Chaoliu Li

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.

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.

Light‐absorbing impurities enhance glacier albedo reduction in the southeastern Tibetan plateau

Light-absorbing impurities (LAIs) in snow of the southeastern Tibetan Plateau (TP) and their climatic impacts are of interest not only because this region borders areas affected by the South Asian atmospheric brown clouds but also because the seasonal snow and glacier melt from this region form important headwaters of large rivers. In this study, we collected surface snow and snowpit samples from four glaciers in the southeastern TP in June 2015 to investigate the comprehensive observational data set of LAIs. Results showed that the LAI concentrations were much higher in the aged snow and granular ice than in the fresh snow and snowpits due to postdepositional processes. Impurity concentrations fluctuated across snowpits, with maximum LAI concentrations frequently occurring toward the bottom of snowpits. Based on the SNow ICe Aerosol Radiative model, the albedo simulation indicated that black carbon and dust account for approximately 20% of the albedo reduction relative to clean snow. The radiative forcing caused by black carbon and dust deposition on the glaciers were between 1.0-141 W m(-2) and 1.5-120 W m(-2), respectively. Black carbon (BC) played a larger role in albedo reduction and radiative forcing than dust in the study area, enhancing approximately 15% of glacier melt. Analysis based on the Fire INventory from NCAR indicated that nonbiomass-burning sources of BC played an important role in the total BC deposition, especially during the monsoon season. This study suggests that eliminating anthropogenic BC could mitigate glacier melt in the future of the southeastern TP. Plain Language Summary In this study, we focused on light-absorbing impurities (LAIs), including black carbon, organic carbon, and mineral dust in glacial surface snow from southeaster Tibetan glaciers. This study showed the concentrations of LAIs, and estimated their impact on albedo reduction. Furthermore, we discussed the potential source of impurities and their impact to the study area. These results provide scientific basis for regional mitigation efforts to reduce black carbon.

Heavy metals in sediments of the Yarlung Tsangbo and its connection with the arsenic problem in the Ganges–Brahmaputra Basin

The Yarlung Tsangbo (YT) is a large river running across southern Tibet and has significant effects on its lower reaches, the Ganges–Brahmaputra Basin. In order to investigate the geochemical features of the YT, 18 surface sediment samples were collected; ten trace element concentrations were measured for bulk sediments and two fine grain size fractions. Meanwhile, basic physicochemical parameters and element concentrations of river water were also analyzed. Results indicated that the river water was alkaline (pH 8.42) and that dissolved oxygen was mainly controlled by river water temperature. Some elements (e.g., Zn and Ni) showed close negative relationship to mean grain size of the sediments. Concentrations of most heavy metals, except As of the YT bulk sediments, were similar to those of Upper Crustal Concentration and its lower reaches, indicating almost no anthropogenic impact. Arsenic of the YT sediments was derived fundamentally from the parent rocks of the YT Basin and was far higher than that of its lower reaches. This indicates that relatively small amounts of As from the study area were transported down to the Brahmaputra River under present, relatively dry climatic conditions. However, more YT sediments might have been transported to its low reaches during the Holocene due to the wet climate, giving high As concentration in Holocene sediments of the Ganges–Brahmaputra Basin. Thus, As transported by the YT may produce important influence on the Ganges–Brahmaputra Basin and contribute to its high As concentration in groundwater.

Observation of an agricultural biomass burning in central and east China using merged aerosol optical depth data from multiple satellite missions

Agricultural biomass burning (ABB) in central and east China occurs every year from May to October and peaks in June. During the period from 26 May to 16 June 2007, one strong ABB procedure happened mainly in Anhui, Henan, Jiangsu and Shandong provinces. This article focuses on analysis of this ABB procedure using a comprehensive set of aerosol optical depth (AOD) data merged by using the optimal interpolation method from the Moderate Resolution Imaging Spectroradiometer, the Multi-angle Imaging Spectroradiometer (MIRS) as well as Sea-viewing Wide Field-of-view Sensor (SeaWiFS)-derived AOD products. In addition, the following additional data are used: fire data from the National Satellite Meteorological Centre of China Meteorological Administration, the mass trajectory analyses from hybrid single-particle Lagrangian integrated trajectory (HYSPLIT) model and ground-based AOD and Ångström data derived from the Aerosol Robotic Network and China Aerosol Remote Sensing Network. The results show that merged satellite AOD values can expand the spatial coverage of agricultural biomass aerosol distributions with good accuracy (R = 0.93, root mean square error = 0.37). Based on the merged AOD images, the highest AOD values were found concentrated in central China as well as in eastern China before 6 June and further extended to northeast China after 12 June. AODs from ground measurement show that eastern China always keeps high AOD values (>1.0), with a maximum exceeding 3.0 and extending as high as nearly 5.0 during this ABB event. With the help of the HYSPLIT model, we analysed the ABB sources and examined how transport paths affect the concentrations of air pollutants in some sites. The results show that Henan, Jiangsu and Anhui provinces are the three main sources in this ABB.

