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We evaluate the sensitivity of tropospheric OH, O₃, and O₃ 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 HO₂, NO₂, and NO₃. Our approach is to couple a global 3D model of tropospheric chemistry (GEOSCHEM) with aerosol fields from a global 3D 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 O₃ → O(¹D) 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 HO₂ accounts for 10–40% of total HO_x radical (≡ OH + peroxy) loss in the boundary layer over polluted continental regions (largely due to …