The nighttime production of OH radicals in the continental troposphere

Chemical pathways involved in the nocturnal production of hydroxyl radical (OH) and associated peroxy radicals (HO 2 and RO 2 ) are quantified for various environmental situations of the continental troposphere by means of numerical simulations. In the nocturnal stable layer, most of the OH radicals are directly produced by VOC+O 3 reactions in rural and semipolluted environments while in urban environments, they result mainly from the radical chain propagation from RO 2 and HO 2 . The radical propagation is mainly driven by NO: the nitrate radical (NO 3 ) plays no role in such processes but is significantly involved in the direct formation of RO 2 radicals. Above the nocturnal stable layer, whatever the environmental situation, OH radicals are mainly due to radical chain propagation in which NO 3 plays a significant role. The nighttime simulated OH concentrations are 3 × 10 4 to 5 × 10 5 and 1 × 10 4 to 5 × 10 4 molec cm −3 for the nocturnal stable and residual layers respectively.