Impact of clouds on the tropospheric ozone budget: Direct effect of multiphase photochemistry of soluble organic compounds

A box model daytime study of the multiphase photochemistry (both gas and aqueous phases) of C1 organic compounds within a non-precipitating cloud on a local scale was performed. In a situation where ozone accumulates in clear sky conditions, as soon as a cloud is formed, the tropospheric ozone budget changes drastically: the net production decreases by a factor of 2 or more, and, depending on the NOx concentrations and the pH values, can actually lead to a chemical net destruction. Both indirect and direct impacts caused by the presence of the liquid phase explain this. The indirect impact is the result of the much higher solubility of HO2 and RO2 radicals than NO and NO2, resulting in lower gas-phase efficiency of the NOx cycles in producing ozone. The direct impact is caused by the very fast reaction of ozone towards O2− radicals within the liquid phase. The relative importance of these two impacts is determined for different NOx concentrations and pH values. When NOx=1 ppbv, the direct impact is of negligible importance at pH=3 (for a total decrease in ozone production of a factor of 2), but it accounts for 6% at pH=4.16 (for a total decrease in ozone production of a factor of 6.6), 16% at pH=5.2 (for a total net chemical destruction of ozone), and 28% at pH=6 (for a total net chemical destruction of ozone). At this NOx level and at pHs=5.2 and 6, despite its small contribution, the effect of the direct impact is the net chemical destruction of ozone. When NOx=0.1 ppbv, both the direct and the indirect impact contribute to the net chemical destruction at all pH values. However, the direct impact now contributes to a larger extent, accounting for 15% at pH=3, and up to 77% at pH=6. The direct impact involves the O3+O2− reaction which leads to the production of aqueous phase OH radicals. The latter species in turn react with dissolved organic compounds to produce additional HO2 and O2− radicals. Therefore, the aqueous phase sink of ozone is auto-catalytic in the presence of soluble organic compounds. The influence varying methanol concentrations has been tested.