Global and regional decreases in tropospheric oxidants from photochemical effects of aerosols

Abstract

We evaluate the sensitivity of tropospheric OH, O 3, and O 3 precursors to photochemical effects of aerosols not usually included in global models: (1) aerosol scattering and absorption of ultraviolet radiation and (2) reactive uptake of HO 2, NO 2, and NO 3. Our approach is to couple a global 3‐D model of tropospheric chemistry (GEOS‐CHEM) with aerosol fields from a global 3‐D aerosol model (GOCART). Reactive uptake by aerosols is computed using reaction probabilities from a recent review (γ HO2 = 0.2, γ NO2 = 10−4, γ NO3 = 10−3). Aerosols decrease the O 3 → O(1 D) photolysis frequency by 5–20% at the surface throughout the Northern Hemisphere (largely due to mineral dust) and by a factor of 2 in biomass burning regions (largely due to black carbon). Aerosol uptake of HO 2 accounts for 10–40% of total HO x radical (≡ OH + peroxy) loss in the boundary layer over polluted continental regions (largely due to sulfate and organic carbon) and for more than 70% over tropical biomass burning regions (largely due to organic carbon). Uptake of NO 2 and NO 3 accounts for 10–20% of total HNO 3 production over biomass burning regions and less elsewhere. Annual mean OH concentrations decrease by 9% globally and by 5–35% in the boundary layer over the Northern Hemisphere. Simulated CO increases by 5–15 ppbv in the remote Northern Hemisphere, improving agreement with observations. Simulated boundary layer O 3 decreases by 15–45 ppbv over India during the biomass burning season in March and by 5–9 ppbv over northern Europe in August, again improving comparison with observations. We find that particulate matter controls would increase surface O 3 over Europe and other industrial regions.