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

Abstract

Abstract. In liquid cloud droplets, superoxide anion (O2(aq)-) is known to quickly consume ozone (O3(aq)), which is relatively insoluble. The significance of this reaction as a tropospheric O3 sink is sensitive to the abundance of O2(aq)- and therefore to the production of its main precursor, the hydroperoxyl radical (HO2(aq)). The aqueous-phase oxidation of oxygenated volatile organic compounds (OVOCs) is the major source of HO2(aq) in cloud droplets. Hence, the lack of explicit aqueous-phase chemical kinetics in global atmospheric models leads to a general underestimation of clouds as O3 sinks. In this study, the importance of in-cloud OVOC oxidation for tropospheric composition is assessed by using the Chemistry As A Boxmodel Application (CAABA) and the global ECHAM/MESSy Atmospheric Chemistry (EMAC) model, which are both capable of explicitly representing the relevant chemical transformations. For this analysis, three different in-cloud oxidation mechanisms are employed: (1) one including the basic oxidation of SO2(aq) by O3(aq) and H2O2(aq), which thus represents the capabilities of most global models; (2) the more advanced standard EMAC mechanism, which includes inorganic chemistry and simplified degradation of methane oxidation products; and (3) the detailed in-cloud OVOC oxidation scheme Jülich Aqueous-phase Mechanism of Organic Chemistry (JAMOC). By using EMAC, the global impact of each mechanism is assessed focusing mainly on tropospheric volatile organic compounds (VOCs), HOx (HOx=OH+HO2), and O3. This is achieved by performing a detailed HOx and O3 budget analysis in the gas and aqueous phase. The resulting changes are evaluated against O3 and methanol (CH3OH) satellite observations from the Infrared Atmospheric Sounding Interferometer (IASI) for 2015. In general, the explicit in-cloud oxidation leads to an overall reduction in predicted OVOC levels and reduces EMAC's overestimation of some OVOCs in the tropics. The in-cloud OVOC oxidation shifts the HO2 production from the gas to the aqueous phase. As a result, the O3 budget is perturbed with scavenging being enhanced and the gas-phase chemical losses being reduced. With the simplified in-cloud chemistry, about 13 Tg yr−1 of O3 is scavenged, which increases to 336 Tg yr−1 when JAMOC is used. The highest O3 reduction of 12 % is predicted in the upper troposphere–lower stratosphere (UTLS). These changes in the free troposphere significantly reduce the modelled tropospheric ozone columns, which are known to be generally overestimated by EMAC and other global atmospheric models.