Abstract. The formation of organic nitrates (ON) in the gas phase and their impact on mass formation of Secondary Organic Aerosol (SOA) was investigated in a laboratory study for α-pinene and β-pinene photo-oxidation. Focus was the elucidation of those mechanisms that cause the often observed suppression of SOA mass formation by NOx, and therein the role of highly oxygenated multifunctional molecules (HOM). We observed that with increasing NOx (a) the portion of HOM organic nitrates (HOM-ON) increased, (b) the fraction of accretion products (HOM-ACC) decreased and (c) HOM-ACC contained on average smaller carbon numbers. Specifically, we investigated HOM organic nitrates (HOM-ON), arising from the termination reactions of HOM peroxy radicals with NOx, and HOM permutation products (HOM-PP), such as ketones, alcohols or hydroperoxides, formed by other termination reactions. Effective uptake coefficients γeff of HOM on particles were determined. HOM with more than 6 O-atoms efficiently condensed on particles (γeff > 0.5 in average) and for HOM containing more than 8 O-atoms, every collision led to loss. There was no systematic difference in γeff for HOM-ON and HOM-PP arising from the same HOM peroxy radicals. This similarity is attributed to the multifunctional character of the HOM: as functional groups in HOM arising from the same precursor HOM peroxy radical are identical, vapor pressures should not strongly depend on the character the final termination group. As a consequence, the suppressing effect of NOx on SOA formation cannot be simply explained by replacement of terminal functional groups by organic nitrate groups. The fraction of organic bound nitrate (OrgNO3) stored in gas-phase HOM-ON appeared to be substantially higher than the fraction of particulate OrgNO3 observed by aerosol mass spectrometry. This result suggests losses of OrgNO3 for organic nitrates in particles, probably due to hydrolysis of OrgNO3 that releases HNO3 into the gas phase but leaves behind the organic rest in the particulate phase. However, the loss of HNO3 alone, could not explain the observed suppressing effect of NOx on particle mass formation from α-pinene and β-pinene. We therefore attributed most of the reduction in SOA mass yields with increasing NOx to the significant suppression of gas-phase HOM-ACC which have high molecular mass and are potentially important for SOA mass formation at low NOx conditions.
Supplementary material to "Atmospheric photo-oxidation of myrcene: OH reaction rate constant, gas phase oxidation products and radical budgets"