The role of clouds in tropospheric photochemistry

We show that photochemical processes in the lower half of the troposphere are strongly affected by the presence of liquid water clouds. Especially CH₂O, an important intermediate of CH₄ (and of other hydrocarbon) oxidation, is subject to enhanced breakdown in the aqueous phase. This reduces the formation of HO _x -radicals via photodissociation of CH₂O in the gas phase. In the droplets, the hydrated form of CH₂O, its oxidation product HCO₂ ^–, and H₂O₂ recycle O₂ ^– radicals which, in turn, react with ozone. We show that the latter reaction is a significant sink for O₃. Further O₃ concentrations are reduced as a result of decreased formation of O₃ during periods with clouds. Additionally, NO _x , which acts as a catalyst in the photochemical formation of O₃, is depleted by clouds during the night via scavenging of N₂O₅. This significantly reduces NO _x -concentrations during subsequent daylight hours, so that less NO _x is available for O₃ production. Clouds thus directly reduce the concentrations of O₃, CH₂O, NO _x , and HO _x . Indirectly, this also affects the budgets of other trace gases, such as H₂O₂, CO, and H₂.