Observed Secondary Organic Aerosol (SOA) and Organic Nitrate Yields From NO3 Oxidation of b-pinene and Limonene

The yields of organic nitrates and of secondary organic aerosol (SOA) particle formation were measured for the reactions NO3 + β-pinene and NO3 + limonene in the atmosphere simulation chamber SAPHIR at Research Center Juelich. These experiments were conducted at low concentrations of NO3 (NO3 + N2O5 < 10 ppb) and monoterpene (peak ~ 15 ppb), with no seed aerosol. SOA formation in both cases was observed to be prompt and substantial, and highly correlated with organic nitrate formation. In the case of limonene oxidation, the sequential oxidation of both double bonds is apparent. The observed gas/aerosol partitioning of organic nitrates can be simulated using an absorptive partitioning model to derive an estimated vapor pressure of the condensing nitrate species of pvap ~ 5 × 10-6 Torr (7 × 10-4 Pa, which constrains speculation about the oxidation mechanism and chemical identity of the organic nitrate. The observations of high aerosol yield (~50% for β- pinene and ~20-30% for limonene) from NO3-initiated oxidation of monoterpenes provide an example of a significant anthropogenic source of SOA from biogenic hydrocarbon precursors. Estimates of the NO3 + monoterpene SOA source strength for California and the globe indicate that NO3 reactions with monoterpenes are likely an important source of organic aerosol on regional and global scales.