Behavior of OH and HO₂ radicals during the Observations at a Remote Island of Okinawa (ORION99) field campaign 2. Comparison between Observations and calculations

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 NO_x 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.