Monday, 12 January 2009
An Environmental Chamber Study of Atmospheric Reactions Involving Hydrogen Oxides and Nitrogen Oxides
Hall 5 (Phoenix Convention Center)
Field observations show discrepancies between measurements and models of the hydroxyl radical (OH) and the hydroperoxyl radical (HO2). An important discrepancy has been observed for the HO2/OH ratio, which is an indicator of the fast chemistry that cycles HOx (e.g., HOx=OH+HO2) between OH and HO2. When comparing HO2/OH to NO, the measured slope of the HO2/OH ratio versus NO is much less than the modeled slope; therefore, the measured ratio is less than modeled for NO less than ~1 ppbv and greater than the modeled for NO greater than 1 ppbv. A common speculation has been that photochemistry involving volatile organic compounds (VOCs) is responsible for this disagreement. This discrepancy has implications for ozone production, as ozone production is proportional to the product of [HO2] and [NO]. A comprehensive study was conducted in a small environmental chamber to explore this discrepancy. The first experiments of NOx-O3 photochemistry confirmed NOx photostationary state in the environmental chamber. The second set of experiments involved the addition of CO and HOx to the NOx-O3 system. Black lights and mercury lamps were used to simulate the radiation of the sun and induce photochemical processes. Standard gas analyzers measured concentrations of NOx, O3, and CO. Penn State's Ground-based Trospospheric Hydrogen Oxide Sensor (GTHOS) measured OH and HO2. The HO2/OH ratio was measured using wide ranges of NO and CO and the behavior was analyzed. The result is that the discrepancy between the measured and modeled HO2/OH ratio is the same in the environmental chamber with the NOx-HOx-CO system as it is in the atmosphere. These studies in a small environmental chamber indicate that the discrepancies between the measured and modeled HO2/OH ratio are due to unknown chemistry in the NOx-HOx chemical system and not to VOC chemistry.