Particle-phase processing of α-pinene NO3 secondary organic aerosol in the dark.

The NO 3 radical represents a significant nighttime oxidant which is present downstream of polluted environments. Existing studies have investigated the formation of secondary organic aerosol (SOA) from NO 3 radicals, focusing on the yields, general composition, and hydrolysis of organonitrates; however, there is limited knowledge about how the composition of NO 3 -derived SOA evolves as a result of particle-phase reactions. Here, SOA was formed from the reaction of α -pinene with NO 3 radicals generated from N 2 O 5 , and the resulting SOA was aged in the dark. The initial composition of NO 3 -derived α -pinene SOA was slightly dependent upon the concentration of N 2 O 5 injected (excess of NO 3 or excess of α -pinene) but was largely dominated by dimer dinitrates (C 20 H 32 N 2 O 8-13). Oxidation reactions (e.g., C 20 H 32 N 2 O 8→ C 20 H 32 N 2 O 9→ C 20 H 32 N 2 O 10) accounted for 60 %–70 % of the particle-phase reactions observed. Fragmentation reactions and dimer degradation pathways made up the remainder of the particle-phase processes occurring. The exact oxidant is not known, although suggestions are offered (e.g., N 2 O 5 , organic peroxides, or peroxynitrates). Hydrolysis of –ONO 2 functional groups was not an important loss term during dark aging under the relative humidity conditions of our experiments (58 %–62 %), and changes in the bulk organonitrate composition were likely driven by evaporation of highly nitrogenated molecules. Overall, 25 %–30 % of the particle-phase composition changes as a function of particle-phase reactions during dark aging, representing an important atmospheric aging pathway. [ABSTRACT FROM AUTHOR]