Early atmospheric contamination on the top of the Himalayas since the onset of the European Industrial Revolution

Sediment record of heavy metals in Xincun Lagoon indicating anthropogenic impact over the last 200 years

Anthropogenic metal pollution is a leading environmental problem in southern China, especially in remote regions where its impact remains poorly understood. This study investigates the historical variation of heavy metal pollution over the last 200 years using a sediment core from Xincun Lagoon, Hainan Island, South China. The temporal evolution of heavy metal pollution aligns with China's socioeconomic development. Prior to the 1950s, heavy metal concentrations were at geochemical background levels, reflecting China's agrarian status. Since the 1950s, the increased heavy metal accumulation may be attributed to intensified human activities linked to rapid urbanization and industrialization. Despite the increase in heavy metal enrichments since the 1950s, Xincun Lagoon currently faces a low ecological risk.

Identifying the natural and agricultural impacts on the glaciochemistry of the Aru ice core on the northwestern Tibetan Plateau

Glaciochemical data sourced from ice cores in polar regions and the Alps have been extensively examined. However, quantitative studies on glaciochemical records of the Tibetan Plateau (TP) are scarce. To address this, we investigated annual variations in the major soluble ions (Ca2+, Mg2+, Na+, K+, NH4+, Cl-, NO3-, and SO42-) in the Aru ice core on the northwestern TP from 1850 to 2016. Applying a positive matrix factorization model, the sources of the major soluble ions and three factors to evaluate natural and agricultural impacts were identified. Factor 1, crustal dust with high loadings of Mg2+ (81.9 %) and Ca2+ (68.7 %), significantly positively correlated with wind speed and significantly negatively correlated with δ18O and net accumulation recorded by the ice core, suggesting that strong winds contributed to crustal dust transport from arid and semi-arid regions of Central Asia and deposition in the Aru glacier. However, relatively warm and wet climate prevented the transport of crustal dust. Factor 2 comprised salt lakes with high dominant loadings of Na+ (75.3 %), SO42- (64.1 %), Cl- (60.8 %), NO3- (52.2 %), and K+ (49.4 %). Declining lake water levels exposed salt lake minerals, which were carried to glaciers under the dynamic conditions of strong winds, whereas warming resulted in an expansion of glacial meltwater and lake water volume, which decreased the contribution of salt lake sediments. Therefore, the contribution of salt lake deposition decreased. Factor 3 was agricultural sources with a high loading of NH4+ (82 %), whose trend aligned closely with the population number and N productions from agricultural sources in South and Central Asia, suggesting that NH3 emissions from agricultural practices are a critical contributor to Factor 3. This study quantified the proportional contribution of natural and agricultural sources to glaciochemical composition, advancing our understanding of glaciochemical records in ice cores from source recognition to quantification.

Metal enrichment in ice-melt water and uptake by chironomids as possible legacy of World War One in the Italian Alps

Relics of World War One (WW1) were buried in alpine glaciers around 100 years ago. Today, these are emerging from the ice due to widespread glacier retreat, and are in direct contact with glacial meltwater-fed streams. To address a possible emergent contamination, we quantified major and trace elements (M-TEs) by mass spectrometry in water and larvae of Diamesa zernyi from three glacial streams fed by glaciers differently impacted by the Italian Austro-Hungarian war, in the Adamello-Presanella mountain range (Italian Alps): Lares and Presena, the two main battlefields, and Amola, 8 km from the front. M-TEs in stream water were interpreted using the crustal enrichment factor (EFc) while larval uptake was quantified by adopting the bioaccumulation factor (BAF). Despite low M-TEs concentrations in the water, in a range between 1 ng L-1 (Ag, Ta) and 1-2 mg L-1 (Al, Fe, Mg), low to moderate enrichments (10 ≥ EFc≥ 6) were observed for Sb and U in Presena and for Ag, As, Bi, Cd, Li, Mo, Pb, Sb and U in Lares. In addition, M-TE mass concentrations in larvae were up to ninety thousand times higher than in water, from 20 to 50 ng g-1 dry weight (d.w.; for Bi, Sb, Ta, Tl) to 1-4 mg g-1 d.w. (for Al, Fe, Na, and Mg). Larvae from Lares accumulated the largest amount of metals and metalloids, including those mostly used in the manufacture of artillery shells (As, Cu, Ni, Pb, Sb; BAFs from 375 to about 11,500). This was expected as most of the WW1 battles in this mountain range were fought on the Lares glacier, where the greatest number of war relics are emerging. These results provide preliminary evidence of water contamination and bioaccumulation of metals and metalloids by glacial fauna as a possible legacy of WW1 in the Alps.

