Peroxy radicals from night-time reaction of N03 with organic compounds

Abstract

REACTIONS of free radicals are the driving force for most chemical processes in the atmosphere. Although hydroxyl (OH) and nitrate (NO3) radicals are the most reactive, peroxy radicals such as HO2 and CH3O2 have an important role as intermediates in tropospheric ozone generation and as precursors to OH radicals1. Moreover, self-reaction of peroxy radicals is the only known gas-phase source of peroxides to the atmosphere. It has long been known that there exist pathways to the formation of radicals—in particular NO3 (refs 2–5) —that do not require sun-light, yet radical chemistry is usually discussed in terms of photochemistry occurring during the day1. The possibility of night-time production of peroxy radicals has received little attention, except perhaps in highly polluted air6–8. Here we show that recent kinetic investigations9,10 combined with field observations of NO3, organic and other relevant species11,12 suggest that tropospheric high night-time concentrations of peroxy radicals—possibly even exceeding daytime values—may occur under certain atmospheric conditions, for instance in coastal areas or forests and in air masses affected by urban emission or biomass burning. The prerequisite for this occurrence is the simultaneous presence in the same air mass of reactive organic species, oxides of nitrogen and ozone.