Understanding the production and interconversion of the hydroxyl radical during the Tropospheric OH Photochemistry Experiment

The hydroxyl radical plays a critical role in the chemistry of the lower atmosphere. Understanding its production, interconversion, and sinks is central to modeling and predicting the chemistry of the troposphere. The OH measurements made during the 1993 Tropospheric OH Photochemistry Experiment provide a detailed look at these mechanisms since NO x , j (O 3 ), RO 2 , HO 2 , nonmethane hydrocarbons (NMHC), and many other relevant species were measured simultaneously. The relationship of OH to NO x and to primary production is extensively examined. Close agreement with theory is shown in the NO x /OH relation with OH concentrations increasing with increasing NO to a maximum at 1–2 ppbv due to conversion of HO 2 to OH, and then OH decreasing with further increasing NO x due to conversion of NO 2 to HNO 3 . Close correlations of OH concentrations with primary production (water, ozone, j (O 3 )) are also shown both on average and on rapid timescales.