Spatial distribution, sources, and direct radiative effect of carbonaceous aerosol along a transect from the Arctic Ocean to Antarctica.

Carbonaceous aerosols (CA) have a high impact on air quality and climate. However, the composition and spatial variability of CA in the marine boundary layer (MBL) remain understudied, especially in the remote regions. Here, atmospheric organic carbon (OC) and elemental carbon (EC) measurements using DRI Model 2001 Thermal/Optical Carbon Analyzer in the MBL were performed during the Chinese Antarctic (2019-2020) and Arctic (2021) research expedition, spanning about 160 latitudes. Due to varying intensities of atmospheric transport from the continents, a significant latitudinal gradient in OC and EC was observed. OC exhibited the highest concentration over the coastal East Asia (CEA), with a mean of 4324 ng m-3 (358-18027 ng m-3), followed by the Arctic Ocean (AO). Similar OC levels were detected over the Southern Ocean (SO) and the Antarctic Ice Sheet (AIS). Similarly, the highest EC was also observed over CEA, with a mean of 867 ng m-3 (71-3410 ng m-3), followed by AO and SO, while the lowest EC appeared over the AIS, with a mean of 30 ng m-3 (2-70 ng m-3). The lower Char-EC/Soot-EC ratios over AO and CEA compared to SO and AIS indicated that fossil fuel combustion contributed more to EC over AO and CEA, while biomass burning played a more significant role in EC levels over SO and AIS. The high OC/EC ratio over AIS was associated with an extremely low EC level and the formation of secondary OC over AIS. SBDART model results suggested that EC had a net warming effect on the atmospheric column, with the highest direct radiative effects (DRE) over AO (5.50 ± 0.15 W m-2, corresponding a heating rate of 0.15 K day-1) and the lowest DRE over SO (1.35 ± 0.04 W m-2, corresponding a heating rate of 0.04 K day-1).

Dynamic of riverine pCO2, biogeochemical characteristics, and carbon sources inferred from δ13C in a subtropical river system.

Rivers significantly contribute to the global carbon budget, but data limitations and uncertainty are hampered by CO2 quantification in the global rivers. Thus, this study estimated riverine pCO2 by employing the pH-alkalinity-temperature method, and dissolved inorganic (DIC), dissolved organic (DOC), particulate organic (POC) carbon, and their isotopes (δ13C) with Chlorophyll-a (Chl a) were measured in river water samples from 26 sampling sites for characterization and source identification in the Yangtze River system. The estimated pCO2 varies from (120 ppm) to (3400 ppm) with an average (1085 ppm) across the Yangtze River and pCO2 is almost three times oversaturated than the ambient air (380 ppm). The downstream sites pronounced elevated pCO2 than the upstream sites. The relationship of δ13CDIC and pCO2 indicated that pCO2 control is seasonally independent. The significant correlations between DOC, POC, and pCO2 revealed that organic carbon influenced pCO2 in the river. The seasonal fluctuations of pCO2 were observed with an average of (762.23 ppm) and (1407.35 ppm) in winter and summer, respectively. δ13CDIC showed that the metabolic process has a negligible influence on DIC, δ13CDIC, and pCO2. δ13CDIC values increased from -8.95‰ to -4.91‰ during summer, whereas winter increased from -19.76‰ to -1.97‰ suggesting that DIC derived from carbonate weathering, dissolution of atmospheric and soil CO2. The δ13CDOC (-30.43‰ to -24.05‰) and δ13CPOC (-29.87‰ to -23.37‰) values confirmed that organic carbon mainly derived from the degradation of organic materials in soil. δ13CDIC revealed that anthropogenic sewage discharge slightly modified DIC composition. Overall, this study provides new insight into recent seasonal fluctuations of the pCO2, DOC, POC, DIC, δ13C, and their inputs. Thus, these variations and inputs of carbon transported by the Yangtze River could have a significant influence not only on the biogeochemical cycle and ecosystem process but also on the global carbon budget.

Heavy metal toxicity, ecological risk assessment, and pollution sources in a hydropower reservoir.