Assessment of elemental distribution and trace element contamination in surficial wetland sediments, Southern Tibetan Plateau

An investigation was performed to measure concentrations of major and trace elements in surficial wetland sediments in the southern Tibetan Plateau in order to assess the sediment quality. Results showed that most of elements have concentrations comparable to those of the Tibetan surface soil (TSS) except for As, Ca, Cs, Sr, and Cu. Correlation analysis indicated that most elements were highly associated with major crustal elements, suggestive of their common natural origin. Sediment quality assessment revealed that the wetlands are unpolluted with most of trace elements except for Cs and As, which are likely associated with organic matters and biological activities. Despite that the wetland sediments are minimally influenced by either local or long-transported anthropogenic pollutants, and no notable trace element pollutants were detected, As was found in elevated concentrations which far exceed the level above which harmful effects on wildlife and humans are likely to be observed.

River water quality across the Himalayan regions: elemental concentrations in headwaters of Yarlung Tsangbo, Indus and Ganges River

Abstract To study the water quality across the Himalayan region, a total of 43 river water samples were collected in September 2012 from the Yarlung Tsangbo (Brahmaputra), Indus and Ganges basins. We measured the common water quality parameters (pH, EC, TDS, and water temperature) and analyzed the element concentrations (As, Cr, S, Sr, Tl, B, Ba, Ca, Al, Fe, K, Mg, Mn, Na, Rb, and Ti) using ICP-AES. The results show that all the rivers have an alkaline environment with pH from 7.9 to 8.9. TDS has a positive relationship with Ca and Mg concentrations. The elemental contents are comparable among the samples from different river basins. The enrichment factor of As in river water across Himalayas showed a high value at 30, indicating the impacts of anthropogenic activities. The river water quality in this region is under the influence of climate change on the glacier melting, which will affect the water chemistry and the seasonal discharge of river runoff.

Deposition and light absorption characteristics of precipitation dissolved organic carbon (DOC) at three remote stations in the Himalayas and Tibetan Plateau, China

The concentrations, depositions and optical properties of precipitation DOC at three remote stations (Nam Co, Lulang and Everest) were investigated in the Himalayas and Tibetan Plateau (HTP). The results showed that their volume-weighted mean DOC concentrations were 1.05±1.01mgCL-1, 0.83±0.85mgCL-1 and 0.86±0.91mgCL-1, respectively, close to those of other remote areas in the world and lower than those of typical polluted urban cities. Combined with precipitation amounts, the DOC depositions at these three stations were calculated to be 0.34±0.32gCm-2yr-1, 0.84±0.86gCm-2yr-1 and 0.16±0.17gCm-2yr-1, respectively. The annual DOC deposition in the HTP was approximately 0.94±0.87TgC, the highest and lowest values appeared in the southeastern and northwestern plateau, respectively. The sources of DOC in the precipitation at these three stations were remarkably different, indicating large spatial heterogeneity in the sources of precipitation DOC over the HTP. Nam Co presented combustion sources from South Asia and local residents, Lulang showed biomass combustion source from South Asia, and Everest was mainly influenced by local mineral dust. The values of the MACDOC at 365nm were 0.48±0.47m2g-1, 0.25±0.15m2g-1, and 0.64±0.49m2g-1, respectively, for the precipitation at the three stations. All of these values were significantly lower than those of corresponding near-surface aerosol samples because precipitation DOC contains more secondary organic aerosol with low light absorption abilities. Additionally, this phenomenon was also observed in seriously polluted urban areas, implying it is universal in the atmosphere. Because precipitation DOC contains information for both particle-bound and gaseous components from the near surface up to the altitude of clouds where precipitation occurs, the MACDOC of precipitation is more representative than that of near-surface aerosols for a given region.

Aged dissolved organic carbon exported from rivers of the Tibetan Plateau

The role played by river networks in regional and global carbon cycle is receiving increasing attention. Despite the potential of radiocarbon measurements (14C) to elucidate sources and cycling of different riverine carbon pools, there remain large regions such as the climate-sensitive Tibetan Plateau for which no data are available. Here we provide new 14C data on dissolved organic carbon (DOC) from three large Asian rivers (the Yellow, Yangtze and Yarlung Tsangpo Rivers) running on the Tibetan Plateau and present the carbon transportation pattern in rivers of the plateau versus other river system in the world. Despite higher discharge rates during the high flow season, the DOC yield of Tibetan Plateau rivers (0.41 gC m-2 yr-1) was lower than most other rivers due to lower concentrations. Radiocarbon ages of the DOC were older/more depleted (511±294 years before present, yr BP) in the Tibetan rivers than those in Arctic and tropical rivers. A positive correlation between radiocarbon age and permafrost watershed coverage was observed, indicating that 14C-deplted/old carbon is exported from permafrost regions of the Tibetan Plateau during periods of high flow. This is in sharp contrast to permafrost regions of the Arctic which export 14C-enriched carbon during high discharge periods.