Peroxy radicals, RO₂, are key species in the atmosphere. They are formed from a reaction of OH radicals with hydrocarbons:

RH + OH + O₂→ RO₂ + H₂O 

　　In polluted environments, RO₂ radicals react predominantly with NO, leading to formation of NO₂, and eventually through photolysis of NO₂ to formation of O₃.

　　At low NOx concentrations such as in the marine boundary layer or the background troposphere, the lifetime of RO₂ radicals increases and other reaction pathways such as self- and cross reaction with other RO₂, with HO₂ or with OH radicals become competitive. To study these reactions, UV absorption spectroscopy has been employed in the past: this technique gives good sensitivity for peroxy radicals, but poor selectivity as these radicals have broad absorption features in the UV. We have established a technique allowing to follow peroxy radicals with a better selectivity compared to UV, and with still good sensitivity by coupling laser photolysis to cw-Cavity Ring Down Spectroscopy in the near IR.

　　I will present some recent results on measurements of rate constants and branching ratios of the self- and cross-reactions of selected peroxy radicals.