Investigation of Organic Peroxy Radical Photolysis as a Source of OH

Abstract:

In some environments, models of atmospheric chemistry calculate values of ambient OH concentrations that reasonably (within a factor of 2) agree with measured OH concentrations. However, it has been found in forested environments (low [NO_x], high [BVOC]) that models have been less successful in calculating OH concentrations, with measured to modeled ratios of up to 10 reported. Enhanced measured to modelled OH concentrations (a factor of 4) have also been observed in more anthropogenically influenced environments with low concentrations of NO_x. These discrepancies suggest the presence of an unknown source of OH. Two candidates for this source are the production of OH following the fast isomerization of organic peroxy (RO₂) radicals and the reaction of RO₂ with hydroperoxy radicals.

The photolysis of RO₂ radicals at ambient wavelengths to yield OH is another possible source of OH that has not been investigated. This process can be quantified by the absorption cross-section of RO₂ radicals for OH production (σ_{RO2, OH}), which is the product of the total absorption cross-section and the quantum yield of OH. In experiments using a three-laser flash photolysis setup, we have measured σ_{RO2, OH} for RO₂ derived from isoprene, ethylene, and methacrolein at wavelengths in the near UV (λ=300–360 nm). The measured values of σ_{RO2, OH} (ranging from 0.05–1.00 x 10^-19 cm^-2) over this wavelength range suggest that RO₂ photolysis is a significant pathway for OH production. To determine the impact of the OH production from RO₂ photolysis on ambient OH concentrations, OH concentrations were calculated with a box model, constrained to the conditions of the OP-3 field campaign in Borneo, that incorporates RO₂ photolysis rates derived from measurements of σ_{RO2, OH}. It was found that RO₂ photolysis increased OH concentrations by approximately 10% over base model chemistry at midday. Measured values of σ_{RO2, OH} and the impact on ambient OH concentrations derived from the modeling will be presented and discussed.