Laboratory simulation for the aqueous OH-oxidation of methyl vinyl ketone and methacrolein: significance to the in-cloud SOA production

Abstract

Abstract. Increasing evidence suggests that secondary organic aerosol (SOA) is formed through aqueous phase reactions in atmospheric clouds. In the present study, the aqueous oxidation of methyl vinyl ketone (MVK) and methacrolein (MACR) via OH radical were investigated under conditions typical of cloud droplets, with an emphasis on the composition and variation of oxygenated organic products. In addition to the small products, high-molecular-weight compounds (HMCs) with an oligomer system was found, interpreted as the ion abundance and time evolution. We observed the SOA yields of 23.8% and 8.8% from MVK–OH and MACR–OH reactions, respectively, for the entire 7 h experiment. Our results provide, for the first time to our knowledge, experimental evidence that aqueous OH-oxidation of MVK contributes to SOA formation. Further, a mechanism primarily involving radical processes was proposed to gain a basic understanding of these two reactions. Based on the assumed mechanism, a specific box model was developed for comparison with the experimental results. The model reproduced the observed profiles of first-generation intermediates, but failed to simulate the kinetics of most organic acids mainly due to the lack of chemical kinetics parameters for HMCs. A sensitivity analysis was performed in terms of the effect of reaction branching ratios on oxalic acid yields and the result indicates that additional pathways involving HMCs chemistry might play an important role in the formation of oxalic acid. We suggest that corresponding experiments are needed for better understanding the behavior of multi-functional products and their contribution to the oxalic acid formation.