Oxidation of low-molecular weight organic compounds in cloud droplets: global impact on tropospheric oxidants and other trace gases

In-cloud aqueous-phase chemistry is known to decrease tropospheric ozone (O3) via O3+O2- with hydroperoxyl radicals (HO2) being the major source of O2-. The significance of this O3 sink is therefore sensitive to the aqueous-phase chemistry of HOx (HOx=HO2+OH). The lack of explicit aqueous-phase chemical kinetics in cloud droplets in most global atmospheric models leads to a general underestimation of this sink. In this study, a detailed aqueous-phase oxidation mechanism for water soluble oxygenated volatile organic compounds (OVOC's) is developed. The mechanism focuses on OVOCs containing up to four-carbon atoms and uses OH and NO3 as the main oxidants during day and night time, respectively. A detailed box-model analysis with CAABA/MECCA is performed to understand the full implications of this new mechanism. Additionally, the newly developed mechanism is implemented into the global atmospheric model ECHAM/MESSy (EMAC), which is capable of representing the described processes explicitly and integrates the corresponding ODE system using a Rosenbrock solver. By using EMAC, the global impact of the proposed mechanism is estimated focusing mainly on tropospheric VOC, O3 and HOx concentrations. This is achieved by performing a detailed Ox and HOx budget analysis in the gas- and aqueous-phase. The resulting changes are evaluated using satellite measurements. In general, the in-cloud OVOC oxidation shifts the HO2 production from the gas-phase to the aqueous-phase. As a result, the O3 budget is perturbed with scavenging being enhanced and the gas-phase chemical losses being reduced. These changes in the free troposphere significantly reduce the modelled tropospheric ozone column, which is known to be generally overestimated by EMAC and global atmospheric model.