The Behavior of the Hydroxyl and Hydroperoxyl Radicals During TexAQS2000

The hydroxyl radical (OH) and the hydroperoxyl radical (HO₂), together called HO_x, are significant reactants in the production of urban pollution, such as occurs in Houston Texas. The mixing ratios of OH and HO₂ and the OH reactivity were measured as part of the multi-investigator study, TexAQS2000, which took place in LaPorte, Texas in August and September 2000. While OH and HO₂ generally show the expected diurnal behavior, three interesting observations emerge. First, simultaneous observations of nighttime HO₂, O₃, and alkenes (e.g., ethene and propene) provide strong evidence for an O₃ + alkene source of HO_x. Second, HO₂ is greater than expected in air containing copious NO, suggesting the presence of unidentified HO_x sources in such air masses. Third, although daytime OH should be in steady state, with its sources and sinks balancing, we find that additional OH sources are required to balance the OH sinks. The evidence supporting these observations and the consequences will be discussed.