The OH and HO2 concentrations observed during the Observations at a Remote Island of Okinawa intensive field campaign (ORION99) were compared with those calculated by using the ancillary observations as input parameters. Detailed comparisons were performed for HO2 with the time resolution of 10 min. During daytime, the observed HO2 concentration levels and variations were basically well reproduced by the model calculations without including heterogeneous processes. On average, the model underestimated daytime HO2 by only 20%. The squared correlation coefficient between observed and calculated HO2 was 0.79. The model underestimated HO2 significantly during a morning period with high NOx concentrations by up to a factor of 3 and for sudden surges observed during noontime on two days by a factor of 1.5. The basically good agreement between measured and calculated HO2 at Okinawa Island was in contrast with the significant model's overestimation of midday HO2 by a factor of 2 at Oki Island. The causes for the difference are discussed. For OH, detailed comparisons were not possible owing to the large uncertainties of measurements and calculations. However, diurnal patterns of OH on several days calculated by a simple steady state model were similar to those observed, indicating the observed OH data were statistically significant when they were averaged hourly. During one night, HO2 concentrations were observed to be 2-5 pptv, positively correlating with NO2. The model significantly underestimated HO2 by up to a factor of 4 and did not reproduce the positive correlation. Currently, no explanations can be given for the nocturnal behaviors.
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