Daytime HO2 concentrations at Oki Island, Japan, in summer 1998: Comparison between measurement and theory

The daytime variation of hydroperoxy (HO2) radical concentration was observ ed by an instrument based on laser-induced fluorescence with NO addition at Oki Island, Japan, in July/August 1998. Although OH was not detected due t o the high detection limit of the instrument, HO2 was determined with the d etection limit of 0.8 parts per trillion by volume (pptv) (S/N=2, integrati on time of 1 min). On average, HO2 showed a maximum concentration of around 9 pptv in the early afternoon hours. During the field campaign, chemical s pecies and meteorological parameters such as O-3, CO, nonmethane hydrocarbo ns(C-2-C-6), NO/NO2, HCHO/CH3CHO, SO2 HNO3 and J(NO2) were also observed. U nfortunately, the absolute value of J((OD)-D-1) was not measured. Model cal culations for radical concentrations were performed and they were compared to the observed hourly HO2 concentrations. On August 9 the calculated HO2 m atched the observation very well within the uncertainties of observations ( +/-26%, 1 sigma) and the model (+/-24%, 1 sigma). This indicates a good per formance of model calculations in estimating HO2 under certain conditions w ith plenty of isoprene. On other days, however, model usually overestimated HO2 by a factor of 2, especially in the hours around noon. It is deduced t hat some important HOx loss chemistry is missing in the model, Although the cause of the discrepancy is not fully understood, possible mechanisms to e xplain the overestimation are studied. A hypothesis with additional loss of HO2 would be more plausible than that with additional OH loss. The additio nal loss rate for HO2 that can reduce the calculated HO2 to the measured le vel was computed for each hour. Its variation correlated well with those of H2O and some photochemical products. The possibilities of HO2 loss on aero sol surface, unknown acceleration of HO2 reactions in the presence of high H2O and HO2 reactions with carbonyl species are discussed.