Atmospheric oxidation mechanism of acenaphthene initiated by OH radicals

Abstract

Gas-phase oxidation of PAHs can result in formation of oxygenated PAHs (OPAHs) and nitro-PAH (NPAHs) as well as a series of secondary pollutants which contribute to formation of SOA. Acenaphthene (ACE) is one of the PAHs with three rings which exists mainly as gaseous form in the atmosphere, where the oxidation is mainly initiated by reaction with OH radical. In this study, the atmospheric oxidation mechanism of ACE initiated by OH radical is investigated using high level quantum chemistry (at the ROCBS-QB3 level) and kinetic calculations, and the overall rate coefficient obtained here is in good agreement with previous experimental values. The reaction starts with OH radical additions to the C3-, C5-site and H atom abstraction from C1/C2 of ACE, and the fates of main adducts formed as ACE-n-OH (ACE3 and ACE5 in short) and R2 are further considered. ACE3 and ACE5 would react with O2 at different sites, of which all the additions are reversible. An overall effective removal rate of ~9 s−1 for ACE5 with O2 are calculated here using steady state approximately, which allows their reaction with NO2 to a relative significant extent in the atmosphere. We predicted significant 4-NACE formation from ACE5. Reaction of ACE3 with O2 is fast enough of ~107 s−1 that 9-NACE is expected to form in smaller quantities. Additionally, a number of cyclic OPAHs and multifunctional compounds are formed in the oxidation of ACE3, ACE5 and R2 under the high/low NOx concentration as observed in previous investigations.