Reactions of NO3 radicals with limonene and α-pinene: Product and SOA formation

Monoterpenes are ubiquitous in forested environments and are subject to reactions with OH radicals, NO3 radicals and O3 both within and above forest canopies. While reactions of monoterpenes with OH and O3 have received considerable attention with regard to their reaction kinetics, reaction products and secondary organic aerosol (SOA) formation those with NO3 have received relatively little consideration. To redress this imbalance and to try and assess the possible importance of NO3+monterpene reactions within forest canopies investigations have been performed on selected monterpenes. The products and aerosol formation from the NO3 radical initiated oxidation of limonene and α-pinene have been investigated in the EUPHORE photoreactor facility, and the secondary chemistry of the observed major products was then investigated in a large volume laboratory photoreactor. In addition to large yields of yet unidentified organic nitrates, pinonaldehyde and endolim have been identified as the major reaction products of the NO3 radical initiated oxidation of α-pinene and limonene, respectively. The aerosol formation has been investigated in the presence of different seed aerosols and also water vapour. The reactions lead to the formation of SOA and the results indicate the importance of the chemical character of the seed aerosol in determining the SOA yield in NO3+monoterpene reactions. The possible pathways leading to SOA formation and also their potential importance are discussed based on the laboratory results on the reaction of NO3 with the two major products.