Elemental composition of the topsoil fine fraction at and around the Tibetan plateau

The <20 μm fractions of crusted topsoils on and around the Tibetan Plateau (TP) were analyzed to take a broad view of the composition of major elements (MEs, Al, Fe, and Mn) and twelve trace elements (TEs, As, Ba, Cd, Co, Cr, Cu, Ni, Pb, Sr, U, V, and Zn) and provide a crustal reference for environmental quality evaluation. The concentrations of most elements were generally higher in the Yarlung Zangbo River watershed (YZRW) and Pamirs but lower in the central Tibetan Plateau (CTP), Qaidam Basin (QB), and Tarim Basin (TB) due to the natural geochemical process. The concentrations of most elements in the five regions were higher than those of the upper continental crust (UCC), which was mainly affected by two natural factors. One was that the wide distribution of shale, schist, and phyllite on the TP led to the high concentrations of As and Cr. The other was that the concentrations of most elements in the <20 μm fractions of crusted topsoils were affected by particle sorting. Cu, Cd, As, and Pb in a few sites of the YZRW were influenced by local traffic emissions, mining operations, and increasing fossil fuel combustion over the past three decades (i.e., the 1980s-2010s). Furthermore, the values of crust‒referenced enrichment factor (EF_ucc) of most TEs (except Ba and Sr) in different environmental media with a high proportion of fine particles, such as aerosols, snow and ice cores, and river and lake sediments were generally 1.2-24.2 times greater than the values of fine fraction (<20 μm)‒referenced enrichment factor (EF _{< 20 μm}), suggesting that the degree of anthropogenic effects on the TP environment should be overestimated in previous assessments using the UCC as the reference.

Composition and sources of heavy metals in aerosol at a remote site of Southeast Tibetan Plateau, China

Knowledge of the elemental composition of aerosols at remote sites is important for evaluating the influence of anthropogenic activities. In this study, the elemental composition and sources of total suspended particles (TSP) at Yaze, a remote site in the southeastern Tibetan Plateau (TP), were investigated. The results showed that the mean elemental concentrations at Yaze were relatively low compared with those in other areas of the TP. Seasonal variations in the studied elements was characterized by low and high concentrations during the monsoon and non-monsoon periods, respectively. The enrichment factors (EFs) for some heavy metals at Yaze were slightly higher than those at Nam Co station (inland TP) but much lower than those at Mt. Yulong (southeastern TP) and in the Indian megacity of Delhi, indicating fewer anthropogenic influences at the study site relative to sites close to severely polluted regions. For the studied elements, three major sources were identified: crustal origins (e.g., Al and Fe), anthropogenic origins (e.g., Zn and Cd) and mixed origins (e.g., As and Bi). Further analysis by potential source contribution functions showed that the local TP was the primary source for elements of crustal origins. Correspondingly, the typical heavy metals were mainly attributed to pollution emitted from anthropogenic activities and transported over long-range from both South and Southeast Asia. This work demonstrates the transport of heavy metals from external sources to remote sites in the southeastern TP. These results are also useful for interpreting the historical profiles of heavy metals in the ice cores of the TP.

A Tibetan ice core covering the past 1,300 years radiometrically dated with 39Ar

Ice cores from alpine glaciers are unique archives of past global and regional climate conditions. However, recovering climate records from these ice cores is often hindered by the lack of a reliable chronology, especially in the age range of 100 to 500 anni (a) for which radiometric dating has not been available so far. We report on radiometric ³⁹Ar dating of an ice core from the Tibetan Plateau and the construction of a chronology covering the past 1,300 a using the obtained ³⁹Ar ages. This is made possible by advances in the analysis of ³⁹Ar using the laser-based detection method atom trap trace analysis, resulting in a twofold increase in the upper age limit of ³⁹Ar dating. By measuring the anthropogenic ⁸⁵Kr along with ³⁹Ar we quantify and correct modern air contamination, thus removing a major systematic uncertainty of ³⁹Ar dating. Moreover, the ⁸⁵Kr data for the top part of the ice core provide information on firn processes, including the age difference between the ice and its enclosed gas. This first application of ³⁹Ar and ⁸⁵Kr to an ice core facilitates further ice cores from nonpolar glaciers to be used for recovering climate records of the Common Era, a period including pronounced anomalies such as the Little Ice Age and the Medieval Warm Period.

