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

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 O 3 and the subsequent reaction of O(¹D) with H 2 O, 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)+H 2 O 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]=10 6 molecules cm −3 , [O 3 ]=50 ppb, 40% relative humidity, and typical operating temperatures, pressures and laser parameters.