HO2 (hydroperoxy) radical of unexpectedly high concentration around 3 ppt was measured by an instrument based on laser-induced fluorescence with NO addition at Oki island, Japan, on the night of August 9/10, 1998. We confirmed that the interference by atmospheric organic peroxy (RO2) radicals was insignificant and concluded that the measured signal originated from nighttime HO2. Model calculations constrained to ancillary measurements indicated that HO2 and RO2 were produced primarily via the reactions of ozone with olefins, especially those with internal olefins, and that NO3 chemistry was relatively unimportant. HO2 concentration was kept high by nighttime NO (~10 ppt) via RO2+NO reactions. Low NO2 (~150 ppt) slowed NO3 production rate. Thus, the high observed HO2 suggests that the reactions of O3 with olefins are important HOx primary production mechanisms in the relatively clean atmosphere. ¿ 1999 American Geophysical Union