Methane emission via sediment and water interface in the Bohai Sea, China

Sediment is recognized as the largest reservoir and source of methane (CH₄) in the ocean, especially in the shallow coastal areas. To date, few data of CH₄ concentration in sediment have been reported in the China shelf seas. In this study, we measured CH₄ concentration in sediment and overlying seawater columns, and conducted an incubation experiment in the Bohai Sea in May 2017. CH₄ concentration was found to be ranged from 3.075 to 1.795 μmol/L in sediment, which was 2 to 3 orders of magnitude higher than that in overlying seawater columns. The surface sediment was an important source of CH₄, while bottom seawater acted as its sink. Furthermore, the net emission rate via sediment water interface (SWI) was calculated as 2.45 μmol/(m²∙day) based on the incubation experiment at station 73, and the earthquake may enhance CH₄ release from sediment to seawater column in the eastern Bohai Sea.     

Spatial and seasonal variability of CO2 flux at the air-water interface of the Three Gorges Reservoir

Difusive carbon dioxide(CO2) emissions from the water surface of the Three Gorges Reservoir, currently the largest hydroelectric reservoir in the world, were measured using floating static chambers over the course of a yearlong survey. The results showed that the average annual CO2 flux was(163.3 ± 117.4) mg CO2/(m2·hr) at the reservoir surface, which was larger than the CO2 flux in most boreal and temperate reservoirs but lower than that in tropical reservoirs. Significant spatial variations in CO2 flux were observed at four measured sites, with the largest flux measured at Wushan(221.9 mg CO2/(m2·hr)) and the smallest flux measured at Zigui(88.6 mg CO2/(m2·hr)); these diferences were probably related to the average water velocities at diferent sites. Seasonal variations in CO2 flux were also observed at four sites, starting to increase in January, continuously rising until peaking in the summer(June-August) and gradually decreasing thereafter. Seasonal variations in CO2 flux could reflect seasonal dynamics in pH, water velocity,and temperature. Since the spatial and temporal variations in CO2 flux were significant and dependent on multiple physical, chemical,and hydrological factors, it is suggested that long-term measurements should be made on a large spatial scale to assess the climatic influence of hydropower in China, as well as the rest of the world.     

Temporal and spatial changes in nutrients and chlorophyll-a in a shallow lake, Lake Chaohu, China: An 11-year investigation

Temporal and spatial changes of total nitrogen (TN), total phosphorus (TP) and chlorophyll-a (Chl-a) in a shallow lake, Lake Chaohu, China, were investigated using monthly monitoring data from 2001 through 2011. The results showed that the annual mean concentration ranges of TN, TP, and Chl-a were 0.08-14.60 mg/L, 0.02-1.08 mg/L, and 0.10-465.90 μg/L, respectively. Our data showed that Lake Chaohu was highly eutrophic and that water quality showed no substantial improvement during 2001 through 2011. The mean concentrations of TP, TN and Chl-a in the western lake were significantly higher than in the eastern lake, which indicates a spatial distribution of the three water parameters. The annual mean ratio of TN:TP by weight ranged from 10 to 20, indicating that phosphorus was the limiting nutrient in this lake. A similar seasonality variation for TP and Chl-a was observed. Riverine TP and NH₄⁺ loading from eight major tributaries were in the range of 1.56×10⁴-5.47×10⁴ and 0.19×10⁴-0.51×10⁴ tons/yr over 2002-2011, respectively, and exceeded the water environmental capability of the two nutrients in the lake by a factor of 3-6. Thus reduction of nutrient loading in the sub-watershed and tributaries would be essential for the restoration of Lake Chaohu.     

Rural vehicle emission as an important driver for the variations of summertime tropospheric ozone in the Beijing-Tianjin-Hebei region during 2014–2019

