Kinetics of OH-initiated oxidation of oxygenated organic compounds in the aqueous phase: new rate constants, structure–activity relationships and atmospheric implications

Abstract

The kinetics of OH oxidation of several organic compounds of atmospheric relevance were measured in the aqueous phase. Relative kinetics were performed using various organic references and OH sources. After validation of the protocol, temperature-dependent rate constants for the reactions of OH radical with ethyl ter-butyl ether ( k 297 K = 1.5 ( ± 1.7 ) × 1 0 9 M - 1 s - 1 , E a / R = 580 (±560) K), n-butyl acetate ( k 297 K = 1.8 (±0.4)×10 9 M −1 s −1, E a / R = 1000 (±200) K), acetone ( k 298 K = 0.11 (±0.05)×10 9 M −1 s −1, E a / R = 1400 (±500) K), methyl ethyl ketone ( k 298 K = 0.81 ( ± 0.18 ) × 1 0 9 M - 1 s - 1 , E a / R = 1200 (±200) K), methyl iso-butyl ketone ( k 298 K = 2.1 ( ± 0.5 ) × 1 0 9 M - 1 s - 1 , E a / R = 1200 (±300) K) and methylglyoxal ( k 298 K = 0.53 ( ± 0.04 ) × 1 0 9 M - 1 s - 1 , E a / R = 1100 (±300) K) were determined. A non-Arrhenius behavior was found for phenol, in good agreement with the contribution of an OH addition to the mechanism, which also includes H-abstraction by OH radicals. Global rate constants of acetaldehyde, propionaldehyde, butyraldehyde and valeraldehyde were studied at 298 K only, as these compounds partly hydrate in the aqueous phase. All the obtained data (except those of phenol) complemented by literature data were used to investigate three methods to estimate rate constants for H-abstraction reactions of OH radicals in aqueous solutions when measured data were not available: Evans-Polanyi-type correlations, comparisons with gas-phase data, structure activity relationships (SAR). The results show that the SAR method is promising; however, the data set is currently too small to extend this method to temperatures other than 298 K. The atmospheric impact of aqueous phase OH oxidation of water-soluble organic compounds is discussed with the determination of their global atmospheric lifetimes, taking into account both gas- and aqueous-phase reactivities. The results show that atmospheric droplets can act as powerful photoreactors to eliminate soluble organic compounds from the atmosphere.