region in the future studies. REEs in the soil samples were found to be consistent with an order of average abundance of the Earth’s crust. In addition, the chondrite-normalized REE distribution of the light REE suggested enrichment of LREE and Eu depletion. Moreover, this study emphasized that soils of the Himalayan region could, in future, be under threat of elemental pollution from long-range transport via atmospheric circulation and deposition.

Polycyclic aromatic hydrocarbons in soils from the Central-Himalaya region: Distribution, sources, and risks to humans and wildlife

The Central Himalayas are not only a natural boundary between China and Nepal but also a natural barrier for transport of air masses from South Asia. In this study, 99 samples of surface soil were collected from five regions of Nepal on the southern side of the Central Himalayas, and 65 samples of surface soil were obtained from the northern side on the edge of the Tibetan Plateau, China (TPC). Concentrations of polycyclic aromatic hydrocarbons (PAHs) in soils were measured to determine their distribution, potential for accumulation, and sources, as well as risks to humans and the environment. Mean concentrations of Σ16PAHs were 2.4×10(2) and 3.3×10(2)ng/g dry mass (dm) in soils collected from the TPC and Nepal, respectively. Significant correlations between concentrations of lower molecular weight PAHs (LMW-PAHs) in soils and altitude were found. Total organic carbon (TOC) in soil was positively but weakly correlated with concentrations of PAHs in the study area, which suggested little role of TOC in adsorption of PAHs. The cities of Kathmandu and Pokhara in Nepal and Nyemo (especially Zhangmu Port), Shigatse, and Lhasa on the TPC, were areas with relatively great concentrations of PAHs in soils. The main sources of PAHs identified by positive matrix factorization were emissions from motor vehicles and combustion of coal and biomass in the Central Himalayas. Calculated total benzo[a]pyrene potency equivalents of 0.23-44ng/gdm and index of additive cancer risk of 3.8×10(-3)-9.2×10(-1) indicated that PAHs in almost all soils investigated posed de minimis risk of additional cancer to residents via direct contact and had no significant risk of additional cancers through consumption of potable water. Mean risk quotient values indicated that 39% of soils had a slight risk to wildlife and the ambient environment of the Central Himalayas.