The influences on air-water exchange and water-particle partitioning of polycyclic aromatic hydrocarbons (PAHs) in semi-enclosed gulf are not well understood. In this study, we investigated polycyclic aromatic hydrocarbons (PAHs) at the air-water and water-particle interface in the Beibu Gulf of the South China Sea in combination with measurements and machine learning (ML) predictions. For 3-ring and most of 4- to 6-ring PAHs, volatilization and deposition fluxes were observed, respectively. Generally, PAHs underwent a strong volatilization process in the Beibu Gulf. The presence of low molecular weight (LMW) PAHs in the particulate phase was not stable, they might desorb and release into the water during particle transport. The drastic change of LWM PAHs between seawater dissolved and particulate phase was indicative of its unique hydrological and geographical environment in the Beibu gulf. Results of interpretable ML approach (XGBoost) indicated that volatilization of 3-ring PAHs was significantly controlled by dissolved PAH concentrations, which not only influenced the rate of air-water exchange but also was involved in the exchange direction. The deposition of high molecular weight (HMW) PAHs affected their particulate water partitioning process resulting in the tendency of HMW PAHs to adsorb with the seawater particulate matters. This study highlighted the synergistic influences of air-water exchange and water-particle partitioning on the environmental fate of PAHs in the semi-enclosed gulf.
 

Polycyclic aromatic hydrocarbons (PAHs); Beibu Gulf; Air-water exchange; Water-particle partitioning; Machine learning approach