Impact of air-sea exchange on the spatial distribution of atmospheric methane in the Dalian Bay and adjacent coastal area, China

Abstract To date, the impact of air-sea exchange on spatial distribution of atmospheric methane (CH4 hereafter) remains less understood in the coastal areas of China. Here we measured the dissolved and atmospheric CH4 in the Dalian Bay and adjacent area in July and August 2014, respectively. Results showed that the study area was a net significant source of atmospheric CH4, with a mean sea-to-air CH4 flux of 170.6 ± 149.5 μmol/(m2·day). We optimized a method to accurately quantify the elevated atmospheric CH4 mole fraction (△CH4 hereafter) caused by air-sea exchange. The calculated △CH4 in the study area ranged from 15.4 to 102.1 nmol/mol, 1.5–10.2 nmol/mol, and 0.03–0.22 nmol/mol at the mixing height of 1, 10, and 471 m, respectively. The △CH4 mole fractions caused by air-sea exchange were positive with sea-to-air CH4 flux and in situ observed atmospheric CH4 mole fraction, while negative with altitude. Under the standard conditions, we defined 50.8 μmol/(m2·day) as the criteria value of sea-to-air CH4 flux which could result in a detectable elevation atmospheric CH4 mole fraction at a height of 10 m.