Radical-driven carbonyl-to-acid conversion and acid degradation in tropospheric aqueous systems studied by CAPRAM

Model studies on the aqueous phase radical-driven processing of carbonyl compounds and acids in clouds and deliquescent particles were performed. The model exposed that aqueous radical conversions of carbonyl compounds and its oxidation products can contribute potentially to the formation of functionalised organic acids. The main identified C2–C4 organic gas phase precursors are ethylene glycol, glycolaldehyde, glyoxal, methylglyoxal and 1,4-butenedial. The aqueous phase is shown to contribute significantly with about 93%/63%, 47%/8%, 31%/4%, 7%/4%, 36%/8% to the multiphase oxidative fate of these compounds under remote/urban conditions. Interestingly, the studies revealed that aqueous chemical processing is not only limited to in-cloud conditions but also proceeds in deliquescent particle phase with significant fluxes. Oxalic acid is shown to be formed preferably in deliquescent particles subsequent to the in-cloud oxidations. Mean aqueous phase oxalate formation fluxes of about 12, 42 and 0.4 ng m−3 h−1 in the remote, urban and maritime scenario, respectively. Additionally, the turnovers of the oxidation of organics such as methylglyoxal by NO3 radical reactions are identified to be competitive to their OH pendants. At the current state of CAPRAM, mean C2–C4 in-cloud oxidation fluxes of about 0.12 and 0.5 μg m−3 h−1 are modelled under the idealised remote and urban cloud conditions. y, turnovers from radical oxidations were compared with those of thermal reactions. It is demonstrated that, based on the sparse kinetic data available organic accretion reaction might be of interest in just a few cases for cloud droplets and aqueous particles but generally do not reach the oxidative conversion rates of the main radical oxidants OH and NO3. Interestingly, oxidation reactions of H2O2 are shown to be competitive to the OH radical conversions in cases when H2O2 is not readily used up by the S(IV) oxidation.