Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 6 6 2006 10.5194/acpd-6-11597-2006 http://www.atmos-chem-phys-discuss.net/6/11597/2006/ http://www.atmos-chem-phys-discuss.net/6/11597/2006/acpd-6-11597-2006.html http://www.atmos-chem-phys-discuss.net/6/11597/2006/acpd-6-11597-2006.pdf 11597 11620 2006-11-20 Near-UV photolysis cross sections of CH₃OOH and HOCH₂OOH determined via action spectroscopy C. M. Roehl Z. Marka J. L. Fry P. O. Wennberg Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA 91125, USA Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125, USA now at: Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA now at: Department of Chemistry, University of California, Berkeley, Berkeley, CA 94720-1460, USA Knowledge of molecular photolysis cross sections is important for determining atmospheric lifetimes and fates of many species. A method and laser apparatus for measurement of these cross sections in the near-ultraviolet (UV) region is described. The technique is based on action spectroscopy, where the yield of a photodissociation product (in this case OH) is measured as a function of excitation energy. For compounds yielding OH, this method can be used to measure near-UV photodissociation cross section as low as 10^−23 cm² molecule^−1. The method is applied to determine the photodissociation cross sections for methyl hydroperoxide (CH₃OOH; MHP) and hydroxymethyl hydroperoxide (HOCH₂OOH; HMHP) in the 305–365 nm wavelength range. The measured cross sections are in good agreement with previous measurements of absorption cross sections.