Insight into atmospheric deposition and spatial distribution of bioavailable iron in the glaciers of northeastern Tibetan Plateau

Iron (Fe) is an essential micronutrient in glacial ecosystems and modulates global biogeochemical cycles. To find out the deposition concentration, multiple origins and release form of iron in various glacier areas of central Asia, this study investigated the total Fe (TFe) and dissolved-Fe (dFe, diameter < 0.45 or <0.2 μm) deposition in glaciers and snowpack of northeast Tibetan Plateau, based on snow and meltwater sampling in ablation period of 2014-2017. The composition and concentration of dFe in the samples were measured, and the spatial distribution and temporal variations of dFe in glacial surface snow and meltwater runoff were investigated. Results showed that average TFe and dFe contents exhibited a generally heterogeneous geographic distribution that varied from north to south. The northern locations in eastern Tianshan Mountains (e.g. Miaoergou Glacier) showed the highest TFe and dFe values, followed by Yuzhufeng Glacier of eastern Kunlun Mountains, whereas the Qilian Mountains locations displayed relatively lower TFe and dFe contents spanning a wide range. Based on the good correlation between TFe and dFe, we infer that aeolian dust and anthropogenic aerosols, and their chemical interactions are likely the important origins for dFe deposition. In meltwater runoff the peak values of dFe release flux appeared in July, with maximum appeared earlier (the early of July) than TFe (the end of July). Moreover, the annual dFe release flux from Laohugou glacier terminus meltwater runoff is estimated to be 1740 kg yr^-1 (with 9256 kg yr^-1 for TFe), and meltwater showed higher mean concentration of dFe than that of glacier snowpack. We also provided a conceptual framework showing the multiple origins and transport dynamics of dissolved Fe along the atmosphere-glacier-meltwater runoff path. Compared to Fe release in other global glacier/ice-sheet, the TP glacier is an important potential dFe reservoir and may have a profound effect on regional downstream ecosystem through Fe biochemistry cycle.

Overestimation of anthropogenic contribution of heavy metals in precipitation than those of aerosol samples due to different treatment methods

Due to increased anthropogenic activities in recent decades, many heavy metal elements have been emitted into the atmosphere and transported to remote regions. The Enrichment factors (EFs) is a normally used method for evaluating the source of heavy metal elements. However, because of some flaws of this method (e.g., higher solubility of heavy metals elements than reference elements in dilute acid), the anthropogenic contributions of some heavy metal elements in the precipitation sample were overestimated. To address this issue, EFs of heavy metal elements of aerosol, precipitation and snowpit samples in a typical remote area of the Tibetan Plateau (TP) were compared. The results showed that the EF values of many heavy metal elements in precipitation and snowpit samples were close to that of aerosol samples treated with dilute acid but usually much higher than those of totally dissolved aerosol samples. Moreover, EF values of most heavy metal elements in the ice core at the margin of the TP were higher than those at central TP, indicating that signal of long-range transport anthropogenic emitted heavy metal elements is weak and may be covered by natural mineral dust sources at glacier region. Therefore, the threshold EF values for determining anthropogenic sources of heavy metal elements in precipitation and ice core samples should be higher than those of aerosols. This study provides new knowledge on investigating anthropogenic sources of heavy metals in precipitation samples at both the TP and other regions of the world.

Microplastics in the high-altitude Himalayas: Assessment of microplastic contamination in freshwater lake sediments, Northwest Himalaya (India)

In this study, we assess the magnitude, type, and sources of microplastic (MP) in lake bottom sediments collected from freshwater Anchar Lake, located in the Kashmir Valley, Northwest Himalaya. The MP identification was done on twenty-four lake bottom sediment samples under a stereo-microscope, and their polymer compositions were characterized using an Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy. The study reveals that 606 ± 360 (average ± SD, n = 24) numbers of MP were present per kilogram of dry sediment samples, with fibers (91%), fragments/films (8%), and pellets (1%) dominating the shape groups. Polyamide (PA, 96%) was the dominant polymer composition present in the sediment samples, followed by polyethylene terephthalate (PET, 1.4%), polystyrene (PS, 1.4%), polyvinyl chloride (PVC, 0.9%), and polypropylene (PP, 0.7%). Polymer Hazard Index (PHI) and Pollution Load Index (PLI) were used to evaluate the quality of sediments. It was noted that high PHI values (>1000) were due to the presence of PVC polymer. According to PLI values, sediments in the Anchar lake are less contaminated with MP. We conclude that MP in the Anchar Lake have a complex source derived mostly from the automobile, textile, and packaging industries.

