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

Abstract

HO 2 (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 (RO 2 ) radicals was insignificant and concluded that the measured signal originated from nighttime HO 2 . Model calculations constrained to ancillary measurements indicated that HO 2 and RO 2 were produced primarily via the reactions of ozone with olefins, especially those with internal olefins, and that NO 3 chemistry was relatively unimportant. HO 2 concentration was kept high by nighttime NO (∼10 ppt) via RO 2 + NO reactions. Low NO 2 (∼150 ppt) slowed NO 3 production rate. Thus, the high observed HO 2 suggests that the reactions of O 3 with olefins are important HOx primary production mechanisms in the relatively clean atmosphere.