New constraints on the global secondary organic aerosol budget (Invited)

The budget of atmospheric secondary organic aerosol (SOA) is very uncertain with recent estimates suggesting a source as large as 1820 Tg (SOA)/yr. We use a dataset of aerosol mass spectrometer (AMS) observations and a global chemical transport model including aerosol microphysics to produce new top-down constraints on the SOA budget. The model includes SOA sources from monoterpenes, isoprene, and lumped anthropogenic and biomass burning volatile organic compounds (VOCs). We tune the SOA yield from each source to produce the best overall match between model and observation. Our optimised model has a global SOA source of 144±90 Tg (SOA)/yr. Our analyses produce as a robust result that (a) a substantial fraction of the SOA is anthropogenic in origin and/or (b) anthropogenic pollution greatly enhances the production of SOA from biogenic VOCs, far beyond our current understanding. Our best estimates are 104±14, 13±6, and 27±13 Tg (SOA)/yr for anthropogenically-controlled, biogenic, and biomass burning SOA respectively. We calculate a global mean aerosol indirect (cloud albedo) forcing due to this anthropogenically-controlled SOA of -0.57 W/m2. The biogenic and biomass sources are unsufficiently constrained due to the limited number of observations in regions and periods that strongly impacted by these sources but remote from anthropogenic pollution. To further improve the constraints by this method, additional observations are needed in the tropics and the Southern Hemisphere.