The effect of NOx on formation of Highly Oxidized Multifunctional Molecules and SOA formation in photochemical system of α-pinene and β-pinene

Highly oxygenated organic molecules (HOM) are formed in the atmosphere by autoxidation, i.e. peroxy radicals can undergo H-shift followed by O₂ addition. A sequence of these very fast steps leads to highly oxygenated peroxy radicals (HOM-RO₂) and finally to stable termination products with O/C>1.
As other RO₂, HOM-RO₂ are terminated by reactions with RO₂, HO₂ and NO_x and in addition form efficiently stable accretion products. In this study, three noticeable effects on HOM formation were found by introducing NO_x in the photochemical system of monoterpenes. One effect is formation of highly oxygenated organic nitrates (HOM-ON) with sufficiently low vapor pressures allowing significant contributios to SOA formation. The second one is dimer suppression, because of competing dimer pathway (HOM-RO₂·+ RO₂·) and organic nitrate pathway (HOM-RO₂·+ NO_x). Thirdly, the reaction between peroxy radicals and NO increases alkoxy radicals in the system. The fragmentation of alkoxy radicals produces volatile compounds that should result in decrease of SOA yield. However, the effect of fragmentation is offset: alkoxy radicals also undergo H-shifts that produce alkyl radicals and after O₂ addition peroxy radicals, that eventually are higher oxygenated.

Because of their low volatility, HOM play a crucial role in new particle formation and secondary organic aerosol (SOA) formation. Suppression of dimers and increased degree of oxidation of the HOM monomer play together with the result of only a small reduction of the SOA yields.