HOx radical regeneration in the oxidation of isoprene

We propose, and quantify from first principles, two novel HOx-regenerating unimolecular reactions in isoprene oxidation, which are estimated to yield in pristine tropical forest conditions about 0.7 HO2 and 0.03 OH radicals per isoprene oxidized; it is further argued that the photolabile coproduct of HO2 can be a major source of OH, with a yield of the order of 1. The newly proposed chemistry could provide a rationalization for the unexpectedly high OH concentrations often observed in forested environments, such as over the Amazon forest in the recent Gabriel campaign.