SOA from limonene: Role of NO₃ in its generation and degradation

The formation of organic nitrates and secondary organic aerosol (SOA) were monitored during the NO₃ + limonene reaction in the atmosphere simulation chamber SAPHIR at Research Center Jülich. The 24-h run began in a purged, dry, particle-free chamber and comprised two injections of limonene and oxidants, such that the first experiment measured SOA yield in the absence of seed aerosol, and the second experiment yields in the presence of 10 μgm^-3 seed organic aerosol. After each injection, two separate increases in aerosol mass were observed, corresponding to sequential oxidation of the two limonene double bonds. Analysis of the measured NO3, limonene, product nitrate concentrations, and aerosol properties provides mechanistic insight and constrains rate constants, branching ratios and vapor pressures of the products. The organic nitrate yield from NO₃ + limonene is ≈30%. The SOA mass yield was observed to be 25-40%. The first injection is reproduced by a kinetic model. PMF analysis of the aerosol composition suggests that much of the aerosol mass results from combined oxidation by both O ₃ and NO₃, e.g., oxidation of NO₃ + limonene products by O₃. Further, later aerosol nitrate mass seems to derive from heterogeneous uptake of NO₃ onto unreacted aerosol alkene.