Laboratory Studies on the Carbon Kinetic Isotope Effects on the Production Mechanism of Particulate Phenolic Compounds Formed by Toluene Photooxidation: A Tool to Constrain Reaction Pathways

TL;DR

This study investigates the carbon isotope effects in phenolic compounds formed from toluene photooxidation, providing insights into reaction pathways and mechanisms of particulate organic matter formation.

Contribution

It introduces compound-specific isotope analysis to constrain reaction pathways of phenolic compound formation during toluene photooxidation.

Findings

Most phenolic products are isotopically depleted by 5-6 permil.

4-nitrophenol's isotope ratio remains unchanged from toluene.

Reaction with methylhydroxycyclohexadienyl radical and NO2 is a likely pathway.

Abstract

Compound-specific stable carbon isotope ratios for phenolic compounds in secondary particulate organic matter (POM) formed by photooxidation of toluene were studied. Secondary POM generated by photooxidation of toluene using a continuous-flow reactor and an 8 cubic meter indoor smog chamber was collected, and then extracted with acetonitrile. Eight phenolic compounds were identified in the extracts by a gas chromatograph coupled with a mass spectrometer, and their compound-specific stable carbon isotope ratios were determined by a gas chromatograph coupled with a combustion furnace followed by an isotope ratio mass spectrometer. The majority of the products, including methylnitrophenols and methylnitrocatechols, were isotopically depleted by 5 to 6 permil compared to the initial isotope ratio for toluene, whereas the isotope ratio for 4_nitrophenol remained the same as the initial isotope ratio for toluene. Based on the reaction mechanisms postulated in literature, stable carbon isotope ratios of these products were calculated. Comparison of the observed isotope ratios with the predicted implies that a reaction channel of methylhydroxycyclohexadienyl radical with NO2 is a possible production pathway for the particulate phenolic compounds.