Concentration, sources, and flux of dissolved organic carbon of precipitation at Lhasa city, the Tibetan Plateau

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.

Multiproxy probing of anthropogenic influences on the different components of dissolved organic matter in coastal rainwater

In an environment that is tightly linked to humankind, how anthropogenic activity affects the quality and quantity of dissolved organic matter (DOM) in atmospheric depositions is not well understood. In this study, dissolved organic carbon (DOC), UV-vis spectra combined with molecular markers, including formic acid (FA), acetic acid (AA) and dissolved black carbon (DBC), were applied to track the temporal variation and influential factors of rainwater DOM at a coastal site. The ranges of DOC, light absorption at 254 nm (a₂₅₄), FA, AA and DBC were 23.2-471 μmol L^-1, 0.16-10.6 m^-1, 0.12-23.5 μmol L^-1, 0.44-37.8 μmol L^-1 and 0.02-4.8 μmol L^-1, respectively. The negative correlations between DOC, a₂₅₄, AA and precipitation amount revealed a dilution effect. The concentrations of all measured DOM components were statistically different among different seasons with the highest value in spring. Higher DOM concentrations also occurred in the rain with backward trajectories influenced by the land. Compared to the nearby riverine DOM, the DOC-specific UV absorbance (SUVA₂₅₄) of rainwater was lower, suggesting lower aromaticity of rainwater DOM. Significant correlations among different DOM components suggest that they shared similar sources or were affected by the same processes, while the significant correlations with anions (SO₄^2-, F^- and NO₃^-) and the ratio of FA to AA all suggested that the direct emission and secondary production from anthropogenic emissions (fossil fuel burning, biomass and biofuel burning) played important roles in regulating the level of DOM concentration in rainwater. Correlations with environmental variables (PM_2.5, CO and NO₂) further confirmed the input from anthropogenic activities. Furthermore, the monthly wet atmospheric deposition fluxes of DOM components (except DBC) can be successfully simulated by monthly precipitation and monthly average values of PM_2.5 and NO₂. Future studies should examine how atmospheric deposition affects the biogeochemical cycles in coastal regions.

Chromophoric dissolved organic carbon cycle and its molecular compositions and optical properties in precipitation in the Guanzhong basin, China

The investigation of water-soluble organic carbon (WSOC), which is important in the biogeochemical cycle of precipitation, can provide a comprehensive view of chromophores within the atmospheric boundary layer. In this work, the optical properties and molecular characteristics of WSOC in precipitation over the Guanzhong Basin (GB) of North China were investigated using ultraviolet-visible (UV-vis) absorption and excitation-emission matrix (EEM) fluorescence spectra, and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) coupled with electrospray ionization (ESI). Furthermore, sources and wet deposition of WSOC were estimated using in-situ measurements and modeling. The light-absorption by WSOC at 250-300 nm (UV region) and 400-550 nm (visible region) was 64.17% and 15.36% relative to the estimated total light-absorption, respectively. Parallel factor (PARAFAC) analysis revealed three types of fluorophores in WSOC at Xi'an (XN), including two humic-like substances (HULIS) and one protein-like substance (PRLIS), with HULIS accounting for 79% of total fluorescence intensity. FT-ICR MS analysis revealed that CHO and CHON were the most abundant components of WSOC at XN, each containing a variety of lignins, protein/amino sugars, and lipids. Moreover, the positive matrix factorization (PMF) model identified the contributions from three main sources (secondary precursors and aerosols, and coal combustion) of WSOC in precipitation at XN. The annual wet deposition flux of WSOC in precipitation at XN was estimated as about 0.63 g C m^-2 yr^-1, lower than that at other polluted cities. These findings add to our understanding of chromophoric dissolved organic carbon budgets, which is critical for accurately assessing the global carbon cycle.

