How well do we understand marine emissions of carbonyl sulfide?

Abstract:

Carbonyl sulfide (OCS) is the most important precursor for stratospheric aerosols in volcanically quiescent periods. To understand its entrainment into the stratosphere, its tropospheric budget has to be well quantified. Marine emissions are suggested to be the largest source of tropospheric OCS, but the magnitude of these emissions is debated. A large missing source is indicated by atmospheric top-down estimates. This missing source is suggested to be located in the tropical oceans. Here we present marine measurements of OCS from the tropical oceans and combine them with a database of previous measurements, in order to determine how well global marine emissions are understood “bottom-up”. We focus on direct emissions of OCS as well as on indirect emissions of CS₂ and DMS. The latter are partly oxidized to OCS in the troposphere. We categorize new and existing datasets of marine concentrations of OCS, CS₂ and DMS according to their number and resolution of measurements, global spatial coverage and consistency across the use of different methods, in order to identify remaining uncertainties. Results indicate that while the distribution and processes for OCS concentration and thus emissions is reasonably well understood, uncertainties remain in the atmospheric conversion of DMS to OCS and in the source strength of CS₂. As a result from measurements and a model consistent with the existing database, significantly higher direct emissions of OCS than previously estimated are very unlikely. Furthermore, we discuss the probability of indirect emissions to account for the missing source on the order of 600-800 Gg S yr^-1.