Distribution characteristics and sea-to-air fluxes of volatile halocarbons in the Changjiang river estuary and its adjacent Marine area in summer

The concentrations of four volatile halocarbons (VHCs) including CFC-11, CH₃I, CH3CCl₃ and C₂Cl₄ in the seawater of the Changjiang River Estuary and its adjacent marine area were determined by purge and trap-chromatography during the summer of 2016. Furthermore, CFC-11, CH₃I, and C2Cl₄ concentrations in the marine atmosphere were also determined. The results showed that the concentrations of four VHCs in the coastal waters were higher than those in the open sea because of the influence of the Changjiang River runoff. In the vertical distributions, the four VHCs had a common feature with the maxima appearing in the 10 m layer. The concentrations of the VHCs were high inside the estuary (Section C) and low outside (Section A6). The distributions of the VHCs were influenced by water mass, biological production and terrestrial input. No correlation was observed between CH₃I and chlorophyll a (Chl a) concentrations, while CFC-11 had significant positive correlations with CH₃I and C₂Cl₄(p<0.01), indicating that the influence of anthropogenic sources on CH₃I was greater than that of natural sources. In the atmosphere, the concentration distributions of CFC-11, CH₃I and C₂Cl₄ showed a downtrend from inshore to offshore regions. The concentration of CFC-11 was lower than the global average, indicating that the emission of CFC-11 in China has been effectively controlled. The results of backward trajectory showed that the diffusion and transport of land-based pollutants from the nearshore was an important source of three kinds of VHCs in the marine atmosphere. The mean sea-to-air flux of CFC-11, CH₃I and C₂Cl₄ were respectively 24.99 nmol/(m²·d), 7.80 nmol/(m²·d), 1.55 nmol/(m²·d), which indicated that the Changjiang River Estuary and its adjacent marine area acted as a source of atmospheric CFC-11, CH₃I and C₂Cl₄ in summer.