Atmospheric aerosol deposition and its potential influence on particulate organic carbon in the South China Sea.

Atmospheric aerosol inputs represent a potential external forcing influencing marine biogeochemical processes in marginal seas. In the monsoon-dominated South China Sea (SCS), long-range transported aerosols may provide a pathway linking atmospheric variability with surface ocean carbon dynamics. However, the potential role of transboundary aerosol inputs in regulating particulate organic carbon (POC) variability remains unclear. We analyze aerosol optical depth (AOD) over the SCS during 2003-2024 using satellite observations. An integrated framework combining trend analysis, backward trajectory modeling, potential source contribution function (PSCF), and concentration weighted trajectory (CWT) was applied to identify aerosol transport pathways and potential source regions. Machine learning methods were further used to determine the dominant environmental drivers controlling aerosol variability. Results show strong monsoon control on aerosol transport. The northeast monsoon brings mixed continental and marine air masses, while marine air masses dominate during the southwest monsoon. The gradient boosting decision tree (GBDT) model achieved strong predictive performance (R2 = 0.81), identifying wind speed (WS) as the most important factor, accounting for approximately 50% of the total feature importance. Atmospheric inorganic nitrogen deposition was estimated to provide approximately 66 mmol N m-2 yr-1, potentially supporting 19.9-31.7% of the nitrogen demand associated with new primary production in the SCS. These results highlight the importance of atmosphere-ocean coupling and transboundary atmospheric inputs in regulating marine carbon dynamics in monsoon-influenced marginal seas.