Larger than expected organic acid yields from the multi-generation oxidation of petrochemical alkenes

Abstract

Alkenes are primary pollutants in petrochemical source atmospheres, and their atmospheric chemistry is of great importance for tropospheric ozone and secondary organic aerosol formation. Hence, combining quantum chemical calculations and kinetic modelling, we investigated the oxidation mechanism and kinetics of 2-butene (BU), as one of the most important alkenes, and its impact on the environment. The mechanism results show that OH addition is the dominant pathway for cis- and trans-isomers of BU, and then the corresponding OH-adducts are attacked by O2 to produce peroxy radicals, which further react with NO to form acetaldehyde and hydroxyalkyl radicals. Different from the one adopted in current atmospheric models, addition of hydroxyalkyl radicals by O2 and NO to form acetic acid proceeds with a smaller barrier than that for H-abstraction by O2 to form acetaldehyde. A lifetime of less than a few hours (<4 h) for BU is estimated in the petrochemical regions. Kinetic modelling demonstrates that oxidation of BU is predicted to yield significant amounts of organic acids (>56 %) in the petrochemical areas, larger than those are currently recognized, even in environments with low NO concentrations. Our results reveal that the OH-initiated oxidation of BU contributes importantly to organic acid budgets, particularly in the petrochemical regions, bridging the gap in organic acid budgets.