Spatial and temporal distribution characteristics of ground-level nitrogen dioxide and ozone across China during 2015–2020

In recent years, the emissions control in nitrogen oxides (NO _x ) was conducted across China, but how the concentrations of NO _x and its product ozone (O _3 ) in the atmosphere varied in space and time remains uncertain. Here, the spatial and temporal distributions of nitrogen dioxide (NO _2 ) and O _3 in 348 cities of China based on the hourly concentrations data during 2015–2020 were investigated, and the relationships among NO _2 , O _3 and meteorological and socioeconomic parameters were explored. It is shown that higher NO _2 and O _3 concentrations were mainly distributed in North, East and Central China, which are economically developed and densely populated regions. The annual mean concentrations of NO _2 increased from 2015 to 2017 but decreased from 2017 to 2020. The annual variations in O _3 generally exhibited an upward trend in 2015–2019 but decreased by 5% from 2019 to 2020. About 74% and 78% of cities had a decline in NO _2 and O _3 in 2020, respectively, compared to 2019, due to the limits of the motorized transports and industrial production activities during COVID-19 lockdown. The monthly mean concentrations of NO _2 showed an unusual decrease in February in all regions due to the reduced emissions during the Chinese Spring Festival holidays. Compared to 2019, the mean concentrations of NO _2 in January, February and March, 2020 during COVID-19 lockdown decreased by 16%, 28% and 20%, respectively; O _3 increased by 13% and 14% in January and February, respectively, but decreased by 2% in March, 2020. NO _2 and O _3 concentrations are likely associated with anthropogenic and natural emissions. In addition, meteorological parameters can affect NO _2 and O _3 concentrations by influencing the production process, the diffusion and local accumulation, and the regional circulations

Spatio-Temporal Distribution of Total Nitrogen and Phosphorus in Dianshan Lake, China: The External Loading and Self-Purification Capability

In this article, long-term data, statistical analysis, and spatial interpolation method were applied to the analyses of the spatial and temporal changes of total nitrogen (TN) and total phosphorus (TP) in Dianshan Lake. We also estimated the self-purification capability of TN and TP in Dianshan Lake. The results showed that interannual variability of the average concentration of TN in Dianshan Lake changed significantly, showing a characteristic increase before a decline, and the average concentration of TN showed an obvious downward trend, especially after 2007. Interannual variability of the average concentration of TP in Dianshan Lake fluctuated, and the average concentration of TP showed a downward trend after 2007. The seasonal variations of TN and TP in Dianshan Lake were similar. Higher TN concentration occurred in winter and spring, while higher TP concentration appeared in summer, autumn, and winter. The spatial distribution of TN and TP in Dianshan Lake were similar, showing a characteristic which decreased from north to south and west to east. The highest TN and TP values were mainly distributed in the inlet monitoring sites, while the lowest TP values were distributed in the outlet monitoring sites. The self-purification capability of TN and TP were about 2289.97 t/yr and 112.16 t/yr, which suggested a deterioration of natural water quality. Our research showed that Dianshan Lake was highly eutrophic and that water quality showed a substantial improvement from 1996 to 2015

Methane Emissions during the Tide Cycle of a Yangtze Estuary Salt Marsh

Methane (CH4) emissions from estuarine wetlands were proved to be influenced by tide movement and inundation conditions notably in many previous studies. Although there have been several researches focusing on the seasonal or annual CH4 emissions, the short-term CH4 emissions during the tide cycles were rarely studied up to now in this area. In order to investigate the CH4 emission pattern during a tide cycle in Yangtze Estuary salt marshes, frequent fixed-point observations of methane flux were carried out using the in-situ static closed chamber technique. The results indicated that the daily average CH4 fluxes varied from 0.68 mgCH4·m−2·h−1 to 4.22 mgCH4·m−2·h−1 with the average flux reaching 1.78 mgCH4·m−2·h−1 from small tide to spring tide in summer. CH4 fluxes did not show consistent variation with both tide levels and inundation time but increased steadily during almost the whole research period. By Pearson correlation analysis, CH4 fluxes were not correlated with both tide levels (R = −0.014, p = 0.979) and solar radiation (R = 0.024, p = 0.865), but significantly correlated with ambient temperature. It is temperature rather than the tide level mainly controlling CH4 emissions during the tide cycles. Besides, CH4 fluxes also showed no significant correlation with the underground pore-water CH4 concentrations, indicating that plant-mediated transport played a more important role in CH4 fluxes compared with its production and consumption

