Measurement of OH reactivity by laser flash photolysis coupled with laser-induced fluorescence spectroscopy

OH reactivity (k’OH) is the total pseudo-first-order loss rate coefficient describing the removal of OH radicals to all 13 sinks in the atmosphere, and is the inverse of the chemical lifetime of OH. Measurements of ambient OH reactivity 14 can be used to discover the extent to which measured OH sinks contribute to the total OH loss rate. Thus, OH 15 reactivity measurements enable determination of the comprehensiveness of measurements used to predict air 16 quality and ozone production, and, in conjunction with measurements of OH radical concentrations, to assess our 17 understanding of OH production rates. 18 In this work, we describe the design and characterisation of an instrument to measure OH reactivity using laser 19 flash photolysis coupled to laser-induced fluorescence (LFP-LIF) spectroscopy. The LFP-LIF technique produces OH 20 radicals in isolation, and thus minimises potential interferences in OH reactivity measurements owing to the 21 reaction of HO2 with NO which can occur if HO2 is co-produced with OH in the instrument. Capabilities of the 22 instrument for ambient OH reactivity measurements are illustrated by data collected during field campaigns in 23 London, UK, and York, UK. 24 We also present the coupling and characterisation of the LFP-LIF instrument to an atmospheric chamber for 25 measurements of OH reactivity during simulated experiments, and provide suggestions for future improvements 26 to OH reactivity LFP-LIF instruments.