Export of Dissolved Organic Carbon from the Source Region of Yangtze River in the Tibetan Plateau

The carbon release and transport in rivers are expected to increase in a warming climate with enhanced melting. We present a continuous dataset of DOC in the river, precipitation, and groundwater, including air temperature, discharge, and precipitation in the source region of the Yangtze River (SRYR). Our study shows that the average concentrations of DOC in the three end-members are characterized as the sequence of groundwater > precipitation > river, which is related to the water volume, cycle period, and river flow speed. The seasonality of DOC in the river is observed as the obvious bimodal structure at Tuotuohe (TTH) and Zhimenda (ZMD) gauging stations. The highest concentration appears in July (2.4 mg L−1 at TTH and 2.1 mg L−1 at ZMD) and the secondary high value (2.2 mg L−1 at TTH 1.9 mg L−1 at ZMD) emerges from August to September. It is estimated that 459 and 6751 tons of DOC are transported by the river at TTH and ZMD, respectively. Although the wet deposition flux of DOC is nearly ten times higher than the river flux, riverine DOC still primarily originates from soil erosion of the basin rather than precipitation settlement. Riverine DOC fluxes are positively correlated with discharge, suggesting DOC fluxes are likely to increase in the future. Our findings highlight that permafrost degradation and glacier retreat have a great effect on DOC concentration in rivers and may become increasingly important for regional biogeochemical cycles.

Seasonal Variations in Dissolved Organic Carbon in the Source Region of the Yellow River on the Tibetan Plateau

Rivers as the link between terrestrial ecosystems and oceans have been demonstrated to transport a large amount of dissolved organic carbon (DOC) to downstream ecosystems. In the source region of the Yellow River (SRYR), climate warming has resulted in the rapid retreat of glaciers and permafrost, which has raised discussion on whether DOC production will increase significantly. Here, we present three-year data of DOC concentrations in river water and precipitation, explore the deposition and transport processes of DOC from SRYR. Results show that annual mean concentrations of riverine DOC ranged from 2.03 to 2.34 mg/L, with an average of 2.21 mg/L. Its seasonal variation is characterized by the highest concentration in spring and summer (2.65 mg/L and 2.62 mg/L, respectively), followed by autumn (1.95 mg/L), and the lowest in winter (1.44 mg/L), which is closely related to changes in river runoff under the influence of precipitation and temperature. The average concentration of DOC in precipitation (2.18 mg/L) is comparable with riverine DOC, while the value is inversely related to precipitation amount and is considered to be the result of precipitation dilution. DOC deposition flux in precipitation that is affected by both precipitation amount and DOC concentration roughly was 86,080, 105,804, and 73,072 tons/year from 2013 to 2015, respectively. DOC flux delivered by the river ranged from 24,629 to 37,539 tons/year and was dominated by river discharge. Although permafrost degradation in SRYR is increasing, DOC yield is not as significant as previously assumed and is much less than other large rivers in the world.

Characteristics of dissolved organic carbon and nitrogen in precipitation in the northern Tibetan Plateau

Dissolved organic carbon (DOC) and nitrogen (N) play essential roles in global C and N cycles. To address the possible role of DOC and N in precipitation and enrich the related global database, the characteristics of DOC and N in precipitation were investigated in a typical remote permafrost region (upper Heihe River Basin) of the northern Tibetan Plateau (TP) from February 2019 to March 2020. The results demonstrated that the average DOC and total dissolved N (TDN) concentrations in the precipitation were 1.41 ± 1.09 μg mL^-1 and 0.84 ± 0.48 μg mL^-1, respectively, with relatively lower concentrations in the summer. The annual DOC and TDN fluxes were estimated to be 6.42 kg ha^-1 yr^-1 and 3.39 kg ha^-1 yr^-1, respectively, indicating that precipitation was a significant factor in C and N deposition. The light-absorbing properties of precipitation DOC from the SUVA₂₅₄ and spectral slope revealed that precipitation DOC containing more aromatic components and lower molecular weights mostly was present during the summer; the mass cross-section (at the wavelength of 365 nm) ranged 0.26-1.84 m² g^-1, suggesting the potential impact of DOC on climatic forcing in the area. The principal component analysis combined with air mass backward trajectories indicated that the air masses from west Siberia, Central Asia, and northwestern China most significantly influenced the precipitation C and N in the study area. The WRF-Chem simulations and aerosol vertical distributions further illustrated the air mass transport pathways, demonstrating that dust and anthropogenic emissions could be transported over the studied area by westerlies and monsoonal winds. In the study basin, the precipitation deposition of DOC and N contributed largely to the riverine DOC and N exportation during the summer and had potential ecological effects. These results highlight the importance of DOC and N deposition from precipitation in the northern TP.

