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 removal process for most organic compounds found in the atmosphere. Efforts to measure the OH radical rate constants of all tropospheric constituents remain incomplete due to the large variety of primary emitted compounds and their tropospheric degradation products. Based on the measured rate constants of approximate 250 molecules with the 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 hydrocarbons), with the exception of aromatic and halogen-containing compounds. Using a new parameterization of the molecular structure, the overall agreement between measured values and those estimated using the SAR developed in this 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 SARs. Rate constants 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 second parameter to alter the reactivity of C-H bonds in β-position to the functional group resulted in markedly better agreement between calculated and measured rate constants, but was not extended due to the limited set of data. This indicates that strong electron withdrawing groups (e.g., nitrate groups) might influence the reactivity of C-H bonds that are not directly adjacent.