Structure-Reactivity Based Estimation of the Rate Constants for Hydroxyl Radical Reactions with Hydrocarbons

The reaction with the OH radical constitutes the singlemost important removal process for most organiccompounds found in the atmosphere. Efforts to measurethe OH radical rate constants of all troposphericconstituents remain incomplete due to the largevariety of primary emitted compounds and theirtropospheric degradation products.Based on the measured rate constants of ≈250molecules with the OH radical, a structure-activityrelationship (SAR) for OH reactions has beendeveloped. The molecules used in the dataset includemost classes of tropospheric compounds (includingalkanes, alkenes, and oxygenated hydrocarbons), withthe exception of aromatic and halogen-containingcompounds. Using a new parameterization of themolecular structure, the overall agreement betweenmeasured values and those estimated using the SARdeveloped in this study is usually very good, with10% of the molecules showing deviations larger than50%. In particular, the estimated rate constants ofethers and ketones are in better agreement withexperimental data than with previous SARs (Kwok andAtkinson, Atmos. Environ.29, 1685–1695,1995). Rate constants of organic nitrates werenot well described by the SAR used in thisstudy. The basic assumption that the additive rateconstant for a chemical group is only influenced byneighbouring functional groups did not allow a goodparameterization for the rate constants of organicnitrates. The use of a second parameter to alter thereactivity of C-H bonds in β-position to thefunctional group resulted in markedly better agreementbetween calculated and measured rate constants, butwas not extended due to the limited set of data. This indicates that strong electron withdrawing groups(e.g., nitrate groups) might influence the reactivityof C-H bonds that are not directly adjacent.