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 + 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 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.