Heavy metal (HM) toxicity, ecological risk, and pollution sources were analyzed using the pollution indexing and statistical methods in the Three Gorges Reservoir (TGR). The average concentration of HM increased in the order of Cr < Ni < As < Cd < Cu < Mn < Pb < Zn < Al < Fe during the recharge period and Cd < Cr < Ni < As < Cu < Pb < Mn < Zn < Al < Fe during the discharge period. Significant spatial variations of Pb, Zn, Cd, As, Mn, Ni, Cr, and Cu were observed at the upstream and downstream sampling sites. Pb sharply increased during the recharge period, ranges (6.93 -148.62 µg/L) and exceeded WHO and USPEA standards limit. HPI, HEI, Cd, WPI indicated low pollution and moderate pollution with the strong influence of Pb and Cd in the discharge and recharging period, respectively. HTML values are below the permissible toxicity load except for Pb. The Pb toxicity removal percentage is 56.47%, suggesting that the lead's toxicity level is high in TGR and requires the removal process. Ecological risk index values indicated that pollution is low in TGR. The potential ecological risk indexes (RI) of 9.07 and 31.60 were obtained for the discharge and recharge period, respectively, indicating low potential ecological risk from heavy metals in TGR. However, RI values revealed that (Pb, Cd, As Cr Ni, Cu Zn, and Mn) were the most ecological risk HMs in TGR. A significant ecological risk of Pb and Cd distribution was observed across the TGR. Multivariate statistical results found that Pb, Cd, Zn, Mn, Ni, As, Cr, Cu mainly originate from industrial wastewater, mining, metals processing, and agricultural runoff. Fe and Al were mainly from bedrock weathering. Pb, Cd, Zn HMs are a threat to the reservoir ecosystem. This study delivered a current status of HM pollution, toxicity, ecological risk, and pollution sources, indicating a vital insight into HM pollution and water security management in the Three Gorges Reservoir.

Reservoir NO3- pollution and chemical weathering: by dual isotopes of δ15N-NO3-, δ18O-NO3- and geochemical constraints.

Reservoir dams alter the nutrient composition and biogeochemical cycle. Thus, dual isotopes of δ18O-NO3- and δ15N-NO-3 and geochemical signatures were employed to study the NO3- pollution and chemical weathering in the Three Gorges Reservoir (TGR), China. This study found that the TGR dam alters the δ15N-NO3- composition and is enriched in the recharge period. Values of δ15N-NO3- varied from 4.5 to 12.9‰ with an average of 9.8‰ in the recharge period, while discharge period δ15N-NO3- ranged from 3.2 to 12.5‰, with an average of 9.3‰. δ18O-NO3- varies (1.2-11.3‰) with an average of 6.5‰ and (2.4-12.4‰) with an average of 7.5‰, in the recharge and discharge periods, respectively. Stable isotopic values sharply decreased from upstream to downstream, indicating the damming effects. δ18O-NO3- and δ15N NO3- confirm that sewage effluents, nitrification of soil organic material, and NH4+ fertilizers were the primary sources of NO3- in the reservoir. Carbonate weathering mainly provides ions to the reservoir. HCO3- + SO42- and Ca2+ + Mg2+ represent 90% of major ions in the TGR. Downstream sampling sites showed low solute concentration during the recharge period, indicating the dam effect on solute concentration. Ca-Mg-Cl-, Ca-HCO3- and Ca-Cl- were the main water types in the TGR. The average percentage of solutes contribution revealed the carbonate weathering, evaporites dissolution, silicate weathering, and atmospheric input were 51.9%, 41%, 7.8%, and 1.7% for the recharge period. In contrast, the discharge period contributed 66.4%, 29.2%, 10%, and 4.3%, respectively. TGR water is moderately suitable for irrigation, and hardness is high in drinking water. This study provides new insight into the dual isotopic approach and geochemical signatures to interpret the NO3- cycle and chemical weathering process under dam effects in the TGR. However, this isotopic application has some limitations in source identification, isotope fractionation, and transformation mechanisms of nitrate. Thus, further studies need to be done on these topics for a better undestanding.

