NO₂ Suppression of Autoxidation-Inhibition of Gas-Phase Highly Oxidized Dimer Product Formation

Atmospheric autoxidation of volatile organic compounds (VOC) leads to prompt formation of highly oxidized multifunctional compounds (HOM) that have been found crucial in forming ambient secondary organic aerosol (SOA). As a radical chain reaction mediated by oxidized peroxy (RO₂) and alkoxy (RO) radical intermediates, the formation pathways can be intercepted by suitable reaction partners, preventing the production of the highest oxidized reaction products, and thus the formation of the most condensable material. Commonly, NO is expected to have a detrimental effect on RO₂ chemistry, and thus on autoxidation, whereas the influence of NO₂ is mostly neglected. Here it is shown by dedicated flow tube experiments, how high concentration of NO₂ suppresses cyclohexene ozonolysis initiated autoxidation chain reaction. Importantly, the addition of NO₂ ceases covalently bound dimer production, indicating their production involving acylperoxy radical (RC(O)OO•) intermediates. In related experiments NO was also shown to strongly suppress the highly oxidized product formation, but due to possibility for chain propagating reactions (as with RO₂ and HO₂ too), the suppression is not as absolute as with NO₂. Furthermore, it is shown how NO reactions with oxidized peroxy radicals lead into indistinguishable product compositions, complicating mass spectral assignments in any RO₂ + NO system. The present work was conducted with atmospheric pressure chemical ionization mass spectrometry (CIMS) as the detection method for the highly oxidized end-products and peroxy radical intermediates, under ambient conditions and at short few second reaction times. Specifically, the insight was gained by addition of a large amount of NO₂ (and NO) to the oxidation system, upon which acylperoxy radicals reacted in RC(O)O₂ + NO₂ → RC(O)O₂NO₂ reaction to form peroxyacylnitrates, consequently shutting down the oxidation sequence.