Model investigation on parameters driving the in-cloud organic acid formation at a tropical remote marine mountain site

Abstract

During September/October 2017, a comprehensive field campaign (MarParCloud) was performed at the Cape Verde Atmospheric Observatory (CVAO) a marine remote background station in the tropics. There, cloud water measurements were performed at the Monte Verde and analyzed for inorganic and organic acids. The cloud water samples were anaylsed for the main inorganic compounds, but also for methane sulfonic and oxalic acid. To understand the origin of these acids, their potential formation pathways were investigated by means of a multiphase chemistry model. For the simulations, the detailed multiphase chemistry framework MCM-CAPRAM was coupled to the box model SPACCIM. A specific time period with long cloud events was selected for the model investigations. The modeled cloud liquid water content (LWC) was adjusted to fit to the measurements. The simulation of multiphase DMS chemistry was achieved through the CAPRAM-DM1.0 module. Based on recently advanced mechanistic insights on DMS chemistry, the MSA formation pathway in the CAPRAM-DM1.0 module was extended. Default simulations with the original CAPRAM-DM1.0 module were considered as benchmark and newer mechanistic findings on DMS oxidation to produce MSA were included stepwise. At the end, the average modeled cloud water concentrations were compared with the average of the all measurement samples. The comparisons reveal that the average modeled oxalic acid concentration is a factor of two lower than the measurements. Moreover, the simulations reveal several model and mechanistic limitations for the formation of MSA. At first, a realistic reproduction of the LWC is a critical point for the MSA formation, because of the dilution of oxidants. Second, the uptake of precursors is key for MSA. A high Henry’s Law (HA) constant of DMSO and especially MSIA results in a much stronger MSA formation. The current implementation of the HAconstant of DMSO, DMSO2 and MSIA in CAPRAM-DM1.0 results into an overestimation of the average MSA cloud water concentration by a factor of 28. By now, the HA constant of MSIA has yet not been determined experimentally. However, quantum chemical calculations by De Jonge et al. (2021) provide a HA constant of MSIA that is one order of magnitude lower. Applying the smaller HA constants of DMSO, DMSO2 and MSIA from De Jonge et al. (2021) leads still to an overestimation, but with a lower factor of 18. Detailed rate analyses were performed to investigate the most important formation pathways of MSA. The only important pathway is the aqueous-phase MSIA oxidation, but interestingly, the reaction with ozone does not always dominate, even model studies often describe it as the dominate one. Additional sensitivity studies are ongoing focusing on more details of the cloud processing. Overall, the present studies highlight the need of further investigations on the aqueous-phase oxidation pathways of DMS to uncover the MSA formation in the troposphere.