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

Matti P Rissanen

doi:10.1021/acsearthspacechem.8b00123

Abstract

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 NOx reactions with oxidized peroxy radicals lead into indistinguishable product compositions, complicating mass spectral assignments in any RO2 + NOx 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.

Highlights

Autoxidation of volatile organic compounds (VOCs) is a rapid process by which volatile, gas-phase hydrocarbon precursors rapidly evolve into very low volatile end-products capable of acting as in situ atmospheric aerosol embryos.[1−5] It is a pseudounimolecular chain of reactions the efficiency of which relies on facile hydrogen abstraction isomerization reactions of the intermediate peroxy radicals (RO2; see Figure 1)
The dimer species constitute all the products observed with more carbon atoms than the parent cyclohexene, that is, a C7 compound would be considered a dimer in this case
Prevention of gas-phase highly oxidized product formation by NO2 was illustrated in the cyclohexene ozonolysis system, supporting the involvement of acylperoxy radicals as the key intermediates in the autoxidation chain reaction

Summary

Introduction

Autoxidation of volatile organic compounds (VOCs) is a rapid process by which volatile, gas-phase hydrocarbon precursors rapidly evolve into very low volatile end-products capable of acting as in situ atmospheric aerosol embryos.[1−5] It is a pseudounimolecular chain of reactions the efficiency of which relies on facile hydrogen abstraction isomerization reactions of the intermediate peroxy radicals (RO2; see Figure 1). If the RO2 structure is right, that is, if the radical has a loosely bound H atom in the carbon backbone that is about 5 to 8 atoms away from the oxygen atom containing the radical center, the peroxy radical can isomerize by an internal hydrogen abstraction reaction (i.e., Hshift) leading to another carbon-centered radical, and another prompt O2 addition.[3,6−8] This forms a hydroperoxyalkylperoxy radical (commonly denoted as OOQOOH) that is potentially able to undergo a second internal isomerization reaction and an O2 addition This chain of reactions repeats until a suitable reaction partner comes along (bimolecular termination), or if through transfer of the radical site the molecule reaches a structure which is prone to decomposition (unimolecular termination). The unimolecular hydrogen shift isomerization reactions of the peroxy radicals constitute the bottlenecks of the oxidation chain

Methods

Results

Conclusion