Weak transport of atmospheric water-insoluble particulate carbon from South Asia to the inner Tibetan Plateau in the monsoon season.

Carbonaceous particles play a crucial role in atmospheric radiative forcing. However, our understanding of the behavior and sources of carbonaceous particles in remote regions remains limited. The Tibetan Plateau (TP) is a typical remote region that receives long-range transport of carbonaceous particles from severely polluted areas such as South Asia. Based on carbon isotopic compositions (Δ14C/δ13C) of water-insoluble particulate carbon (IPC) in total suspended particle (TSP), PM2.5, and precipitation samples collected during 2020-22 at the Nam Co Station, a remote site in the inner TP, the following results were achieved: First, fossil fuel contributions (ffossil) to IPC in TSP samples (28.60 ± 9.52 %) were higher than that of precipitation samples (23.11 ± 8.60 %), and it is estimated that the scavenging ratio of IPC from non-fossil fuel sources was around 2 times that from fossil fuel combustion during the monsoon season. The ffossil of IPC in both TSP and PM2.5 samples peaked during the monsoon season. Because heavy precipitation during the monsoon season scavenges large amounts of long-range transported carbonaceous particles, the contribution of local emissions from the TP largely outweighs that from South Asia during this season. The results of the IPC source apportionment based on Δ14C and δ13C in PM2.5 samples showed that the highest contribution of liquid fossil fuel combustion also occurred in the monsoon season, reflecting increased human activities (e.g., tourism) on the TP during this period. The results of this study highlight the longer lifetime of fossil fuel-sourced IPC in the atmosphere than that of non-fossil fuel sources in the inner TP and the importance of local emissions from the TP during the monsoon season. The findings provide new knowledge for model improvement and mitigation of carbonaceous particles.

Key toxic components and sources affecting oxidative potential of atmospheric particulate matter using interpretable machine learning: Insights from fog episodes.

Fog significantly affects the air quality and human health. To investigate the health effects and mechanisms of atmospheric fine particulate matter (PM2.5) during fog episodes, PM2.5 samples were collected from the coastal suburb of Qingdao during different seasons from 2021 to 2022, with the major chemical composition in PM2.5 analyzed. The oxidative potential (OP) of PM2.5 was determined using the dithiothreitol (DTT) method. A positive matrix factorization model was adopted for PM2.5. Interpretable machine learning (IML) was used to reveal and quantify the key components and sources affecting OP. PM2.5 exhibited higher oxidative toxicity during fog episodes. Water-soluble organic carbon (WSOC), NH4+, K+, and water-soluble Fe positively affected the enhancement of DTTV (volume-based DTT activity) during fog episodes. The IML analysis demonstrated that WSOC and K+ contributed significantly to DTTV, with values of 0.31 ± 0.34 and 0.27 ± 0.22 nmol min-1 m-3, respectively. Regarding the sources, coal combustion and biomass burning contributed significantly to DTTV (0.40 ± 0.38 and 0.39 ± 0.36 nmol min-1 m-3, respectively), indicating the significant influence of combustion-related sources on OP. This study provides new insights into the effects of PM2.5 compositions and sources on OP by applying IML models.

Attributing Atmospheric Phosphorus in the Himalayas: Biomass Burning vs Mineral Dust.

Atmospheric phosphorus is a vital nutrient for ecosystems whose sources and fate are still debated in the fragile Himalayan region, hindering our comprehension of its local ecological impact. This study provides novel insights into atmospheric phosphorus based on the study of total suspended particulate matter at the Qomolangma station. Contrary to the prevailing assumptions, we show that biomass burning (BB), not mineral dust, dominates total dissolved phosphorus (TDP, bioavailable) deposition in this arid region, especially during spring. While total phosphorus is mainly derived from dust (77% annually), TDP is largely affected by the transport of regional biomass-burning plumes from South Asia. During BB pollution episodes, TDP causing springtime TDP fluxes alone accounts for 43% of the annual budget. This suggests that BB outweighs dust in supplying bioavailable phosphorus, a critical nutrient, required to sustain Himalayas' ecological functions. Overall, this first-hand field evidence refines the regional and global phosphorus budget by demonstrating that BB emission, while still unrecognized, is a significant source of P, even in the remote mountains of the Himalayas. It also reveals the heterogeneity of atmospheric phosphorus deposition in that region, which will help predict changes in the impacted ecosystems as the deposition patterns vary.

