Validation of the calibration of a laser-induced fluorescence instrument for the measurement of OH radicals in the atmosphere

An assessment of the accuracy of OH concentrations measured in a smog chamber by a calibrated laser-induced fluorescence (LIF) instrument has been made, in the course of 9 experiments performed to study the photo-oxidation of benzene, toluene, 1,3,5-trimethylbenzene, paraxylene, ortho-cresol and ethene at the European Photoreactor facility (EUPHORE). The LIF system was calibrated via the water photolysis/ozone actinometry approach. OH concentrations were inferred from the instantaneous rate of removal of each hydrocarbon species (measured by FTIR or HPLC) via the appropriate rate coefficient for their reaction with OH, and compared with those obtained from the LIF system. Good agreement between the two approaches was found for all species with the exception of 1,3,5-trimethylbenzene, for which OH concentrations inferred from hydrocarbon removal were a factor of 3 lower than those measured by the LIF system. From the remaining 8 experiments, an overall value of 1.15+/-0.13 (+/-1sigma) was obtained for [OH](LIF)/[OH](HydrocarbonDecay), compared with the estimated uncertainty in the accuracy of the water photolysis/ozone actinometry OH calibration technique of 26% (1sigma).