Sensitivity of photodissociation rate coefficients and O3 photochemical tendencies to aerosols and clouds

[1] We examine the sensitivity of the daily integrated photodissociation rate (DIPR) coefficients of O3(O D) and NO2, the daily averaged O3 photochemical tendency, and OH concentration to variations of clouds and of four aerosol components: black carbon (BC), mineral dust, sea salt, and sulfate. Clouds and BC aerosols are found to be the most important. A comparison between the sensitivity study and some representative observations showed that the global average cloud reduction of DIPR at the surface level is 20% and it is 30% in storm track zones. At the surface level the average negative BC impact is 10% in urban areas and could reach 40% in some heavily polluted urban areas. Mineral dust aerosol, which is the next most important, can reduce the photochemistry at the surface level by over 17% in some seasons over the desert or along its long-range transport paths. The negative impact of sulfate aerosols is around 2% at the surface level, and the impact is as positive as 4% at the top of the sulfate aerosol layer in some urban areas. BC, sulfate, and mineral dust all have much smaller impacts away from their source regions. The impact of sea-salt aerosol is generally less than 1%. While the fractional impacts on O3 production and destruction and OH concentration do not depend much on NOx, the magnitude of the impact on O3 chemical tendency and OH concentration depends strongly on the concentration of NOx: When NOx is very low, the impacts are also very small even if DIPRs are strongly affected. Different mixing states of absorbing and scattering aerosol components, external, coating, and internal, are studied. It is found that with the same amount of each component, the external mixing state produces the weakest impact while the internal mixing state produces the strongest impact. Coating has almost the same impact as internal mixing. There is very little synergy between cloud and absorbing aerosols when clouds are located above the aerosol. The aerosol absorption is strengthened when clouds and aerosol are located in the same layer. The synergy is strong when clouds are located below the absorbing aerosol, where the impact of absorbing aerosol dominates.