Stable carbon isotope ratios of ethane over the North Pacific: Atmospheric measurements and global chemical transport modeling

The atmospheric mixing ratios of ethane and its stable carbon isotope ratios (delta C-13) were measured over the North Pacific (2 degrees N to 38 degrees N, 140 degrees E to 90 degrees W) during oceanographic cruises in summer and autumn. The measured mixing ratios were relatively low (mostly < 1 ppbv) over the North Pacific, whereas elevated ethane levels (> 1 ppbv) were observed over the western North Pacific near Japan, with lower delta C-13 values (approximately -25 parts per thousand), suggesting recent emissions from neighboring source regions. The most C-13-enriched values of ethane (approximately -16 parts per thousand) were observed over the western equatorial Pacific rather than the central and eastern equatorial Pacific. This is likely caused by the kinetic isotope effect (KIE) for the removal of ethane during the atmospheric transport from potential upwind source regions to the most remote region under the prevailing trade easterly winds. The measurements were compared with the results of a global chemical transport model including two ethane isotopologues ((C2H6)-C-12 and (C2H6)-C-13). The model-estimated delta C-13 values were too high compared with the observations. It is likely that this discrepancy is partly due to an approximately 40% overestimation of the reported KIE for the reaction between ethane and OH radicals.