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 HOx -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, NOx , 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 NOx -concentrations during subsequent daylight hours, so that less NOx is available for O3 production. Clouds thus directly reduce the concentrations of O3, CH2O, NOx , and HOx . Indirectly, this also affects the budgets of other trace gases, such as H2O2, CO, and H2.