Tropospheric ozone (O₃) pollution is increasing in the Beijing-Tianjin-Hebei (BTH) region despite a significant decline in atmospheric fine aerosol particles (PM_2.5) in recent years. However, the intrinsic reason for the elevation of the regional O₃ is still unclear. In this study, we analyzed the spatio-temporal variations of tropospheric O₃ and relevant pollutants (PM_2.5, NO₂, and CO) in the BTH region based on monitoring data from the China Ministry of Ecology and Environment during the period of 2014–2019. The results showed that summertime O₃ concentrations were constant in Beijing (BJ, 0.06 µg/(m³•year)) but increased significantly in Tianjin (TJ, 9.09 µg/(m³•year)) and Hebei (HB, 6.06 µg/(m³•year)). Distinct O₃ trends between Beijing and other cities in BTH could not be attributed to the significant decrease in PM_2.5 (from -5.08 to -6.32 µg/(m³•year)) and CO (from -0.053 to -0.090 mg/(m³•year)) because their decreasing rates were approximately the same in all the cities. The relatively stable O₃ concentrations during the investigating period in BJ may be attributed to a faster decreasing rate of NO₂ (BJ: -2.55 µg/(m³•year); TJ: -1.16 µg/(m³•year); HB: -1.34 µg/(m³•year)), indicating that the continued reduction of NO_x will be an effective mitigation strategy for reducing regional O₃ pollution. Significant positive correlations were found between daily maximum 8 hr average (MDA8) O₃ concentrations and vehicle population and highway freight transportation in HB. Therefore, we speculate that the increase in rural NO_x emissions due to the increase in vehicle emissions in the vast rural areas around HB greatly accelerates regional O₃ formation, accounting for the significant increasing trends of O₃ in HB.     

Methane and nitrous oxide emissions from a subtropical coastal embayment (Moreton Bay, Australia)

Surface water methane (CH₄) and nitrous oxide (N₂O) concentrations and fluxes were investigated in two subtropical coastal embayments (Bramble Bay and Deception Bay, which are part of the greater Moreton Bay, Australia). Measurements were done at 23 stations in seven campaigns covering different seasons during 2010-2012. Water-air fluxes were estimated using the Thin Boundary Layer approach with a combination of wind and currents-based models for the estimation of the gas transfer velocities. The two bays were strong sources of both CH₄ and N₂O with no significant differences in the degree of saturation of both gases between them during all measurement campaigns. Both CH₄ and N₂O concentrations had strong temporal but minimal spatial variability in both bays. During the seven seasons, CH₄ varied between 500% and 4000% saturation while N₂O varied between 128 and 255% in the two bays. Average seasonal CH₄ fluxes for the two bays varied between 0.5 ± 0.2 and 6.0 ± 1.5 mg CH₄/(m²·day) while N₂O varied between 0.4 ± 0.1 and 1.6 ± 0.6 mg N₂O/(m²·day). Weighted emissions (t CO₂-e) were 63%-90% N₂O dominated implying that a reduction in N₂O inputs and/or nitrogen availability in the bays may significantly reduce the bays' greenhouse gas (GHG) budget. Emissions data for tropical and subtropical systems is still scarce. This work found subtropical bays to be significant aquatic sources of both CH₄ and N₂O and puts the estimated fluxes into the global context with measurements done from other climatic regions.     

Turnover of dissolved organic carbon fuels nocturnal CO2 emissions from a headwater catchment reservoir, Southeastern China: Effects of ecosystem metabolism on source partitioning of CO2 emissions

Dam reservoirs in headwater catchments, as critical zones for their proximity to terrestrial sources, play important roles in dissolved organic carbon (DOC) cycling. However, the effects of ecosystem metabolism (EM) on DOC cycling are not well known. Here, in-situ diurnal and monthly observations were conducted to measure EM (including gross primary production (GPP), ecosystem respiration (ER) and heterotrophic respiration (HR)), DOC turnover and CO₂ emissions in a headwater catchment reservoir in Southeastern China in 2020. Our study showed the nocturnal CO₂ emission rate was about twice as high as in daytime, and was strongly driven by EM. The values for DOC turnover velocity ranged from 0.10 to 1.59 m/day, and the average DOC turnover rate was 0.13 day⁻¹, with the average removal efficiency of 12%. The contribution of respired DOC to daily CO₂ emissions ranged from 17% to 61%. The accumulated efficiencies were estimated to be 13% for the selected 15 reservoirs throughout the Changjiang River network, corresponding to about 0.34 Tg C/year of the respired DOC. The modified CO₂ flux was 0.75 Tg C/year, and respired DOC accounted for about 45% of total emitted CO₂ from the 15 larger reservoirs. Our research emphasizes the necessity of incorporating the effects of EM into studies of reservoir DOC removal and CO₂ emissions.     

