Formaldehyde Chemical Actinometer for the Troposphere: Measurements of Photolysis Rate Coefficient, H2 and H02 Branching Ratios, and Comparison with other Ultraviolet Ligh

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9422307 Heikes The photolysis of formaldehyde is an important intermediate step in the atmospheric oxidation of hydrocarbons and leads to the direct formation of carbon monoxide, hydrogen, and perhydroxyl radicals. The later two are produced through two different photolysis channels. The perhydroxyl channel is significant to the net photochemical production of odd-hydrogen, ozone, hydrogen peroxide, and organic hydroperoxides. Uncertainties in the photolysis rate and in the branching between the two channels lead directly to uncertainties in the oxidizing power of the atmosphere. These rates are currently estimated using modeled actinic fluxes of light, laboratory derived formaldehyde absorption cross sections, and laboratory derived quantum yields. Atmospheric gases, aerosols, and clouds confound the actinic flux calculation. This together with measurement uncertainties in the laboratory data may lead to significant errors in the photolysis rates and radical production rates from the photolysis of formaldehyde. In this project, a chemical actinometer will be constructed to measure directly the photolysis rate of formaldehyde and the product distribution. Ambient formaldehyde concentrations will also be measured. These measurements will be used in conjunction with theoretical models to test our understanding of formaldehyde photolysis and evaluate its strength as a perhydroxyl radical source. Measured photolysis rates and branching ratios will be correlated with narrow-band and broad-band UV irradiance measurements. Correlations identified will permit the use of more simple irradiance measurements to estimate formaldehyde photolysis properties.