Spatial distributions and sea-to-air fluxes of non-methane hydrocarbons in the atmosphere and seawater of the Western Pacific Ocean.

During an oceanographic campaign in the western Pacific Ocean from 12 August to 3 October 2014, the concentrations of five non-methane hydrocarbons (NMHCs) were measured in marine atmosphere and seawater. The average mixing ratios of ethane, ethylene, propane, propylene, and isoprene were 1.109 ± 0.359, 0.658 ± 0.137, 0.711 ± 0.377, 0.429 ± 0.139, and 0.255 ± 0.201 ppbv, respectively. In general, atmospheric concentrations of ethane and propane showed significant decrease from the inshore area to the open ocean, while ethylene and propylene exhibited decreasing trends from low latitudes to high latitudes. The results suggested that atmospheric ethane and propane with long lifetimes were more likely influenced by the air mass transported from continent, while ethylene, propylene and isoprene with short lifetimes were mainly derived from the surface seawater. The average concentrations of ethane, ethylene, propane, propylene, and isoprene in the surface seawater were 6.6 ± 5.8, 51.9 ± 23.5, 15.4 ± 4.3, 17.2 ± 3.8, and 23.5 ± 8.6 pmol L, respectively. A significant positive correlation was observed between ethane and propane (R = 0.45, n = 39, p < 0.001), implying that their production and removal pathways in the surface seawater were similar. High concentrations of isoprene were observed in the waters with high Chl-a values, suggesting that the biological process was a controlling factor. The estimated sea-to-air fluxes of ethane, propane, ethylene, propylene, and isoprene were in the range of 0.1-24.9, 4.2-235, 1.0-43.8, 1.5-90.2, and 2.1-149 nmol m d, respectively. This study is of great importance to the contribution to the atmospheric NMHCs from the western Pacific Ocean and provides data supporting for global NMHCs emission estimates.