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

The daytime variation of hydroperoxy (HO₂) radical concentration was observed 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 to the high detection limit of the instrument, HO₂ was determined with the detection limit of 0.8 parts per trillion by volume (pptv) (S/N=2, integration 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 species and meteorological parameters such as O₃, CO, nonmethane hydrocarbons(C₂-C₆), NO/NO₂, HCHO/CH₃CHO, SO₂, HNO₃ and J(NO₂) were also observed. Unfortunately, the absolute value of J(O¹D) was not measured. Model calculations for radical concentrations were performed and they were compared to the observed hourly HO₂ concentrations. On August 9 the calculated HO₂ matched the observation very well within the uncertainties of observations (± 26%, 1σ) and the model (± 24%, 1σ). This indicates a good performance of model calculations in estimating HO₂ under certain conditions with 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 that some important HO_x loss chemistry is missing in the model. Although the cause of the discrepancy is not fully understood, possible mechanisms to explain the overestimation are studied. A hypothesis with additional loss of HO₂ would be more plausible than that with additional OH loss. The additional loss rate for HO₂ that can reduce the calculated HO₂ to the measured level was computed for each hour. Its variation correlated well with those of H₂O and some photochemical products. The possibilities of HO₂ loss on aerosol surface, unknown acceleration of HO₂ reactions in the presence of high H₂O, and HO₂ reactions with carbonyl species are discussed.