Reactions of lactones with tropospheric oxidants: A kinetics and products study

Tropospheric lifetimes of two lactones, gamma-valerolactone (GVL) and alpha-methyl gamma-butyrolactone (AMGBL) in terms of their reactions with two important tropospheric oxidants- OH and Cl - are estimated, after determining the respective rate coefficients of their reactions in the gas phase using relative rate method. Values of the rate coefficients of the reactions of GVL (kOH = (1.95 ± 0.58) x 10−12; kCl = 2.26 ± 0.53 × 10−11 cm3molecule−1s−1) and AMGBL (kOH = 1.81 ± 0.43 × 10−12; kCl = 3.42 ± 0.63 × 10−11 cm3molecule−1s−1) at 298 K imply that reaction with OH is the dominant reaction in the ambient conditions, and that reaction with Cl atom becomes relevant under marine boundary layer (MBL) conditions, reducing the tropospheric lifetimes to almost half. The tropospheric life times of GVL and AMGBL based on their reaction with OH under ambient conditions are comparable, 71.2 and 76.7 h respectively. However, the products of the reactions are found to be different. In GVL, a promising second generation biofuel component, only ring opening reactions are important, acetic acid and CO2 being the only observed volatile products. In AMGBL, additional ring oxidized products are also formed, of which 3,4-dihydro-3-methyl-2,5-furandione is the most dominant one. The absence of such ring oxidized products in GVL is assigned to the difference in the preferred position of H atom abstraction, based on the computed rate coefficients of individual channels reported in the literature. This suggests that the impact in terms of aerosol generation in the troposphere may also be different for GVL and AMGBL, which differ only in the position of methyl substitution.