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

The OH and HO2 concentrations observed during the Observations at a Remote Island of Okinawa intensive field campaign (ORION99) were compared with tho se calculated by using the ancillary observations as input parameters. Deta iled 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 heter ogeneous processes. On average, the model underestimated daytime HO2 by onl y 20%. The squared correlation coefficient between observed and calculated HO2 was 0.79. The model underestimated HO2 significantly during a morning p eriod with high NO, concentrations by up to a factor of 3 and for sudden su rges 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 i n contrast with the significant model's overestimation of midday HO2 by a f actor 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 sever al days calculated by a simple steady state model were similar to those obs erved, indicating the observed OH data were statistically significant when they were averaged hourly. During, one night, HO2 concentrations were obser ved to be 2-5 pptv, positively correlating with NO2. The model significantl y underestimated HO2 by up to a factor of 4 and did not reproduce the posit ive correlation. Currently, no explanations can be given for the nocturnal behaviors.