Chemistry of OH and HO2 radicals observed at Rishiri Island, Japan, in September 2003: Missing daytime sink of HO2 and positive nighttime correlations with monoterpenes

Abstract

We used laser-induced fluorescence to measure concentrations of OH and HO2 at Rishiri Island, Japan, during September 2003. The average maximum daytime concentration were 2.7 × 106 cm-3 for OH and 5.9 pptv for HO2. The observed concentrations were compared to those predicted by a photochemical box model constrained by ancillary observations. During the daytime, the model overestimated HO2 levels by an average of 89% and OH levels by an average of 35%. This overestimate of OH was rectified when the model was constrained by observed HO2 levels, suggesting that loss processes of HO2 were missing in the model. We calculated the loss rates of HO2 required to bring the modeled HO2 levels into agreement with observed levels. We then studied processes that are capable of explaining the loss rates, including halogen chemistry, heterogeneous loss of HO2 on aerosol surfaces, and the possibility of more rapid HO2 + RO2 reactions than expected. In the nighttime, most of the observed hourly averaged OH and 10-min-averaged HO2, concentrations were statistically significant and fell in the ranges (0.7-5.5) × 105 cm-3 and 0.5-4.9 pptv, respectively. Both HO2 and OH concentrations showed strong positive correlations with total monoterpene concentrations, strongly suggesting that the radicals were produced via reactions of monoterpenes. The median nighttime modeled-to-observed ratios were 1.29 and 0.56 for HO2 and OH, respectively. These ratios dropped to 0.49 and 0.29 during the evening of 25 September, possibly related to the presence of unmeasured olefinic species or chemical reactions involving RO2 that are poorly represented in the model. Copyright 2007 by the American Geophysical Union.