Identification and Quantitation of Aerosol Products of the Reaction of β-Pinene with NO3 Radicals and Implications for Gas- and Particle-Phase Reaction Mechanisms.

Substantial amounts of gas- and particle-phase organic nitrates have been reported in field studies of atmospheric chemistry conducted around the world, and it has been proposed that a significant fraction of these may be formed from the nighttime reaction of monoterpenes with NO3 radicals. In the study presented here, β-pinene (a major global monoterpene emission) was reacted with NO3 radicals in an environmental chamber and the molecular and functional group composition of the resulting secondary organic aerosol (SOA) was determined using a variety of methods. Eight products, which comprised ∼95% of the SOA mass, were identified and quantified. More than 90% (by mass) of these consisted of acetal heterodimers and heterotrimers that were apparently formed through acid-catalyzed reactions in phase-separated particles. The molar yield of the major oligomer was 16.7%, and the yields of the other six and the single monomer ranged from 1.1% to 2.9%, for a total yield of 30.7%. From these analyses it was determined that the yields of the two major monomer building blocks were 25.9% and 23.6%, and that those of the other four ranged from 2.0% to 4.8%, for a total monomer yield of 62.4%. The measured SOA mass yield was 88.9% and the O/C, N/C, and H/C ratios, molecular weight, and density of the SOA calculated from the results of functional group analysis of the bulk SOA were 0.40, 0.11, 1.79, 217 g mol-1, and 1.21 g cm-3, respectively, similar to values estimated from results of molecular analysis. The results demonstrate the combined importance of RO2• + RO2• reactions, alkoxy radical decomposition and isomerization, and acid-catalyzed particle-phase reactions in the NO3 radical-initiated oxidation of β-pinene and subsequent formation of SOA and should be useful for understanding reactions of other monoterpenes and for developing models for the laboratory and atmosphere.