Ozone Reaction Rates Critical to Mesospheric Data Interpretation

Sri International, Menlo Park CA

Investigators

Abstract

Measurements of mesospheric emissions rely on kinetic interpretations to determine the key photochemical reactants of the upper atmosphere. The largest uncertainty in deriving hydrogen (H) concentrations is the ~50% error in the important heat release reaction H + O3 that produces the Meinel band emissions and removes O3 (ozone). Reducing this uncertainty will also increase confidence in modeling comparisons and interpreting responses to atmospheric variations and dynamics. Because the recommended rate constant for this reaction relies on just two older temperature-dependent studies, this research makes use of laser photolysis/laser-induced fluorescence (LIF) measurements over a wide temperature range (140-400 K). Photolysis of O3/H2 or HCl mixtures will be coupled with time-dependent LIF diagnostics for H reactant disappearance and OH (v=9) product appearance. The new measurements should remove the unacceptably large error term. Modeling of key species such as H, O, and O3 is also affected by such rate constant uncertainties and by issues raised about specific rate constants in examining model disagreements with data. A second issue related to the H + O3 reaction above concerns the O3 concentration and the longstanding O3 model deficit problem. Thus, in the light of new theoretical information, the investigators will examine a second critical O3 reaction rate constant, its formation by O + O2 recombination. Recent quantum trajectory calculations show the presence of long-lived resonant collisions, which should be capable of producing unexpected, increased rate constants at the very low pressures pertaining to the atmosphere but not accessed in the laboratory. This work will consistently incorporate the quantum results into kinetic rate theory using master equation calculations to map this pressure dependence and its uncertainty and implications. The importance of a new concept to basic rate theory will be introduced for small molecule recombinations, namely the role of rare long-lived collisional resonances in very low-pressure reactions. These conditions are very difficult to probe experimentally. Two important aspects of atmospheric models, and conclusions drawn as a result, will be improved. The project will also contribute to the training and research experiences of a postdoctoral fellow and summer undergraduate students. Improving two key rate constants controlling the ozone chemistry of the upper atmosphere will have broader impacts as a result of the improved modeling of this region that results. Two specific problems will be addressed at atmospheric conditions. The results will benefit atmospheric modelers, kineticists, and those analyzing aeronomy data, including data from current satellite missions. Concentrations of H atoms derived from emissions data will be much more precise.