Photochemistry of important molecules and radicals in the atmosphere

The oxidizing capacity of the troposphere is controlled by radical reactions, where the OH radical plays the dominant role. For this reason it is important to know OH sources / sinks. Knowledge on this can be critically assessed from measurement of the OH concentration, [OH], in the atmosphere, where the majority of OH measurements have been made with the low pressure laser induced fluorescence (LIF) FAGE technique (Fluorescence Assay by Gaseous Expansion). Atmospheric chemical models are used to indicate how well these sources / sinks are understood. In general, these models do a reasonable job in predicting the [OH] in remote and urban environments, where the chemistry is simple and NO dominated, respectively. In pristine, forested, tropical environments, where the chemistry is dominated by isoprene emitted from trees, models do a poor job and under predict the [OH], with differences in the measured and modelled [OH] up to a factor of 10 reported, see Figure 1. This dramatic enhancement in the measured [OH] compared to modelled has also been observed in less pristine environments, where there is a wider range of volatile organic hydrocarbons, including isoprene, but low concentrations of nitrogen oxides, [NOX].