Role of Scirpus mariqueter on Methane Emission from an Intertidal Saltmarsh of Yangtze Estuary

The role of wetland plant (Scirpus mariqueter) on methane (CH4) emissions from a subtropical tidal saltmarsh of Yangtze estuary was investigated over a year. Monthly CH4 flux and pore-water CH4 concentration were characterized using static closed chamber technique and pore-water extraction. Measured chamber CH4 fluxes indicated that saltmarsh of the Yangtze estuary acted as a net source of atmospheric CH4 with annual average flux of 24.0 mgCH4·m−2·day−1. The maximum chamber CH4 flux was in August (91.2 mgCH4·m−2·day−1), whereas the minimum was observed in March (2.30 mgCH4·m−2·day−1). Calculated diffusion CH4 fluxes were generally less than 6% of the chamber fluxes. Significant correlations were observed between the chamber CH4 flux and rhizospheric pore-water CH4 concentration (11–15 cm: p < 0.05, R = 0.732; 16–20 cm: p < 0.05, R = 0.777). In addition, chamber CH4 fluxes from July to September constituted more than 80% of the total annual emission and were closely correlated with aboveground biomass yield of S. mariqueter. The results indicated that S. mariqueter transportation was the dominant CH4 emission pathway and it provided an efficient route for the belowground CH4 to escape into the atmosphere while avoiding oxidation, leading to CH4 emissions

Quality of Sediment Organic Matter Determines the Intertidal N2O Response to Global Warming

Estuaries and coasts are areas of intense biogeochemical cycling and are sensitive to global climate change. However, the effect of temperature increase on the emissions of the powerful greenhouse gas nitrous oxide (N2O) in different estuarine and coastal areas is still uncertain. In this study, we used laboratory incubation experiments to investigate increasing temperatures (12, 25, and 35 degrees C) and tidal effects on N2O fluxes in intertidal sediments from the East China coast (ECC). Overall, the ECC acts as a net source of atmospheric N2O and exhibited considerable spatial variability over three orders of magnitude (from -0.17 to 8.4 mu mol m(-2) h(-1)). The warming promoted N2O emissions in most intertidal areas, while reducing N2O emissions at some sampling sites. In addition, the overall effect of flooding on N2O emissions changed from a positive to a negative effect with increasing temperature. By combining the sediment properties of all the sampling sites, we found that large differences in N2O emissions at the same amended floodwater nitrogen concentration were due to the quality and quantity of sediment organic matter. Sediment derived mainly from marine sources emitted more N2O than sediment derived from terrestrial sources. This suggests that the mangroves, salt marshes, and intertidal zones of estuaries have a mitigating effect on N2O emissions due to their elevated terrestrial organic matter inputs. This research improves our understanding of the impact of future global climatic changes on intertidal N2O fluxes, which can inform future studies and models and can be used to constrain intertidal N2O emissions

The Carbon Stock and Sequestration Rate in Tidal Flats From Coastal China

Tidal flats form around the estuarine and coastal zone by continuous terrigenous sediment transport and deposition processes. Now a large body of published carbon research work frame within the vegetated area (mangrove forests, sea grass bed, and salt marshes). Nonvegetated tidal flats, which are characterized by predominantly silts and clays sediment, were generally impressed with low carbon stock due to their meager primary productivity. However, these regions may be a potentially important carbon sink, given their high burial rate, expanding areal coverage, and detrial organic carbon derived from watershed and adjacent vegetated area. Low carbon densities (<0.01 g cm(-3)) were found in Chinese tidal flat sediments by the study, but the carbon sequestration rates ranged from 35 to 361 g C m(-2) yr(-1), which were comparable to rates of worldwide vegetated coastal areas. The high rates can be ascribed to rapid sedimentation rates (1-2 cm yr(-1)) during the past several decades. The highest areal carbon stocks were located at tidal flat sites proximal to mangrove forests. The majority of carbon stocks (100 cm) was found in the unvegetated tidal flats instead of in the vegetated tidal flats. The former occupied 87% of the entire tidal area, 6.7 times larger than the latter. Tidal flats in coastal China store 78.07 Tg C (100 cm), accounting for nearly 80% of the C deposited in entire coastal tidal area. The future carbon sequestration rates of Chinese tidal flats are facing uncertainties under the pressures of reduced fluvial sediment loads from major rivers