Exploring natural aerosol formation from DMS oxidation and implications for aerosol forcing
Atmospheric aerosol influences the Earth’s overall radiative balance, both directly via scattering and absorbing insolation or outgoing radiation reflected by the Earth’s surface, and indirectly through promoting cloud formation and changing cloud properties. To quantify the radiative forcing requires the comparison of the net radiative budgets of a "polluted" atmosphere versus a "cleaner" preindustrial one. The accurate determination of aerosol radiative forcing thus partly depends on our ability to model the poorly understood preindustrial atmosphere. Here, we focus on modeling the oxidation of dimethyl sulfide (DMS) – a primary natural precursor of non-sea-salt sulfate. Based on previous laboratory studies, we extend the simple DMS oxidation scheme used in the Community Atmospheric Model with chemistry (CAM-chem) version 6 (and similarly in many global ESMs) by adding new gas- and aqueous-phase reactions and including intermediate compounds, e.g., methanesulfonic acid (MSA). This implementation delays the formation of the sulfate, changes the spatial distribution of sulfate aerosol, allows more time for the intermediates to undergo dry or wet deposition, and eventually reduces the effective sulfate yield from DMS. We explore whether a more comprehensive DMS oxidation scheme improves the ability of the model to capture observations of DMS, sulfate, and other relevant species from a series of airborne and in situ measurements. Finally, we perform a series of simulations under present-day and preindustrial climate and emission scenarios to characterize the impact of a more comprehensive DMS scheme on the estimation of aerosol indirect effect.