Dust dominates glacier darkening across majority of the Tibetan Plateau based on new measurements.

Rapid retreat and darkening of most glaciers in the Tibetan Plateau (TP) are enhanced by the deposition of light-absorbing particles (LAPs). Here, we provided new knowledge on the estimation of albedo reduction caused by black carbon (BC), water-insoluble organic carbon (WIOC), and mineral dust (MD), based on a comprehensive study of snowpit samples from ten glaciers across the TP collected in the spring of 2020. According to the albedo reductions caused by the three LAPs, the TP was divided into three sub-regions: the eastern and northern margins, Himalayas and southeastern TP, and western to inner TP. Our findings indicated that MD had a dominant role in causing snow albedo reductions across the western to inner TP, with comparable effects to WIOC but stronger effects than BC in the Himalayas and southeastern TP. BC played a more important role in the eastern and northern margins of the TP. In conclusion, the findings of this study emphasize not only the important role of MD in glacier darkening across majority of the TP but also the influence of the WIOC in enhancing glacier melting which indicates the dominant contribution of non-BC components in the LAP-related glacier melting of the TP.

Wildfire-Derived Nitrogen Aerosols Threaten the Fragile Ecosystem in Himalayas and Tibetan Plateau.

Himalayas and Tibetan Plateau (HTP) is important for global biodiversity and regional sustainable development. While numerous studies have revealed that the ecosystem in this unique and pristine region is changing, their exact causes are still poorly understood. Here, we present a year-round (23 March 2017 to 19 March 2018) ground- and satellite-based atmospheric observation at the Qomolangma monitoring station (QOMS, 4276 m a.s.l.). Based on a comprehensive chemical and stable isotope (15N) analysis of nitrogen compounds and satellite observations, we provide unequivocal evidence that wildfire emissions in South Asia can come across the Himalayas and threaten the HTP's ecosystem. Such wildfire episodes, mostly occurring in spring (March-April), not only substantially enhanced the aerosol nitrogen concentration but also altered its composition (i.e., rendering it more bioavailable). We estimated a nitrogen deposition flux at QOMS of ∼10 kg N ha-1 yr-1, which is approximately twice the lower value of the critical load range reported for the Alpine ecosystem. Such adverse impact is particularly concerning, given the anticipated increase of wildfire activities in the future under climate change.

Optimizing the physiological pharmacokinetic model to rank the risks of persistent organic pollutants towards fish on the Tibetan Plateau.

Persistent organic pollutants (POPs) could pose adverse risks towards fish in aquatic environments. However, related risk assessments in remote regions are lacking. In this study we investigated three kinds of POPs in four common fish species (n = 62) from high-altitude rivers and lakes on the Tibetan Plateau. The results showed that the lipid weight concentrations of organochlorine pesticides (OCPs), polycyclic aromatic hydrocarbons (PAHs), and perfluoroalkyl substances (PFAS) in fish muscle followed the order Σ13PAHs (24.5-3354 ng/g) > Σ11PFAS (2.48-164 ng/g) > Σ7OCPs (1.61-82.2 ng/g), which is comparable to that found in other remote regions. The physiologically based pharmacokinetic (PBPK) model was optimized using physiological parameters specific to the sampled Tibetan fish to generate accurate effective concentration (EC) thresholds. Based on these measured concentrations and newly simulated EC thresholds, the ecological risk ratios for selected toxic POPs (dichlorodiphenyltrichloroethane (DDT), pyrene (Pyr), and perfluorooctane sulfonate (PFOS)) ranged from 8.53 × 10-8 to 2.03 × 10-5. Racoma tibetanus and Schizothorax macropogon were the most vulnerable Tibetan fish species. All the risk ratios were far below 1, indicating that there was no risk of POPs towards Tibetan fish. However, the risk ratios for emerging POPs (i.e., PFOS) were 2-3 orders of magnitude higher than for legacy POPs (i.e., DDT and Pyr), suggesting that monitoring of emerging POPs should be reinforced. Our study sheds light on the risk assessment of wildlife exposed to POPs in remote regions with limited toxicity data.

Organic aerosols in the inland Tibetan Plateau: New insights from molecular tracers.

