Volatile halocarbons in the marine atmosphere and surface seawater: Diurnal and spatial variations and influences of environmental factors

Abstract

The concentrations of trichlorofluoromethane (CFC-11), methyl iodide (CH3I), dibromomethane (CH2Br2), bromodichloromethane (CHBrCl2), dibromochloromethane (CHBr2Cl), and bromoform (CHBr3) were determined in the marine atmosphere and surface seawater of the East China Sea (ECS) and the South Yellow Sea (SYS) between March and April 2017. Mean mixing ratios (range) of CFC-11, CH3I, CH2Br2, CHBrCl2, CHBr2Cl, and CHBr3 were 192.6 (179.4–226.9), 2.0 (0.7–4.5), 1.0 (0.3–8.0), 0.8 (0.2–5.0), 0.7 (0.2–3.0), and 5.6 (1.1–25.3) pptv, respectively. Anthropogenic activities contributed to elevated atmospheric concentrations of CFC-11, while a combination of biological processes in the seawater and terrestrial sources contributed to elevated atmospheric concentrations of CH3I and bromoalkanes. Mean (range) concentrations of CFC-11, CH3I, CH2Br2, CHBrCl2, CHBr2Cl, and CHBr3 in the surface seawater were 9.47 (4.43–21.28), 4.65 (3.70–6.05), 12.07 (9.63–31.07), 5.45 (1.06–30.04), 13.51 (4.36–54.29), and 15.80 (1.21–34.51) pmol L−1, respectively. Productions of CH3I and bromoalkanes in the surface seawater might be influenced by phytoplankton, picophytoplankton, heterotrophic bacteria, dissolved organic carbon, and nutrients. CFC-11 in the atmosphere and seawater showed little diurnal variation. Compared with weak variation in the seawater, CH3I in the atmosphere showed a significant diurnal variation with a range of 1.2–5.6 pptv. The variation of bromoalkanes concentrations in the atmosphere corresponded to the water concentrations, with the highest value occurring at mid-day. In the current study, the emissions of CH3I, CH2Br2, and CHBr3 from the ECS and the SYS accounted for about 1.1%, 1.8%, and 0.01% of the annual global ocean emissions, respectively, highlighting the importance of the coastal areas to the reactive bromine and iodine in the atmosphere.