Methane reduction of a perennially waterlogged rice paddy

The ridge cultivation has great potential for reducing CH₄ emissions from the perennially waterlogged rice paddies．This was strongly supported by the data from our field experiments carried out during an entire rice growing season of 1992．Compared with the normal cultivation，the ridge cultivation reduced 30．8%more CH₄ emissions，and it did not show any negative effects on rice productivity．All of these suggest that in the regions wit h a vast areas of perennially waterlogged paddies，ridge cultivation should be a very promising option for both CH₄ reduction and sustainable rice productivity．     

Integrated biological–physical process for biogas purification effluent treatment

Biogas purification via water scrubbing produces effluent containing dissolved CH₄, H₂S, and CO₂, which should be removed to reduce greenhouse gas emissions and increase its potential for water regeneration. In this study, a reactor built with air supplies at the top and bottom was utilized for the treatment of biogas purification effluent through biological oxidation and physical stripping processes. Up to 98% of CH₄ was removed through biological treatment at a hydraulic retention time of 2 hr and an upper airflow rate of 2.02?L/day. Additionally, a minimum CH₄ concentration of 0.04% with no trace of H₂S gas was detected in the off gas. Meanwhile, a white precipitate was captured on the carrier showing the formation of sulfur. According to the developed mathematical model, an upper airflow rate of greater than 2.02?L/day showed a small deterioration in CH₄ removal performance after reaching the maximum value, whereas a 50?L/day bottom airflow rate was required to strip the CO₂ efficiently and raise the effluent pH from 5.64 to 7.3. Microbiological analysis confirmed the presence of type 1 methanotroph communities dominated by Methylobacter and Methylocaldum. However, bacterial communities promoting sulfide oxidation were dominated by Hyphomicrobium.     

Metals in sediment/pore water in Chaohu Lake: Distribution, trends and flux

Nine metals, Cd, Cu, Ni, Pb, As, Cr, Zn, Fe, and Mn in sediment and pore water from 57 sampling sites in Chaohu Lake (Anhui Province, China) were analyzed for spatial distribution, temporal trends and diffuse flux in 2010. Metals in the surface sediment were generally the highest in the western lake center and Nanfei-Dianbu River estuary, with another higher area of As, Fe, and Mn occurring in the Qiyang River estuary. Metal contamination assessment using the New York sediment screening criteria showed that the sediment was severely contaminated in 44% of the area with Mn, 20% with Zn, 16% with Fe, 14% with As, and 6% with Cr and Ni. An increasing trend of toxic metals (Cd, Cu, Ni, Pb, As, Cr, Zn) and Mn with depth was shown in the western lake. Compared with metal content data from the sediment survey conducted in 1980s, the metal content of surface sediment in 2010 was 2.0 times that in the 1980s for Cr, Cu, Zn, and As in the western lake, and less than 1.5 times higher for most of the metals in the eastern lake. Among the metals, only Mn and As had a widespread positive diffuse flux from the pore water to overlying water across the whole lake. The estimated flux in the whole lake was on average 3.36 mg/(m². day) for Mn and 0.08 mg/(m². day) for As, which indicated a daily increase of 0.93 μg/L for Mn and 0.02 μg/L for As in surface water. The increasing concentration of metals in the sediment and the flux of metals from pore water to overlying water by diffusion and other physical processes should not be ignored for drinking-water sources.     

Chemical characterization and source identification of PM2.5 in Luoyang after the clean air actions

Luoyang is a typical heavy industrial city in China, with a coal-dominated energy structure and serious air pollution. Following the implementation of the clean air actions, the physicochemical characteristics and sources of PM_2.5 have changed. A comprehensive study of PM_2.5 was conducted from October 16, 2019 to January 23, 2020 to evaluate the effectiveness of previous control measures and further to provide theory basis for more effective policies in the future. Results showed that the aerosol pollution in Luoyang in autumn and winter is still serious with the average concentration of 91.1 μg/m³, although a large reduction (46.9%) since 2014. With the contribution of nitrate increased from 12.5% to 25.1% and sulfate decreased from 16.7% to 11.2%, aerosol pollution has changed from sulfate-dominate to nitrate-dominate. High NO₃⁻/SO₄²⁻ ratio and the increasing of NO₃⁻/SO₄²⁻ ratio with the aggravation of pollution indicating vehicle exhaust playing an increasingly important role in PM_2.5 pollution in Luoyang, especially in the haze processes. Secondary inorganic ions contributed significantly to the enhancement of PM_2.5 during the pollution period. The high value of Cl⁻/Na⁺ and EC concentration indicate coal combustion in Luoyang is still serious. The top three contributor sources were secondary inorganic aerosols (33.3%), coal combustion (13.6%), and industrial emissions (13.4%). Close-range transport from the western and northeastern directions were more important factors in air pollution in Luoyang during the sampling period. It is necessary to strengthen the control of coal combustion and reduce vehicle emissions in future policies.