Concentration, sources and wet deposition of dissolved nitrogen and organic carbon in the Northern Indo-Gangetic Plain during monsoon

Precipitation represents an important phenomenon for carbon and nitrogen deposition. Here, the concentrations and fluxes of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) with their potential sources were analyzed in wet precipitation during summer monsoon from the Northern Indo-Gangetic Plain (IGP), important but neglected area. The volume-weighted mean (VWM) concentration of DOC and TDN were 687.04 and 1210.23 µg/L, respectively. Similarly, the VWM concentration of major ions were in a sequence of NH₄⁺ > Ca²⁺ > SO₄^2- > Na⁺ > K⁺ > NO₃^- > Cl^- > Mg²⁺ > F^- > NO₂^-, suggesting NH₄⁺ and Ca²⁺ from agricultural activities and crustal dust played a vital role in precipitation chemistry. Moreover, the wet deposition flux of DOC and TDN were 9.95 and 17.06 kg/(ha⋅year), respectively. The wet deposition flux of inorganic nitrogen species such as NH₄⁺-N and NO₃^--N were 14.31 and 0.47 kg/(ha⋅year), respectively, demonstrating the strong influence of emission sources and precipitation volume. Source attribution from different analysis suggested the influence of biomass burning on DOC and anthropogenic activities (agriculture, animal husbandry) on nitrogenous species. The air-mass back trajectory analysis indicated the influence of air masses originating from the Bay of Bengal, which possibly carried marine and anthropogenic pollutants along with the biomass burning emissions to the sampling site. This study bridges the data gap in the less studied part of the northern IGP region and provides new information for policy makers to deal with pollution control.

Dissolved organic carbon in eastern Canadian lakes: Novel patterns and relationships with regional and global factors

Long-term patterns in dissolved organic carbon (DOC) concentrations in 49 eastern Canadian lakes from four sites were re-examined with a ~ 35-year (~1980-2015) dataset. The study sites were Dorset (number of lakes, n = 8), Experimental Lakes Area (ELA, n = 4), Kejimkujik (n = 26) and Yarmouth (n = 11). Lake DOC patterns were synchronous within each site. However, comparisons of DOC patterns across sites showed that they were synchronous only between the Kejimkujik and Yarmouth locations. Hence, these two sites were pooled into a single Nova Scotia site (NS). Increases in DOC concentration were evident in Dorset, Ontario from 1988 (r² = 0.78, p < 0.001) and NS from 2000 (r² = 0.43, p = 0.006). DOC at the ELA in northwestern Ontario had a different pattern compared to the other sites, i.e., DOC had increased earlier (1983-2000), and then, unlike Dorset and NS, neither an increase nor decrease was detected between 2001 and 2015 (p = 0.78). Precipitation and sulfur deposition explained the greatest variance in DOC patterns at the Dorset and NS sites (i.e., precipitation: 21-49% and sulfur deposition: 24-54%). Precipitation was the most important driver of DOC at the ELA. Our results indicate that all the sites have gone through a process of increasing DOC, but at different times. The stabilizing pattern at the ELA since 2001 may suggest that DOC concentrations in ELA lakes have reached, or are approaching a new equilibrium, a phenomenon that was not observed at the other sites. Also, the increase in DOC was not always associated with declining sulfur deposition (e.g., ELA). Therefore, we conclude that there was considerable variation in DOC patterns across this large geographic region of Canada and potential drivers of these patterns were not consistent across these diverse sites.

