Distributions of C2–C6 hydrocarbons over the western North Pacific and eastern Indian Ocean

Atmospheric nonmethane hydrocarbons (NMHCs: C2–C6) were measured over the western North Pacific and eastern Indian Ocean between 25°N and 40°S during oceanographic cruise from December 1996 to February 1997 using a fused-silica-lined stainless-steel canister and GC technique. Averaged mixing ratios of individual NMHCs were 0.61 ppbv (ethane), 0.42 ppbv (ethylene), 0.17 ppbv (acetylene), 0.24 ppbv (propane), 0.60 ppbv (propylene), 0.07 ppbv (i-butane), 0.13 ppbv (n-butane), 0.04 ppbv (i-pentane), 0.06 ppbv (n-pentane), and 0.06 ppbv (n-hexane). Although there are few reported data available for comparison in these regions of marine boundary layer, mixing ratios of the NMHCs are within the range of previous results reported in the similar latitudes from the Atlantic and Pacific Ocean. NMHCs with lifetimes more than a week (C2–C4 alkanes and acetylene) showed a significant latitudinal decrease from north to south in the Northern Hemisphere, suggesting an important source strength in East Asian region. In contrast, NMHCs with lifetimes less than a week (alkenes and C5–C6 alkanes) did not show any significant latitudinal trends. Based on a comparison with the results of the backward trajectories for 2 days, light alkenes were suggested to be derived from ocean surfaces.