Spatiotemporal distribution and environmental control factors of halocarbons in the Yangtze River Estuary and its adjacent marine area during autumn and spring.

The oceanic production and release of volatile halocarbons (VHCs) to the atmosphere play a vital role in regulating the global climate. In this study, seasonal and spatial variations in VHCs, including trichlorofluoromethane (CFC-11), methyl iodide (CHI), dibromomethane (CHBr), and bromoform (CHBr), and environmental parameters affecting their concentrations were characterized in the atmosphere and seawater of the Yangtze River Estuary and its adjacent marine area during two cruises from October 17 to October 26, 2019 and from May 12 to May 25, 2020. Significant seasonal variations were observed in the atmosphere and seawater because of seasonal differences in the prevalent monsoon, water mass (Yangtze River Diluted Water), and biogenic production. VHCs concentrations were positively correlated with Chl-a concentrations in the surface water during autumn. The average sea-to-air fluxes of CHI, CHBr and CHBr in autumn were 19.7, 4.0, and 7.6 nmol m d, respectively, while those in spring were 6.3, 6.4, and -3.6 nmol m d. In the ship-based incubation experiments, ocean acidification and dust deposition had no significant effects on VHCs concentrations. The concentrations of CHBr and CHBr were significantly positively correlated with phytoplankton biomass under lower pH condition (M3: pH 7.9). This result indicated that CHBr and CHBr concentrations were mainly related to the biological release.