Spatial and temporal distribution and sea-to-air fluxes of volatile halocarbons in the East China Sea in spring and autumn

The production and release of volatile halogenated hydrocarbons (VHCs) in the ocean play a crucial role in regulating global climate. The extensive voyage surveys in the East China Sea were conducted on board the R/V “Xiangyanghong 18” from April 1 to 19, 2021 and on board “Dongfanghong 3” from October 9 to 30, 2021. This study conducted the temporal and spatial distributions of 4 VHCs including Trichlorofluoromethane (CFC-11), bromomethane (CH₃Br), iodomethane (CH₃I), and dibromomethane (CH₂Br₂) in the seawater and atmosphere of the East China Sea in spring and autumn, and the environmental parameters affecting their concentrations were also studied. The distribution of VHCs in seawater and atmosphere and the sea-to-air fluxes were compared seasonally. The results showed that the distributions of VHCs in the atmosphere and seawater exhibited significant seasonal changes, which were influenced by the source of air mass, sea surface temperature, water mass, seasonal differences in biological release, and river input. The high concentrations of VHCs in surface seawater mainly appeared in the coastal waters of Fujian and Zhejiang provinces and some stations in the open sea. The distribution of CFC-11 was mainly affected by oil and gas production platform and terrigenous air mass transportation. Phytoplankton release was the main source of CH₂Br₂ in autumn surface seawater, while CH₃I was mainly affected by river input and sea surface temperature. Atmospheric CH₃Br and CH₃I showed a significant positive correlation in both seasons. The mean sea-to-air fluxes of CFC-11, CH₃Br, CH₃I and CH₂Br₂ in spring were −535.17, 10.17, 16.69 and 24.18 nmol/(m²∙d), respectively, while those in autumn were −1621.03, 41.96, 94.32 and 57.23 nmol/(m²∙d). These results indicated that the East China Sea was the sink of CFC-11 in the atmosphere but the source of CH₃Br, CH₃I and CH₂Br₂. Lower sea temperature led to higher sea-to-air flux of CFC-11 in spring than in autumn, and higher wind speed and surface seawater concentration were the main reasons for the higher sea-to-air fluxes of CH₃Br, CH₃I and CH₂Br₂ in autumn than in spring.