Behavior of OH and HO2 radicals during the Observations at a Remote Island of Okinawa (ORION99) field campaign 1.: Observation using a laser-induced fluorescence instrument

The hydroxyl (OH) and hydroperoxyl (HO2) radicals were measured by a laser-induced fluorescence instrument at Cape Hedo, Okinawa Island, Japan, in summer 1999 during the Observations at a Remote Island of Okinawa (ORION99) field campaign. The field deployment of the instrument and its calibrations are described in detail. From the frequent calibrations during the field campaign, it was shown that the instrument, utilizing an optical fiber to transmit the laser light to the detection cell located on a tower, had a sufficiently stable sensitivity to OH and HO2 in order to trace their diurnal and day-to-day variations. The detection limit of the instrument was typically around 4 x 10(6) radicals cm(-3) with an integration time of 1 min. We could not examine fast OH variations. However, hourly averaged OH concentrations during daytime were statistically significant. The HO2 concentrations were higher and the detailed variations were detected. On average, OH and HO2 showed daytime maxima of around 4 x 10(6) radicals cm(-3) and of around 17 pptv, respectively. The median, tenth, and ninetieth percentiles of the measured daytime HO2/OH concentration ratio were 76, 32, and 143, respectively. The power law dependence of HO2 on J((OD)-D-1) was about 0.5 when the NO concentration was lower than 300 pptv and was about 1 when the NO concentration was higher than 1 ppbv, which was consistent with the known radical chemistry in the lower troposphere.