Structure-reactivity based estimation of the rate constants for hydroxyl radical reactions with hydrocarbons

The reaction with the OH radical constitutes the single most important remo val process for most organic compounds found in the atmosphere. Efforts to measure the OH radical rate constants of all tropospheric constituents rema in incomplete due to the large variety of primary emitted compounds and the ir tropospheric degradation products. Based on the measured rate constants of approximate to 250 molecules with t he OH radical, a structure-activity relationship (SAR) for OH reactions has been developed. The molecules used in the dataset include most classes of tropospheric compounds (including alkanes, alkenes, and oxygenated hydrocar bons), with the exception of aromatic and halogen-containing compounds. Usi ng a new parameterization of the molecular structure, the overall agreement between measured values and those estimated using the SAR developed in thi s study is usually very good, with 10% of the molecules showing deviations larger than 50%. In particular, the estimated rate constants of ethers and ketones are in better agreement with experimental data than with previous S ARs (Kwok and Atkinson, Atmos. Environ. 29, 1685-1695, 1995). Rate constant s of organic nitrates were not well described by the SAR used in this study . The basic assumption that the additive rate constant for a chemical group is only influenced by neighbouring functional groups did not allow a good parameterization for the rate constants of organic nitrates. The use of a s econd parameter to alter the reactivity of C-H bonds in beta-position to th e functional group resulted in markedly better agreement between calculated and measured rate constants, but was not extended due to the limited set o f data. This indicates that strong electron withdrawing groups (e.g., nitra te groups) might influence the reactivity of C-H bonds that are not directl y adjacent.