First Cavity Ring-Down Spectroscopy HO 2 Measurements in a Large Photoreactor
Résumé
Abstract The HO 2 radical is one of the most important intermediate species in atmospheric chemistry. We report on the development of a new photoreactor with first in-situ measurement of HO 2 radical photostationary concentrations using continuous wave cavity ring-down spectrometry (cw-CRDS). Characterization of the actinic photon flux was carried out by NO 2 actinometry. Photolysis of Cl 2 /methanol mixtures in air under UV light allowed the measurement of HO 2 photostationary concentrations of a few 10 10 molecules cm -3 with an HO 2 detection limit of 1.5 × 10 10 molecules cm -3 at 6638.207 cm -1 . The feasibility of HO 2 direct measurement in a reaction chamber is demonstrated through the measurement of the HO 2 overall loss at different pressures showing the importance of HO 2 diffusion and wall loss in such low pressure quartz reactor. The rate coefficient for the HO 2 +HO 2 reaction has been measured at 6.6, 24 and 118 mbar and found to be in good agreement with the recommended value.