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

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.