High particulate carbon deposition in Lhasa-a typical city in the Himalayan-Tibetan Plateau due to local contributions

The Himalayan-Tibetan Plateau is a typical remote region with sparse air pollution. However, air pollution in cites of the inner Himalayan-Tibetan Plateau is relatively serious due to emissions from local residents. Carbonaceous aerosols are not only an important component of air pollutants that affect the health of local residents but also an important trigger of climate change. In this study, the annual wet and dry deposition rates of carbonaceous particles were investigated in Lhasa-a typical city in the Himalayan-Tibetan Plateau, by collecting precipitation and dry deposition samples and analyzing with a thermal-optical measurement protocol. The results showed that the in-situ annual wet deposition rates of water-insoluble organic carbon (WIOC) and black carbon (BC) were 169.6 and 19.4 mg m^-2 yr^-1, respectively, with the highest and lowest values occurring in the monsoon and non-monsoon periods, respectively. Both precipitation amounts and concentrations of WIOC and BC contributed to wet deposition rates. The dry deposition rates of WIOC and BC in Lhasa had an opposite seasonal variation to that of wet deposition, with annual average deposition rates of 2563.9 and 165.7 mg m^-2 yr^-1, respectively, which were much higher than those of the nearby glacier region and remote area. These values were also much higher than the results from modeling and empirical calculations, indicating that Lhasa is a high pollution point that cannot capture by models. The results in this study have significant implications for the transport of local emissions in Lhasa to the nearby remote and glacier regions.

Carbonaceous matter in glacier at the headwaters of the Yangtze River: Concentration, sources and fractionation during the melting process

Carbonaceous matter has an important impact on glacial retreat in the Tibetan Plateau, further affecting the water resource supply. However, the related studies on carbonaceous matter are still scarce in Geladaindong (GLDD) region, the source of the Yangtze River. Therefore, the concentration, source and variations of carbonaceous matter at Ganglongjiama (GLJM) glacier in GLDD region were investigated during the melting period in 2017, which could deepen our understanding on carbonaceous matter contribution to glacier melting. The results showed that dissolved organic carbon (DOC) concentration of snowpit samples (283 ± 200 μg/L) was much lower than that of precipitation samples (624 ± 361 μg/L), indicating that large parts of DOC could be rapidly leached from the snowpit during the melting process. In contrast, refractory black carbon (rBC) concentration measured by Single Particle Soot Photometer of snowpit samples (4.27 ± 3.15 μg/L) was much higher than that of precipitation samples (0.97 ± 0.49 μg/L). Similarly, DOC with high mass absorption cross-section measured at 365 nm value was also likely to enrich in snowpit during the melting process. In addition, it was found that both rBC and DOC with high light-absorbing ability began to leach from the snowpit when melting process became stronger. Therefore, rBC and DOC with high light-absorbing ability exhibited similar behavior during the melting process. Based on relationship among DOC, rBC and K⁺ in precipitation, the main source of carbonaceous matter in GLJM glacier was biomass burning during the study period.

Atmospheric wet deposition of dissolved organic carbon to a typical anthropogenic-influenced semi-enclosed bay in the western Yellow Sea, China: Flux, sources and potential ecological environmental effects

Dissolved Organic Carbon (DOC) is a key organic compound in wet precipitation, but few data are available in China marginal seas. To probe the concentration, deposition flux, seasonality, source and potential ecological environmental effects of precipitation DOC, in this study, one-year precipitation samples were collected at Jiaozhou Bay (JZB), a typical anthropogenic-influenced semi-enclosed bay in the western Yellow Sea for the first time from June 2015 to May 2016. The concentrations of DOC in precipitation were highly variable with a volume-weighted mean (VWM) concentration of 3.63 mg C L^-1, which was mostly higher than those in other areas. DOC concentrations were lower in wet season than that in dry season due to the dilution from more amount of rainfall. The wet deposition flux of DOC was calculated to be 3.15 g C m^-2 yr^-1 with 68.7% of which occurred in wet season mainly owing to the promoting of more rainfall amount. Besides, local emissions together with the long-range transport of pollutants were other factors controlling precipitation DOC. Fossil fuel combustion particularly coal burning was considered to be the leading source of precipitation DOC based on correlation analysis with some generally accepted indicators. Wet deposition dominates the external input of DOC at JZB by comparison with riverine input with a percentage of 54%. Heavy storm may exert enrichment effect on DOC levels in the surface water of JZB, and then promote the secondary productivity. This study emphasizes that wet deposition is an important process that should be seriously considered in the models of global/regional carbon biogeochemical cycling.

