THE NIGHTTIME PRODUCTION OF OH RADICALS IN THE CONTINENTAL TROPOSPHERE

Chemical pathways involved in the nocturnal production of hydroxyl rad ical (OH) and associated peroxy radicals (HO2 and RO2) are quantified for various environmental situations of the continental troposphere by means of numerical simulations. In the nocturnal stable layer, most o f 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 RO2 and HO2. The radic al propagation is mainly driven by NO: the nitrate radical (NO3) plays no role in such processes but is significantly involved in the direct formation of RO2 radicals. Above the nocturnal stable layer, whatever the environmental situation, OH radicals are mainly due to radical ch ain propagation in which NO3 plays a significant role. The nighttime s imulated OH concentrations are 3x10(4) to 5x10(5) and 1x10(4) to 5x10( 4) molec cm(-3) for the nocturnal stable and residual layers respectiv ely.