Vertical distribution of non methane hydrocarbons in the remote marine boundary layer

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Abstract
The marine production and vertical atmospheric distribution of light nonmethane hydrocarbons (NMHC) from C2 to C6 in the marine boundary layer was studied at the Hao atoll, in the intertropical South Pacific. NMHC vertical profiles were performed up to 1600 m altitude by using balloon and remote controlled aircraft‐borne devices simultaneously with surface air and seawater samplings. Seawater was found to be supersaturated in NMHC. Despite the concentration variations of the seawater its relative composition is well reproducible, within a factor of 2. The average NMHC atmospheric composition at sea level is similar to that of seawater. However, the results obtained display a very large short time variability of the NMHC atmospheric concentrations, which seems to result mainly from variations in the marine emission rates and/or advective processes of transport. It was initially expected that the proportions of short‐lived species should rapidly decrease in altitude, because of their chemical decay, mainly by oxidation by OH radicals. However, it appeared that the proportions of the different NMHC measured remained relatively constant up to the maximum altitude sampled during the campaign, i.e., about 1600 m. This suggests that transport processes in the atmosphere were always very rapid, relative to the chemical processes, even for short‐lived NMHC. However, for reactive species, simple calculations based on the hypothesis of a steady state in thin atmospheric layers show that the photochemistry involved is consistent with usual OH concentrations varying from 1.6×107 rad cm−3 at noon to 4×104 rad cm−3 at sunset.