Photochemical conversion of toluene in simulated atmospheric matrix and characterization of large molecular weight products by +APPI FT-ICR MS

Abstract

The lack of kinetic parameters of VOC photochemical conversion especially in the presence of NO and O3, and product information, will result in the incomplete understanding for the occurrence of haze. In the present study, the photochemical conversion of toluene, one of most significant hydrocarbons of VOCs, in toluene, toluene/NO, toluene/O3 and reaction systems was assessed for up to 4 days in a smog chamber at 278 K and 308 K. The results indicated that the addition of NO and O3 promoted the conversion of toluene. The first-order kinetic rate constants of toluene assessed in the toluene reaction system at 278 K and 308 K were 0.12 and 0.19 day−1, respectively. The second-order kinetic rate constants of toluene assessed in the toluene/NO were 0.1 × 10−13 cm3 molecule−1 day−1 (at 278 K) and 0.9 × 10−13 cm3 molecule−1 day−1 (at 308 K), and those in the toluene/O3 reaction systems were 0.06 × 10−12 cm3 molecule−1 day−1 (at 278 K) and 0.11 × 10−12 cm3 molecule−1 day−1 (at 308 K). The small difference of second-order kinetic rate constants between 278 K and 308 K obtained for the toluene/O3 system when compared with the toluene/NO system indicated the reduced reaction sensitivity of O3 to the temperature. Several dozens of large molecule products containing up to O6 heteroatoms were identified by positive ion atmospheric pressure photoionization (+APPI) coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), with double bond equivalents up to 18, and carbon numbers ranging within 12–38, respectively. The findings presented herein may provide a new train of thought for the photochemical reaction process of toluene and its conversion in ambient air.