Detailed Dynamics of Atmospheric Photoreactions

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9725549 Vaida The hydoxyl radical (OH) is the primary means by which hydrocarbons and carbon monoxide are oxidized in the atmosphere. There is a discrepancy between model calculations of the time dependence of the OH increase following sunrise and observations, particularly in the lower stratosphere. At dawn, the models consistently underestimate the measured OH concentration; in particular, at daybreak there is a sudden increase in the measured OH concentration, amounting to close to 20 percent of the maximum amount (occurring at noon), which is not predicted by models. In response to these uncertainties, the project links theory and laboratory experiments to study the detailed dynamics of atmospheric photoreactions. Specifically, two areas of inquiry are outlined as potentially providing atmospheric free radicals with solar photons of much lower energy than assumed in standard models. These areas involve: 1) investigation of a general mechanism involving visible excitation of overtone transitions of the OH stretching vibrations in key atmospheric chemical species (nitric acid, pernitric acid, hydrogen peroxide, water) using a combination of experimental and theoretical techniques; and 2) examination of the vibrational and electronic spectra of molecular complexes through laboratory investigations. The outcome of this work is expected to be the improved parameterizations of small molecule photoreactions and molecular cluster dynamics for inclusion in chemical models of the atmosphere.