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

To date, the impact of air-sea exchange on spatial distribution of atmospheric methane (CH 4 hereafter) remains less understood in the coastal areas of China. Here we measured the dissolved and atmospheric CH 4 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 CH 4 , with a mean sea-to-air CH 4 flux of 170.6 ± 149.5 μmol/(m2·day). We optimized a method to accurately quantify the elevated atmospheric CH 4 mole fraction (△CH 4 hereafter) caused by air-sea exchange. The calculated △CH 4 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 △CH 4 mole fractions caused by air-sea exchange were positive with sea-to-air CH 4 flux and in situ observed atmospheric CH 4 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 CH 4 flux which could result in a detectable elevation atmospheric CH 4 mole fraction at a height of 10 m. Image 106245 • Coastline is an intensive ventilated area for CH 4 in the study area. • A method was improved for quantifying the impact of air-sea exchange on atmospheric CH 4. • Air-sea exchange induced atmospheric CH 4 mole fraction decreasing with increasing altitude. • A criterion flux value was defined for studying the impact of air-sea CH 4 exchange. [ABSTRACT FROM AUTHOR]