The reactions of ozone with alkenes: An important source of HO x in the boundary layer

The reactions of ozone with alkenes have been shown recently to lead to the direct production of OH radicals. Organic peroxy radicals (RO 2 ) probably accompany the production of OH. In this paper, we draw attention to the potential importance of these reactions in the primary production of HO x (HO x = OH, HO 2 and RO 2 ) radicals in various regions of the boundary layer. The reactions of ozone with anthropogenic alkenes are shown to be the most important source of HO x in many urban settings during the day and evening, and a significant source at night. The majority of HO x comes from trace quantities of alkenes with internal double bonds. Reaction of O 3 with isoprene and terpenes can be an important source of HO x in forested regions; we show that these reactions are the dominant radical source in the late afternoon and into the night. This additional HO x source is expected to increase predicted OH concentrations compared to those calculated by models that do not include the O 3 ‐alkene source.