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

Abstract

Nighttime chemistry of OH, HO2, and organic peroxy radicals is examined in different environmental cases of the lower continental troposphere. The scenarios cover a large range of NOx concentrations and various mixtures of anthropogenic and biogenic VOCs. For each scenario the primary formation rate of radicals species is calculated and the evolution of the radical pool is investigated by calculating the efficiency of the propagation reactions versus the termination reactions in the RO2 → H02 → OH cycle. Finally, the net OH formation rates as well as the relative contributions of the NO3− and O3‐initiated mechanisms in the OH production are calculated. We found that the radical initiation is driven by ozonolysis of VOC in urban and environments with low NOx concentrations, while both NO3 and O3 contribute to the radical initiation in rural sites with moderated NOx levels. The nighttime formation of OH radicals exhibits similarities between the different scenarios. In most of the cases, OH formation at night results mainly from VOC + O3 reactions in the mixing layer as well as the upper layer of the troposphere. 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 moderated levels of NOx. In the mixing layer the interconversion among radicals (RO2 → HO2 and HO2 → OH conversions), when it happens at night, is essentially driven by NO. The resulting OH fluxes vary from 0.2–1×107 to 1×106 molecule cm−3 s−1 in the urban scenario in the mixing and top layer, respectively. In rural/remote situations, nighttime fluxes range from 1×106 molecule cm−3 s−1 in the mixing layer to 1×105 molecule cm−3 s−1 in the upper layer.