Organic Peroxyl Radical Photolysis in the Near-Infrared:  Effects on Tropospheric Chemistry

We investigate the effects of near-infrared photolysis of organic peroxyl radicals (RO 2 ) on tropospheric chemistry. We propose that the excitation of an RO 2 to its lowest excited electronic state with near-infrared (near-IR) light is followed by intramolecular reactions that produce hydroxyl (OH) or hydroperoxyl (HO 2 ) radicals. Spectra to this low-lying state have recently been obtained, but absorption cross sections for this electronic transition and yields of the resulting photoproducts have not been directly measured. We suggest a limiting range of cross sections from estimates for the same transition in HO 2 and by comparison to other allowed electronic transitions. On the basis of a thermochemical assessment, we propose that OH and an aldehyde are the principal photoproducts of near-IR photolysis of RO 2 . These photolysis reactions are included in a model of the troposphere with a standard photochemical mechanism and conditions appropriate to remote, rural, and urban locations. Inclusion of RO 2 photolysis has a small effect on any of the major tropospheric chemical constituents if lower limit estimates of the absorption cross sections are used. Midrange or upper limit cross section estimates result in significant departures from the currently accepted photochemical scheme. These studies provide a clear need for further measurements of RO 2 absorption cross sections and photoproduct yields, which are the principal uncertainties.