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The formation of organic nitrates (ONs) in the gas phase and their impact on mass formation of secondary organic aerosol (SOA) was investigated in a laboratory study for -pinene and -pinene photooxidation. Focus was the elucidation of those mechanisms that cause the often observed suppression of SOA mass formation by , and therein the role of highly oxygenated multifunctional molecules (HOMs). We observed that with increasing concentration (a) the portion of HOM organic nitrates (HOM-ONs) increased, (b) the fraction of accretion products (HOM-ACCs) decreased, and (c) HOM-ACCs contained on average smaller carbon numbers.

Specifically, we investigated HOM organic nitrates (HOM-ONs), arising from the termination reactions of HOM peroxy radicals with , and HOM permutation products (HOM-PPs), such as ketones, alcohols, or hydroperoxides, formed by other termination reactions. Effective uptake coefficients  of HOMs on particles were determined. HOMs with more than six O atoms efficiently condensed on particles ( on average), and for HOMs containing more than eight O atoms, every collision led to loss. There was no systematic difference in  for HOM-ONs and HOM-PPs arising from the same HOM peroxy radicals. This similarity is attributed to the multifunctional character of the HOMs: as functional groups in HOMs arising from the same precursor HOM peroxy radical are identical, vapor pressures should not strongly depend on the character of the final termination group. As a consequence, the suppressing effect of on SOA formation cannot be simply explained by replacement of terminal functional groups by organic nitrate groups.

According to their  …