Aerosols affect the radiative forcing of the global climate and cloud properties. Organic aerosols are among the most important, yet least understood, components of the sensitive Tibetan Plateau atmosphere. Here, the concentration of and the seasonal and diurnal variations in biomass burning and biogenic aerosols, and their contribution to organic aerosols in the inland Tibetan Plateau were investigated using molecular tracers. Biomass burning tracers including levoglucosan and its isomers, and aromatic acids showed higher concentrations during winter than in summer. Molecular tracers of primary and secondary biogenic organic aerosols were more abundant during summer than those in winter. Meteorological conditions were the main factors influencing diurnal variations in most organic molecular tracers during both seasons. According to the tracer-based method, we found that biogenic secondary organic aerosols (38.5 %) and fungal spores (14.4 %) were the two dominant contributors to organic aerosols during summer, whereas biomass burning (15.4 %) was an important aerosol source during winter at remote continental background site. Results from the positive matrix factor source apportionment also demonstrate the importance of biomass burning and biogenic aerosols in the inland Tibetan Plateau. During winter, the long-range transport of biomass burning from South Asia contributes to organic aerosols. In contrast, the precursors, biogenic secondary organic aerosols, and fungal spores from local emissions/long-range transport are the major sources of organic aerosols during summer. Further investigation is required to distinguish between local emissions and the long-range transport of organic aerosols. In-depth insights into the organic aerosols in the Tibetan Plateau are expected to reduce the uncertainties when evaluating aerosol effects on the climate system in the Tibetan Plateau.

Diagnostic ratio of nitrated phenols as a new method for the identification of pollution emission sources.

Nitrated phenols (NPs) are emitted from biomass burning and vehicles emissions, or produced by oxidation of phenolic precursors. Previous studies have investigated the emission factors of NPs from various primary emission sources. However, there is no study on the source apportionment method for the diagnostic ratio of NPs. In this study, a new source apportionment method is established using a diagnostic ratio of NPs. Two categories (methyl-nitrocatechols and methyl-nitrophenols) of NP diagnostic ratios, are proposed for source apportionment of primary aerosols. In order to show the accuracy of this source apportionment method, it was applied to the source apportionment of atmospheric NPs in both urban (Kathmandu, Nepal) and remote areas (Lulang, Tibetan Plateau, China). The results show that biomass burning is a common emission source for atmospheric NPs in Kathmandu and Lulang, with vehicle emissions being another important emission source. The atmospheric NPs in the urban area of Kathmandu are commonly from gasoline motorbike emissions, while the atmospheric NPs in Lulang derive from diesel vehicles, throughout the year. The conclusions of the source apportionment study were consistent with the actual vehicle types of local residents in Kathmandu and Lulang, which further proves the reliability of the NP diagnostic ratios method.

Concentrations, sources, fluxes, and absorption properties of carbonaceous matter in a central Tibetan Plateau river basin.

Carbonaceous matter (CM) (such as water-insoluble organic carbon (WIOC), black carbon (BC), and water-soluble organic carbon (WSOC)) has a significant impact on the carbon cycle and radiative forcing (RF) of glacier. Precipitation samples and glacier's snow/ice samples (snowpit, surface snow, and granular ice) (Xiao dongkemadi Glacier) were collected at the Dongkemadi River Basin (DRB) in the central Tibetan Plateau (TP) between May and October 2016 to investigate the characteristics and roles of CM in the TP River Basin. WIOC, BC, and WSOC concentrations in precipitation were relatively higher than that in snowpit, but lower than that in surface snow/ice, with the wet deposition fluxes of 0.10 ± 0.002, 0.04 ± 0.001, and 0.12 ± 0.002 g C m-2 yr-1 at DRB, respectively. The positive matrix factorization model identified four major sources (biomass burning source, secondary precursors, secondary aerosol, and dust source) of CM in precipitation at DRB. Two source areas (South Asia and the interior of TP) contributing to the pollution at DRB were identified using a potential source contribution function model, a concentration-weighted trajectory method, and the back-trajectory model. Moreover, the light-absorption by WSOC in the ultraviolet region was 23.0%, 12.1%, and 3.4% relative to the estimated total light-absorption in precipitation, snowpit, and surface snow/ice, respectively. Optical indices analysis revealed that WSOC in snowpit samples presented higher molecular weight, while presented higher aromatic and higher molecule sizes in surface snow/ice and precipitation samples, respectively. RF by WSOC relative to that of BC was estimated to be 17.6 ± 17.6% for precipitation, 10.9 ± 5.8% for snowpit, and 10.7 ± 11.6% for surface snow/ice, respectively, during the melt season in the central TP River Basin. These results help us understand how CM affects glaciers, and they can be utilized to create policies and recommendations that efficiently reduce emissions.

