Influence of Human Activity on Wet Deposition and Nitrogen Species in Coastal and Marine Environments

In this study, 101 and 102 rainwater samples were collected from April 2020 to February 2024 in a coastal and marine area, respectively. The results show that the Cl/Na ratios in both study areas were lower than the seawater value (1.17), suggesting chloride depletion. The chloride depletion rates in both areas decreased after the COVID-19 lockdown period. The molar ratio of NH4+ to non-sea-salt sulfate (nss-SO42−) was 1.54, with a mixture of (NH4)2SO4 and NH4HSO4 in the coastal area, and 0.83, with NH4HSO4 as the main form, in the marine area. A decreasing trend attributed to high O3 and relative humidity (RH) levels occurred in 2022. Among the dissolved organic nitrogen (DON) species, DON accounted for 24% and 32% of the total dissolved nitrogen (TDN) in the coastal and marine areas, respectively, indicating a greater relative contribution under lower anthropogenic influence. On the basis of the correlation between the species and source analysis results, NO3− mainly originated from fossil fuel combustion, NH4+ originated from agricultural emissions and secondary aerosols, and DON originated from secondary aerosols via combustion processes and natural emissions. In terms of the flux, due to lockdown activities, the Dissolved Inorganic Nitrogen (DIN) flux decreased significantly from 40.6 to 19.3 mmol m−2 yr−1 in the coastal area and from 27.7 to 15.1 mmol m−2 yr−1 in the marine area. Additionally, a slight decrease occurred in the DON flux, from 21.6 to 19.3 mmol m−2 yr−1 and from 27.7 to 15.1 mmol m−2 yr−1, respectively. Regarding new production, based on nitrogen input, the level in the coastal area decreased from 5.83 to 2.10 g C m−2 yr−1, and that in the marine area decreased from 3.92 to 1.55 g C m−2 yr−1, indicating a significant reduction during the COVID-19 lockdown.