Linking atmospheric pollution to cryospheric change in the Third Pole region: current progress and future prospects

The Tibetan Plateau and its surroundings are known as the Third Pole (TP). This region is noted for its high rates of glacier melt and the associated hydrological shifts that affect water supplies in Asia. Atmospheric pollutants contribute to climatic and cryospheric changes through their effects on solar radiation and the albedos of snow and ice surfaces; moreover, the behavior and fates within the cryosphere and environmental impacts of environmental pollutants are topics of increasing concern. In this review, we introduce a coordinated monitoring and research framework and network to link atmospheric pollution and cryospheric changes (APCC) within the TP region. We then provide an up-to-date summary of progress and achievements related to the APCC research framework, including aspects of atmospheric pollution's composition and concentration, spatial and temporal variations, trans-boundary transport pathways and mechanisms, and effects on the warming of atmosphere and changing in Indian monsoon, as well as melting of glacier and snow cover. We highlight that exogenous air pollutants can enter into the TP's environments and cause great impacts on regional climatic and environmental changes. At last, we propose future research priorities and map out an extended program at the global scale. The ongoing monitoring activities and research facilitate comprehensive studies of atmosphere-cryosphere interactions, represent one of China's key research expeditions to the TP and the polar regions and contribute to the global perspective of earth system science.

A comparison of various approaches used in source apportionments for precipitation nitrogen in a mountain region of southwest China

Six different approaches are applied in the present study to apportion the sources of precipitation nitrogen making use of precipitation data of dissolved inorganic nitrogen (DIN, including NO₃^- and NH₄⁺), dissolved organic nitrogen (DON) and δ¹⁵N signatures of DIN collected at six sampling sites in the mountain region of Southwest China. These approaches include one quantitative approach running a Bayesian isotope mixing model (SIAR model) and five qualitative approaches based on in-situ survey (ISS), ratio of NH₄⁺/NO₃^- (R_N), principal component analysis (PCA), canonical-correlation analysis (CCA) and stable isotope approach (SIA). Biomass burning, coal combustion and mobile exhausts in the mountain region are identified as major sources for precipitation DIN while biomass burning and volatilization sources such as animal husbandries are major ones for DON. SIAR model results suggest that mobile exhausts, biomass burning and coal combustion contributed 25.1 ± 14.0%, 26.0 ± 14.1% and 27.0 ± 12.6%, respectively, to NO₃^- on the regional scale. Higher contributions of both biomass burning and coal combustion appeared at rural and urban sites with a significant difference between Houba (rural) and the wetland site (p < 0.05). The R_N method fails to properly identify sources of DIN, the ISS and SIA approach only respectively identifies DON and DIN sources, the PCA only tracks source types for precipitation N, while the CCA identify sources of both DIN and DON in precipitation. SIAR quantified the contributions of major sources to precipitation NO₃^- but failed for precipitation NH₄⁺ and DON. It is recommended to use ISS and SIAR in combination with one or more approaches from PCA, CCA and SIA to apportion precipitation NO₃^- sources. As for apportioning precipitation NH₄⁺ sources, more knowledge is needed for local ¹⁵N databases of NH₃ and DON and ¹⁵N fractional mechanisms among gaseous, liquid and particulate surfaces in this mountain region and similar environments.

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 MAC_DOC 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 MAC_DOC of precipitation is more representative than that of near-surface aerosols for a given region.