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.

An overestimation of light absorption of brown carbon in ambient particles caused by using filters with large pore size.

As an important component of carbonaceous particles, organic carbon (OC) plays a significant role in radiative forcing in the atmosphere. Recently, the warming effect of light-absorbing OC has been emphasized. Water-soluble organic carbon (WSOC) is commonly used as a surrogate to investigate the light absorption of OC. Thus far, filters with 0.45 µm (PS1) and 0.20 µm pore sizes (PS2) are both used to investigate the light absorption of WSOC, which may cause large divergent results. In this study, we found that the light absorption ability of WSOC treated with PS1 was higher than that of PS2 due to the extinction of suspended particles (e.g., black carbon) with particle size between 0.20 µm and 0.45 µm, although the concentrations of WSOC treated with PS1 and PS2 were very close. This phenomenon was more remarkable at visible wavelengths, resulting in an overestimation of the warming effect of WSOC by 9%-22% for aerosol samples treated by PS1, with the highest values occurring in samples heavily influenced by fossil fuel burning emissions. An overestimation of WSOC light absorption treated by PS1 occurred in the investigated ambient aerosol samples from three sites, so it may be a general phenomenon that also exists in other regions of the world. Therefore, to achieve the actual solar radiative forcing of OC in the atmosphere, it is recommended to use PS2 in the future, and reported data of WSOC treated by PS1 should be re-evaluated.

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.

Century-long record of polycyclic aromatic hydrocarbons from tree rings in the southeastern Tibetan Plateau.

Limited studies have been carried out on the historical variations of atmospheric polycyclic aromatic hydrocarbons (PAHs), especially in remote regions of the world. In this study, century-long record of PAHs (1916-2018) were reconstructed from tree rings in the remote southeastern Tibetan Plateau (TP). The total concentrations of 15 PAHs varied from 27.5 to 6.05 × 102 ng/g dry weight (dw), with a mean value of 1.40 × 102 ng/g dw. Higher levels of PAHs were observed during World War Ⅱ and the Peaceful Liberation of Tibet, and increasing trends were observed starting from rapid industrialization in India. Both the isomer ratios and the positive matrix factorization model results indicated biomass and coal combustion were the dominant sources of PAHs. The carcinogenic risk of PAHs to local residents was assessed, which might have been negligible in most past periods and lower than in other regions of the world. Nevertheless, since the beginning of the 21st century, the cancer risk has been increasing year by year, indicating more actions are needed to reduce emissions of PAHs. This study provides an idea for reconstructing the pollution history of PAHs at the global scale.

Carbonaceous matter in the atmosphere and glaciers of the Himalayas and the Tibetan plateau: An investigative review.

Carbonaceous matter, including organic carbon (OC) and black carbon (BC), is an important climate forcing agent and contributes to glacier retreat in the Himalayas and the Tibetan Plateau (HTP). The HTP - the so-called "Third Pole" - contains the most extensive glacial area outside of the polar regions. Considerable research on carbonaceous matter in the HTP has been conducted, although this research has been challenging due to the complex terrain and strong spatiotemporal heterogeneity of carbonaceous matter in the HTP. A comprehensive investigation of published atmospheric and snow data for HTP carbonaceous matter concentration, deposition and light absorption is presented, including how these factors vary with time and other parameters. Carbonaceous matter concentrations in the atmosphere and glaciers of the HTP are found to be low. Analysis of water-insoluable organic carbon and BC from snowpits reveals that concentrations of OC and BC in the atmosphere and glacier samples in arid regions of the HTP may be overestimated due to contributions from inorganic carbon in mineral dust. Due to the remote nature of the HTP, carbonaceous matter found in the HTP has generally been transported from outside the HTP (e.g., South Asia), although local HTP emissions may also be important at some sites. This review provides essential data and a synthesis of current thinking for studies on atmospheric transport modeling and radiative forcing of carbonaceous matter in the HTP.

