A master equation study of laser-generated interference in the detection of hydroxyl radicals using laser-induced fluorescence

Abstract

Concentrations of OH in the atmosphere are extremely small, and for any quantitative measurement technique it is imperative that the observed signal originates solely from ambient OH. A master equation model has been formulated to calculate the contribution to the OH concentration from laser photolysis of O3 and the subsequent reaction of O(1D) with H2O, for conditions typical of instruments designed to measure OH by laser-induced fluorescence. The model incorporates the quantum state specific production of OH from the O(1D)+H2O reaction and rotational energy transfer, and calculates the temporal behaviour of all rotational states. For detection at 308 nm and 282 nm, whilst probing the lowest rotational level, N'=1, the model predicts that interference from laser generated OH accounts for 0.4% and ~10% respectively, of the total LIF signal, for [OH]=106 molecules cm-3, [O3]=50 ppb, 40% relative humidity, and typical operating temperatures, pressures and laser parameters.