Reservoir CO2 evasion flux and controlling factors of carbon species traced by δ13CDIC at different regulating phases of a hydro-power dam.

As the world largest hydropower reservoir, the Three Gorges Reservoir (TGR) significantly impacted on the carbon cycle since reservoirs are sources of carbon sink. This study was carried out to investigate the effects of damming on the carbon cycle. δ13CDIC and δ13CDOC were used to trace the origin of dissolved organic (DOC) and inorganic carbon (DIC). The estimated CO2 evasion flux in two regulating phases (discharge and recharge) with averages of 111 mg/m2 h and 264 mg/m2 h, respectively. At the basin scale, average CO2 flux was about 188 mg/m2 h and varies from -158 mg/m2 h to 1092 mg/m2 h. The highest average pCO2 (1294 ppmv) was observed during the discharge period, which was oversaturated than atmospheric equilibrium value; hence, the TGR act as a considerable sink of atmospheric carbon. The δ13CDIC varies from -8.95‰ to 0.00‰ with mean -1.87‰; these enrich isotope values indicated that metabolic process (photosynthesis and respiration) and the rapid kinetics of carbonate weathering by soil CO2 control the pCO2. The low pCO2 of reservoir water caused the rapid dissolution of CO2 from the atmosphere during the recharge period. The δ13CDOC varies between -30.64‰ to -23.05‰, which is similar to the values of C3 vegetation; thus, the source of DOC would be the degradation of soil organic matter. Overall, this study revealed the δ13CDIC signature coupled with soil CO2 dissolution and admixture of atmospherically equilibrated waters resulting in the sink of atmospheric CO2 of the reservoir and impoundment of the dam alters the carbon cycle and aquatic carbon budget in TGR. The findings of this study provide a global image on the contribution of reservoirs to the carbon cycle and aquatic carbon budget. Coupling with isotope signatures and elemental concentrations, investigation of the biogeochemical cycle of the carbon can be effectively traced.

Tracing controlling factors of riverine chemistry in a headwater tributary of the Yangtze River, China, inferred from geochemical and stable isotopic signatures.

The Jialing River is the second largest headwater tributary of the Yangtze River in China, therefore, the river water has been contaminated and water quality is deteriorated. Hence, this study aims to find the main controling factors of riverine chemistry. 52 water samples were collected for the determination of major ions and environmental isotopes of δ18O and δ2H. Stoichiometry of geochemical data with mixing end members and multivariate statistical analysis were employed with integrated GIS approach for data interpretations. The δ18O and δ2H of the Jialing River Basin (JRB) were used to define the origin of river water from meteoric water and water in the spring season is affected by high evaporation and evaporates dissolution. The average TDS 301 mg/L that is higher than the Yangtze River. In the JRB, 80% of the anion in water samples represented HCO3- (207 mg/L) and SO42- (80 mg/L) while 80% of the cations were accounted by Ca2+ (59.8 mg/L) and Mg2+ (17.9 mg/L). The water chemistry mainly derived from the water rock interaction. Piper plot indicated that Ca-Mg-HCO3- was the most dominant water type and most ions derived from carbonate weathering by H2SO4 and H2CO3. The stoichiometry results further confirmed carbonate weathering is dominant than silicate weathering. Evaporate ions were modified by anthropogenic sources. Agricultural inputs are higher than the industry and atmospheric inputs. Redundancy analysis showed that most contributive land-use type in explaining riverine chemistry was the cultivate land (62.6, 66.4, and 67.9%) at all buffer scales of 30, 20, and 10 km, respectively. Forest and grasslands mostly correlate with Ca2+, Mg2+, Cl-, SO42-, EC, pH, and HCO3- while anthropogenic land-use types such as cultivated and construction lands correlate with Na+, K+, Cl-, and NO3-. These results revealed that the lithology of the basin mainly controlled the upstream water chemistry while downstream riverine chemistry was controlled by both lithology and anthropogenic inputs. Nevertheless, this study suggested that explicitly determining the controlling factors of riverine chemistry involves a complex process and combination of different chemical constituents and factors on river water. However, this study managed to provide useful information to further understanding of the geochemical process in JRB.