A modeling study of the nighttime radical chemistry in the lower continental troposphere 2. Origin and evolution of HOx

Nighttime chemistry of OH, HO2, and organic peroxy radicals is examined in different environmental cases of the lower continental troposphere. The sce narios cover a large range of NOx concentrations and various mixtures of an thropogenic and biogenic VOCs. For each scenario the primary formation tale of radicals species is calculated and the evolution of the radical pool is investigated by calculating the efficiency of the propagation reactions ve rsus the termination reactions in the RO2--> HO2--> OH cycle. Finally, the net OH formation rates as well as the relative contributions of the NO3- an d O-3-initiated mechanisms in the OH production are calculated. We found th at the radical initiation is driven by ozonolysis of VOC in urban and envir onments with low NOx concentrations, while both NO3 and O-3 contribute to t he radical initiation in rural sites with moderated NOx levels. The nightti me formation of OH radicals exhibits similarities between the different sce narios. In most of the cases, OH formation at night results mainly from VOC +O-3 reactions in the mixing layer as well as the upper layer of the tropos phere. The VOC+NO3 reactions were found to account for a maximum of 25% in OH production in scenarios with high levels of biogenic compounds and moder ated levels of NOx. In the mixing layer the interconversion among radicals (RO2--> HO2 and HO2--> OH conversions),when it happens at night, is essenti ally driven by NO. The resulting OH fluxes vary from 0.2-1x10(7) to 1x10(6) molecule cm(-3) s(-1) in the urban scenario in the mixing and top layer, r espectively. In rural/remote situations, nighttime fluxes range from 1x10(6 ) molecule an in the mixing Layer to 1x10(5) molecule cm(-3) s(-1) in the u pper layer.