Laboratory Measurements of HOx Radical Rate Constants

Measurements of chemically active species in the mesosphere and upper stratosphere - O3, OH, HO2 - are poorly predicted by model calculations and some rate constant changes for OH + O (1), OH + HO2 (2), and O + HO2 (3) have been proposed as remedies. We have performed a series of laser photolysis experiments on mixtures of O3, N2, O2, and H2O or H2, using laser induced fluorescence measurements of OH or O atom decay rates to provide new determinations of these rate constants. Our method relies upon complete photodissociation of ozone at 248 nm to produce known amounts of O atoms at temperatures of 230 K to 385 K. For (1), we measure k = 1.36 x 10(-11) e (261/T) cm3/molec/s in agreement with recommended values. A second experiment which requires computer modeling of results was designed to be sensitive to k(2)*k(3)/k(1). Initial analysis of decays suggests a 15% increase in k(3) and 15% decrease in k(2) from NASA panel values. These differences are within uncertainties and are insufficient to modify model predictions significantly. Research supported by the NASA Geosciences ITM Program and NSF Aeronomy Program.