Atmospheric and kinetic studies of OH and HO_2 by the FAGE technique

A new FAGE setup has recently been built at the University of Lille,France.It permits the quantification of OH and HO 2 in the atmosphere with a detection limit of 3×10 5 molecules/(cm 3 ·min) for OH and 1×10 6 molecules/(cm 3 ·min) for HO 2.Its coupling to a photolysis cell enables the measurement of the total reactivity of the hydroxyl radical in ambient air and kinetic studies in laboratory.Two configurations have been considered:one with the photolysis cell at 90° to the FAGE nozzle,the other on line with the FAGE nozzle.The two configurations have been tested and validated by measuring the well known rate constants of OH with CH 4,C 3 H 8 and CO.The advantages and drawbacks of each configuration have been evaluated.The "on line" configuration limits losses and permits measurements over a larger reactivity range but is affected by OH formation from the laser beam striking the FAGE nozzle,thus limiting the ability to carry out energy dependence studies which can,in contrast,be successfully performed in the 90° configuration.