Potential environmental risk of trace elements in fly ash and gypsum from ultra-low emission coal-fired power plants in China

The ultra-low emission retrofitting (ULE) in China's coal-fired power plants (CFPPs) enhances removal efficiencies of trace elements, which may increase their contents in fly ash and gypsum. However, their potential environmental risks in these wastes have been scarcely evaluated. Experiments indicated that the trace elements in fly ash and gypsum accounted for approximately 92.9-98.2% of the total outputs. Most trace elements in these wastes existed mainly as mobile/leachable forms, except for the Hg in fly ash (residual form). We comprehensively evaluated the potential environmental risks of trace elements in fly ash and gypsum from ULE CFPPs in China using a modified risk assessment approach that integrates a trace element enrichment model for waste, and chemical speciation datasets. We found that nationally, trace elements in gypsum represented low levels of potential risk, even after ULE. However, the potential moderate environmental risk of fly ash has aroused attention because of trace element pollution, where Hg and Cd contributed the major risks. The relatively high risks from fly ash are mainly distributed in Guangxi, Hunan and Hebei provinces. The disposal of fly ash in these areas should be given special attention in the future.

Lead Isotope Evidence for Enhanced Anthropogenic Particle Transport to the Himalayas during Summer Months

The Indo-Gangetic Plain (IGP) is one of the most highly polluted regions of the world, yet the temporal pattern of transport of anthropogenic aerosols from this region to the Himalayas is poorly constrained. On the basis of the seasonal variation of planetary boundary layer heights, air mass back trajectory analysis, and year-long time-series data for ²⁰⁸Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, ²⁰⁶Pb/²⁰⁴Pb, and ¹⁴³Nd/¹⁴⁴Nd from aerosols collected over a high-altitude station, we demonstrate that anthropogenic Pb transport to the glacierized catchment has a seasonal pattern. The Pb isotope data reveal that during winter, the thinned boundary layer traps up to 10 ± 7% more coal-derived Pb in the IGP. In contrast, in nonwinter months, a thicker boundary layer and enhanced subtropical westerly winds result in efficient Pb transport to the Himalayas. As Pb isotope ratios are robust conservative chemical tracers and Pb is predominantly derived from anthropogenic sources, these observations suggest that enhanced transport of anthropogenic aerosols to the glacierized catchment of the Himalayas coincides with higher near-surface temperatures in the summer, creating positive feedback that enhances melting. Our results further suggest that >50% of Pb in the Himalayan aerosols originates from the resuspension of historic Pb derived from phased out leaded gasoline, highlighting the importance of legacy Pb stored in the Indo-Gangetic Plains.

A case study using 2019 pre-monsoon snow and stream chemistry in the Khumbu region, Nepal

This case study provides a framework for future monitoring and evidence for human source pollution in the Khumbu region, Nepal. We analyzed the chemical composition (major ions, major/trace elements, black carbon, and stable water isotopes) of pre-monsoon stream water (4300-5250 m) and snow (5200-6665 m) samples collected from Mt. Everest, Mt. Lobuche, and the Imja Valley during the 2019 pre-monsoon season, in addition to a shallow ice core recovered from the Khumbu Glacier (5300 m). In agreement with previous work, pre-monsoon aerosol deposition is dominated by dust originating from western sources and less frequently by transport from southerly air mass sources as demonstrated by evidence of one of the strongest recorded pre-monsoon events emanating from the Bay of Bengal, Cyclone Fani. Elevated concentrations of human-sourced metals (e.g., Pb, Bi, As) are found in surface snow and stream chemistry collected in the Khumbu region. As the most comprehensive case study of environmental chemistry in the Khumbu region, this research offers sufficient evidence for increased monitoring in this watershed and surrounding areas.

A Perspective of the Cumulative Risks from Climate Change on Mt. Everest: Findings from the 2019 Expedition

In 2019, the National Geographic and Rolex Perpetual Planet Everest expedition successfully retrieved the greatest diversity of scientific data ever from the mountain. The confluence of geologic, hydrologic, chemical and microbial hazards emergent as climate change increases glacier melt is significant. We review the findings of increased opportunity for landslides, water pollution, human waste contamination and earthquake events. Further monitoring and policy are needed to ensure the safety of residents, future climbers, and trekkers in the Mt. Everest watershed.