Light absorption and fluorescence characteristics of water-soluble organic compounds in carbonaceous particles at a typical remote site in the southeastern Himalayas and Tibetan Plateau.

Carbonaceous particles play an important role in climate change, and an increase in their emission and deposition causes glacier melting in the Himalayas and the Tibetan Plateau (HTP). This implies that studying their basic characteristics is crucial for a better understanding of the climate forcing observed in this area. Thus, we investigated characteristics of carbonaceous particles at a typical remote site of southeastern HTP. Organic carbon and elemental carbon concentrations at this study site were 1.86 ± 0.84 and 0.18 ± 0.09 µg m-3, respectively, which are much lower than those reported for other frequently monitored stations in the same region. Thus, these values reflect the background characteristics of the study site. Additionally, the absorption coefficient per mass (α/ρ) of water-soluble organic carbon (WSOC) at 365 nm was 0.60 ± 0.19 m2 g-1, with the highest and lowest values corresponding to the winter and monsoon seasons, respectively. Multi-dimensional fluorescence analysis showed that the WSOC consisted of approximately 37% and 63% protein and humic-like components, respectively, and the latter was identified as the component that primarily determined the light absorption ability of the WSOC, which also showed a significant relationship with some major ions, including SO2-4, K+, and Ca2+, indicating that combustion activities as well as mineral dust were two important contributors to WSOC at the study site.

Sedimentary ancient DNA metabarcoding delineates the contrastingly temporal change of lake cyanobacterial communities.

Harmful cyanobacterial blooms consisting of toxic taxa can produce a wide variety of toxins to threaten water quality, ecosystem functions and services. Of greater concern was the changing patterns of cyanobacterial assemblage were not well understood due to the lack of long-term monitoring data over the temporal scale. Biodiversity change in cyanobacterial community and paleoenvironmental variables over the past 170 years in Lake Chenghai were investigated based on sedimentary ancient DNA metabarcoding and traditional paleolimnological analysis. The results showed species richness and homogenization of cyanobacterial assemblage increased in the most recent decades, which were synchronized with the growth of artificial fertilization and decline in precipitation. Cyanobacterial co-occurrence network analysis revealed more complex interactions and weak community stability after the change point of ∼1987, while the rare cyanobacterial genera such as Anabaena, Planktothrix, Oscillatoria and Microcystis were identified to be keystone taxa affecting cyanobacterial assemblage. Furthermore, an increase of toxin-producing cyanobacterial taxa was significantly and positively associated with TN and TP, as well as TN/IP and TN/TP, which was verified by quantitative real-time PCR of mcyA and rpoC1 genes. Threshold in total nitrogen (TN) concentration should be targeted no more than 0.60 mg/L to alleviate nuisance cyanobacterial blooms in Lake Chenghai. These findings reinforce the comprehensive understanding for the long-term dynamics of cyanobacterial assemblage responding to environmental change, which could contribute to proactively regulate environmental conditions for avoiding undesirable ecological consequences.

A new method for extraction of methanol-soluble brown carbon: Implications for investigation of its light absorption ability.

As an important component of organic carbon (OC), brown carbon (BrC) plays a significant role in radiative forcing in the atmosphere. Water-insoluble OC (WIOC) generally has higher light absorption ability than water-soluble OC (WSOC). The mass absorption cross-section (MAC) of WIOC is normally investigated by dissolving OC in methanol. However, all the current methods have shortcomings due to neglecting the methanol insoluble particulate carbon that is detached from the filter and suspended in methanol extracts, which results in MAC uncertainties of the methanol-soluble BrC and its climate warming estimation. In this study, by investigating typical biomass combustion sourced aerosols from the Tibetan Plateau and ambient aerosols from rural and urban areas in China, we evaluated the light absorption of extractable OC fraction for the existing methods. Moreover, a new method was developed to overcome the methanol insoluble particulate carbon detachment problem to achieve more reliable MAC values. We found that OC can be dissolved in methanol in a short time (e.g., 1 h) and ultrasonic treatment and long-term soaking do not significantly increase the extractable OC fraction. Additionally, we proved that methanol insoluble particulate carbon detachment in methanol does exist in previous methods, causing overestimation of the BrC mass extracted by methanol and thus the underestimation of MAC values. We therefore recommend the newly developed extraction method in this study to be utilized in future related studies to quantitatively obtain the light absorption property of methanol-soluble BrC.