Atmospheric fate of methacrolein. 2. Formation of lactone and implications for organic aerosol production.

We investigate the oxidation of methacryloylperoxy nitrate (MPAN) and methacrylicperoxy acid (MPAA) by the hydroxyl radical (OH) theoretically, using both density functional theory [B3LYP] and explicitly correlated coupled cluster theory [CCSD(T)-F12]. These two compounds are produced following the abstraction of a hydrogen atom from methacrolein (MACR) by the OH radical. We use a RRKM master equation analysis to estimate that the oxidation of MPAN leads to formation of hydroxymethyl-methyl-α-lactone (HMML) in high yield. HMML production follows a low potential energy path from both MPAN and MPAA following addition of OH (via elimination of the NO(3) and OH from MPAN and MPAA, respectively). We suggest that the subsequent heterogeneous phase chemistry of HMML may be the route to formation of 2-methylglyceric acid, a common component of organic aerosol produced in the oxidation of methacrolein. Oxidation of acrolein, a photo-oxidation product from 1,3-butadiene, is found to follow a similar route generating hydroxymethyl-α-lactone (HML).