New insights into particle-bound trace elements in surface snow, Eastern Tien Shan, China

Trace elements (TEs) in the insoluble particles of surface snow are less affected by melting processes and can be used as environmental proxies to reveal natural and anthropogenic emissions. Here the first comprehensive study of the 16 TEs (Al, As, Ba, Bi, Cr, Cu, Fe, Mn, Ni, Pb, Sn, Sr, Ti, U, V, and Zn) in insoluble particles (>0.45 μm) from surface snow samples collected at Urumqi Glacier No. 1 (UG1), Eastern Tien Shan, China, from February 2008 to January 2010 were presented. Results show that concentrations of most insoluble particulate TEs (TEs _insol) in the snow were higher in summer while lower in winter, due to the increasing particle inputs and melting processes. The abundances of As, Cr, Cu, Ni, Pb, and Zn in some samples were higher than those in surrounding urban soils, which might due to these TEs have further anthropogenic input beyond the already contaminated re-suspended urban soil particles and TEs were mainly enriched in particles with small grain size. Based on enrichment factor (EF) and principal component analysis (PCA), our results suggest that eight TEs (Al, Fe, Ti, Ba, Mn, Sr, U, and V) mainly came from mineral dust, while the remaining eight TEs (As, Bi, Cr, Cu, Ni, Pb, Sn, and Zn) were affected by coal combustion, mining and smelting of non-ferrous metals, traffic emissions, and the steel industry. The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model suggests that pollutants might originate from Xinjiang province, Kazakhstan, and Kyrgyzstan. Moreover, UG1 received more significant inputs of particle-bound pollutants in summer than in winter due to the stronger convection and the prevailing valley wind that transports pollutants from the city of Urumqi.

Decoupling Natural and Anthropogenic Mercury and Lead Transport from South Asia to the Himalayas

Mercury (Hg) and lead (Pb) accumulation since the Industrial Revolution has been generally observed to increase concurrently in lake sedimentary records around the world. Located downwind during the monsoon season from the rapidly developing South Asia, the Himalayas and the Tibetan Plateau are expected to receive direct anthropogenic Hg and Pb loadings, yet the source, pathway, and effects of such transport remain poorly known due to logistic challenges in accessing this region. When studying the sediment record from Lake Gokyo (4750 m above sea level (a.s.l.)) in the Himalayas, we find remarkably different Hg and Pb accumulation trends over the past 260 years. Although Hg accumulation has continued to increase since the Industrial Revolution, Pb accumulation peaked during that time and has been decreasing since then. Stable isotope analysis reveals that the decoupling trends between these two elements are due to different sources and pathways of Hg and Pb in the region. Both δ²⁰²Hg and Δ¹⁹⁹Hg have been increasing since the Industrial Revolution, suggesting that anthropogenic Hg emissions from South Asia have been continuously increasing and that the Indian monsoon-driven wet deposition of atmospheric Hg is the dominant pathway for Hg accumulation in the sediments. In contrast, analysis of ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁷Pb ratios suggests that Pb accumulation in the sediments originates primarily from natural sources and that the decreasing trend of Pb accumulation is most likely due to a weakening input of atmospheric mineral dust by the westerlies. These decoupling trends highlight the ongoing issue of transboundary Hg transport to the Himalayas and the Tibetan Plateau that are source waters for major freshwater systems in Asia and calls for regional and international collaborations on Hg emission controls in South Asia.

Early atmospheric contamination on the top of the Himalayas since the onset of the European Industrial Revolution

Because few ice core records from the Himalayas exist, understanding of the onset and timing of the human impact on the atmosphere of the "roof of the world" remains poorly constrained. We report a continuous 500-y trace metal ice core record from the Dasuopu glacier (7,200 m, central Himalayas), the highest drilling site on Earth. We show that an early contamination from toxic trace metals, particularly Cd, Cr, Mo, Ni, Sb, and Zn, emerged at high elevation in the Himalayas at the onset of the European Industrial Revolution (∼1780 AD). This was amplified by the intensification of the snow accumulation (+50% at Dasuopu) likely linked to the meridional displacement of the winter westerlies from 1810 until 1880 AD. During this period, the flux and crustal enrichment factors of the toxic trace metals were augmented by factors of 2 to 4 and 2 to 6, respectively. We suggest this contamination was the consequence of the long-range transport and wet deposition of fly ash from the combustion of coal (likely from Western Europe where it was almost entirely produced and used during the 19th century) with a possible contribution from the synchronous increase in biomass burning emissions from deforestation in the Northern Hemisphere. The snow accumulation decreased and dry winters were reestablished in Dasuopu after 1880 AD when lower than expected toxic metal levels were recorded. This indicates that contamination on the top of the Himalayas depended primarily on multidecadal changes in atmospheric circulation and secondarily on variations in emission sources during the last 200 y.