Nighttime observation of the HO2 radical by an LIF instrument at Oki island, Japan, and its possible origins

HO2 (hydroperoxy) radical of unexpectedly high concentration around 3 ppt w as measured by an instrument based on laser-induced fluorescence with NO ad dition at Oki island, Japan, on the night of August 9/10, 1998. We confirme d that the interference by atmospheric organic peroxy (RO2) radicals was in significant and concluded that the measured signal originated from nighttim e HO2. Model calculations constrained to ancillary measurements indicated t hat HO2 and RO2 were produced primarily via the reactions of ozone with ole fins, especially those with internal olefins, and that NO3 chemistry was re latively unimportant. HO2 concentration was kept high by nighttime NO (simi lar to 10 ppt) via RO2 + NO reactions. Low NO2 (similar to 150 ppt) slowed NO3 production rate. Thus, the high observed HO2 suggests that the reaction s of O-3 With olefins are important HOx primary production mechanisms in th e relatively clean atmosphere.