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

Abstract

Abstract. In liquid cloud droplets, superoxide anion (O−2(aq)) is known to quickly consume ozone (O3(aq)), which is relatively insoluble. The significance of this reaction as tropospheric O3 sink is sensitive to the abundance of O−2(aq) and therefore to the production of its main precursor, 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 atmospheric model ECHAM/MESSy (EMAC), 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) via 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 of predicted OVOCs 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 a−1 of O3 are scavenged, which increases to 336 Tg a−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.

Tropospheric ozone production and chemical regime analysis during the COVID-19 lockdown over Europe

“…Similar to the HOOVER campaigns, NO, O 3 and CO can be well approximated by the model simulations which we present in Figure S2 and S3 of the Supplement. Tropospheric ozone is slightly overestimated, which we attribute to the simplified representation of multiphase chemistry (clouds) in the present model version, which underpredicts chemical ozone loss (Rosanka et al, 2021).…”

Organic pollutants from tropical peatland fires: regional influences and its impact on lower stratospheric ozone

“…SCAV calculates the transfer of species into and out of rain and cloud droplets using the Henry's law equilibrium, acid dissociation equilibria, oxidation-reduction reactions, heterogeneous reactions on droplet surfaces, and aqueous-phase photolysis reactions (Tost et al, 2006). As mentioned earlier and as demonstrated by Rosanka et al (2020b), in-cloud OVOC oxidation significantly influences the atmospheric composition. However, the ordinary differential equations (ODE) systems resulting from the combination of gas-phase and in-cloud aqueous-phase suffer from (1) a higher stiffness due to fast acid-base equilibria and phase-transfer reactions, and (2) load imbalance on High-Performance Computing (HPC) systems due to the sparsity of clouds.…”

“…Many oxygenated VOCs (OVOCs) have a high solubility and quickly partition and react in cloud droplets influencing radical concentrations and the atmospheric composition in general (Herrmann et al, 2015). Rosanka et al (2020b) showed that the in-cloud OVOC oxidation has a significant impact on the predicted concentrations of VOCs, key oxidants, and O 3 .…”