Effects of below-cloud gas scavenging on raindrop chemistry over remote ocean regions

Abstract

A particularly simple case in which raindrop chemistry may be studied is in rain over remote ocean regions, where natural spatial and temporal chemical concentrations are relatively unperturbed. Recent observations of the stable molecules and ions in marine air and marine rain have been used as initial conditions for a computational model of raindrop chemistry over the North Atlantic and Equatorial Pacific Oceans. The model incorporates below-cloud gas scavenging as well as an extensive set of inorganic and organic chemical reactions. The results include the following: Photochemical processes are central to the free radical chemistry of raindrops over the oceans. Within raindrops, the concentrations of species may be controlled by their initial concentrations prior to fall (as is the case for NO 3 −), by their rate of incorporation from the gas phase (as with RO 2·), or by chemical equilibria or reactions (as with O 2 −). This control sometimes differs in different geographical areas; for example, aqueous organic acids are produced about equally by incorporation and by reaction in the Equatorial Pacific calculations but are produced primarily by reaction in the North Atlantic calculations. Of the inorganic ions in these calculations, ammonium changes the most (as much as 20 %) during raindrop fall. However, differences in solar flux and gas concentration influences on raindrop chemistry are insufficient to explain observed differences in inorganic ion concentrations in North Atlantic and Equatorial Pacific raindrops. Organic thermochemical and photochemical processes have the potential to play interesting and important roles in raindrop chemistry over remote ocean regions, particularly if formaldehyde and alkyl hydroperoxide concentrations in the gas phase are large.