HO 2 /OH and RO 2 /HO 2 ratios during the Tropospheric OH Photochemistry Experiment: Measurement and theory

Ambient concentrations of the hydroxyl (OH), hydroperoxyl (HO 2 ), and total peroxy (ΣRO 2 ) radicals were measured as part of the Tropospheric OH Photochemistry Experiment at Idaho Hill, Colorado, during August and September of 1993. OH radicals were measured using ion‐assisted mass spectroscopy and low‐pressure laser‐induced fluorescence (LIF) detection techniques. HO 2 was measured using chemical conversion and LIF detection of OH. ΣRO 2 radicals were measured using a chemical amplifier technique. The simultaneous measurements of these key species provide an opportunity to test our present understanding of the fast photochemistry of the troposphere. Measured HO 2 /OH ratios were typically between 15 and 80, and agreed well with predictions under conditions where NO mixing ratios were greater than 100 pptv. However, under clean conditions the measured ratio was a factor of 3–4 lower than predicted. The RO 2 /HO 2 ratio was typically a factor of 4–15 larger than predicted by present theories of tropospheric chemistry. A steady state model was used in an attempt to analyze the discrepancies between the measured HO 2 /OH and RO 2 /HO 2 ratios with present theories of hydrocarbon oxidation in the troposphere.