Diurnal variation of non-methane hydrocarbons in the subantarctic atmosphere

Abstract

Environmental context. The ocean surface is known to be supersaturated in some non-methane hydrocarbons and particularly alkenes. This oceanic source, though small on a global scale, can be a dominant component of the background atmosphere in remote areas. Attempts have been made to quantify this source, in order to estimate its magnitude in the budgets of these gases in the water column and the atmosphere. A main difficulty is to determine the production processes involved under the effects of plankton activity and solar and UV radiation penetration in the water column. Abstract. Non-methane hydrocarbons (NMHCs) play a key role in the photochemistry of the remote atmosphere. They are oxidised by OH radicals and subsequently lead to a net formation of peroxy radicals, which have a crucial role in the budget of tropospheric ozone. Whereas in polluted areas, the effect of light hydrocarbons results in a net formation of ozone, in non- or low-polluted areas, the self-reaction of peroxy radicals dominates and leads eventually to ozone destruction, which in turn acts significantly on the OH budget. In remote environments, the origin of the NMHC background level is clearly attributed to a marine production, but a great uncertainty still exists about its geographical, seasonal and diurnal variability. Here, we present in situ measurements of NMHCs and particularly of alkenes in subantarctic areas, which show very systematic diurnal trends in agreement with an origin clearly dependent on photochemical processes on the surface seawater. The diurnal variability of alkene atmospheric mixing ratios appears strongly related to solar irradiance at the ocean surface. The magnitude of this marine source is deduced from a simple 1-D model of the alkene budget in the marine boundary layer. It appears that the required source must be approximately one order of magnitude greater than the source deduced from concentration measured at 1-m depth, and consistent with probable high concentration gradients close to the ocean surface.