Stable carbon isotopic compositions of light hydrocarbons over the western North Pacific and implication for their photochemical ages

Abstract

Light hydrocarbons collected over the western North Pacific in canisters during an oceanographic cruise in May 1999 were analyzed for the mixing ratios and the stable carbon isotopic compositions (δ13C). Their mixing ratios decreased during the passage of a warm front. By contrast, they increased after the passage of a cold front. The δ13C values of ethane were found to be in a range of −26 to −19‰ and to be systematically heavier when the mixing ratios (range of 1.7–0.6 ppbv) became lower. The δ13C values of ethane over the open ocean generally showed heavier values than those previously reported in the source regions, suggesting an isotopic fractionation during a long‐range atmospheric transport. In contrast, δ13C values of i‐ and n‐butanes varied in a wide range (−37 to −16‰ and −37 to −21‰, respectively); however, their averaged values are comparative to the isotopic source compositions. Likewise, no systematic relationships were found between the mixing ratios and the δ13C values of the butane isomers. For n‐butane the observed relationship was substantially different from that predicted by kinetic isotope effect by the reaction with OH radicals probably due to the contribution of effective mixing process. From the changes in the δ13C values for ethane and n‐butane the averaged photochemical ages were calculated to be 49 ± 26 days and 2 ± 6 days, respectively. The difference in the estimated ages is attributed to the different reactivity of hydrocarbons. The average ages for n‐butane were found to be consistent with numbers of days, which the air mass took for the transport from the continent to the sampling site over the ocean, on the basis of backward trajectory analysis. This study suggests that the photochemical ages are useful for assessing the importance of recent emission in the marine atmosphere.