(263d) Oxidized Organic Compounds Formed from Chlorine-Initiated Oxidation of Toluene

Oxidation of small aromatic compounds can affect the formation of ozone and organic aerosol. Most of the recent literature has focused on their reaction with hydroxyl radicals. However, recently, Cl-initiated photochemistry has been shown to significantly affect ozone formation in urban cities. In this work, we quantify the formation of secondary organic aerosol (SOA) from Cl-initiated photo-oxidation of small aromatic compounds including toluene, m-xylene and 1, 2, 4-trimethylbenzene under both low NOx and high NOx conditions. A high resolution time-of-flight chemical ionization mass spectrometer (HR-Tof-CIMS) was used to measure gas-phase products formed during each experiment. Major products observed during the Cl-initiated photo-oxidation of toluene included benzaldehyde, benzyl alcohol, benzyl hydroperoxide, phenol, cresol, epoxide, methylbutanedial, methylhexanedial, benzaldehyde, butanedial and bicyclic peroxy radical (Figure 1). Toluene-Cl photochemistry also produces secondary OH radicals, and typical toluene-OH products were also observed in these experiments.

Toluene-OH oxidation has been studied in the past, and it has been difficult to form highly oxidized SOA in environmental chamber experiments. In the work reported here we achieved SOA oxidation states as high as 0.24 using Cl-initiated photo-oxidation reactions, which occur at a faster timescale than OH-initiated reactions. The SOA oxidation states were inferred from mass spectra measured by an Aerosol Chemical Speciation Monitor (ACSM) and a High Resolution Aerosol Mass Spectrometer (HR-AMS) and were consistently higher than those reported previously. The addition of NO_x resulted in a higher organic aerosol oxidation state. Overall, our results indicate that toluene-Cl reaction under both low and high NOx conditions can yield highly oxidized organic aerosol and that this reaction pathway has the ability to alter SOA formation in the atmosphere.

Figure 1: Panel A shows major gas-phase products from toluene-Cl and toluene-OH pathways as measured by the HR-Tof-CIMS; Panel B shows common gas-phase products from both pathways as measured by the HR-Tof-CIMS; Panel C shows uncorrected organic aerosol formation as observed from the HR-AMS. Panels A, B and C all represent data obtained from low NO_x-toluene-Cl system. The thick black line represents additional injection of chlorine during the experiment.