The reactivity of peroxy radicals

Peroxy radicals, RO2, are key species in the atmosphere. They are formed from a reaction of OH radi-cals with hydrocarbons: RH + OH + O2 -> RO2 + H2OIn 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 life-time of RO2 radicals increases and other reaction pathways such as self- and cross reaction with other RO2 or with HO2 radicals become competitive.To study the reactivity of peroxy radicals, 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, but with still good sensitivity by coupling laser photolysis to cw-Cavity Ring Down Spectroscopy in the near IR. Two identical cw-CRDS paths are installed in a recently constructed temperature-controlled photolysis reactor in a small angle with respect to the Excimer photolysis beam, leading to an overlap of around 35 cm between the photolyzed volume and the detection volume. A third detection path for UV absorption measurements is installed in a slightly larger angle, leading to an overlap of around 20 cm between photolysis and absorption volume.Here, we will present the first results obtained in the new reactor: the reaction between RO2 radicals and NO2. This reaction leads in an equilibrium reaction to the formation of RO2NO2 species. If the lifetime of these RO2NO2 are long enough, they will be transported and become a NOx source in remote environments. Therefore, determination of rate- and equilibrium constants of such reactions is important. In this work, two RO2 radicals have been generated simultaneously by 248nm laser photolysis of acetone, leading to roughly 1/3 CH3C(O)O2 radicals and 2/3 CH3O2 radicals. Time-resolved decays have then been observed for both radicals in the presence of different NO2 concentrations. The detection of both RO2 radicals is done simultaneously by high sensitivity cw-CRDS. RO2 concentrations can be decreased to a level where self-reaction becomes negligible at still excellent S/N ratio, making the measurement of RO2 + NO2 reaction straightforward. NO2 is quantified by UV-multipass absorption spectroscopy at 532nm in the photolysis reactor, and concentrations are compared with the calculated ones from the use of calibrated flowmeters.