Atmospheric sink of styrene, α-methylstyrene, trans-β-methylstyrene and indene: Rate constants and mechanisms of Cl atom-initiated degradation

Abstract

The kinetics and products of the oxidation of four aromatic compounds, i.e. styrene, α-methylstyrene, trans-β-methylstyrene and indene, with Cl atoms were determined at atmospheric pressure and room temperature. Kinetic experiments were carried out in a 400 L Teflon reaction chamber using GC-FID for the analysis of reactants and products were determined using a 56 L quartz-glass reactor coupled to FTIR spectrophotometer. The rate constants at 298 K, using different reference compounds, were (in units of cm3 molecule−1 s−1): kstyrene = (1.29 ± 0.52)×10−10, kα-methylstyrene = (1.55 ± 0.27)×10−10, ktrans-β-methylstyrene = (1.09 ± 0.23)×10−10 and kindene = (1.01 ± 0.30)×10−10. Observations with FTIR suggest that the main reaction is the addition of the Cl to the aliphatic chain of the aromatic molecules. We found benzaldehyde, benzoyl chloride, formaldehyde and formyl chloride from styrene; acetophenone, formaldehyde and formyl chloride from α-methylstyrene; and benzaldehyde, formaldehyde and acetyl chloride from trans-β-methylstyrene as the main oxidation products. DFT theoretical calculations were performed in order to shed light on the identification of the reaction products. To the best of our knowledge, this work represents the first determination of the rate coefficients and products for the reaction of the Cl atoms with these compounds, except for the rate constant of styrene which has been studied previously. The loss processes of the title compounds in the atmosphere are mostly controlled by reactions with OH radicals during the day and with NO3 at night, but in coastal areas and some polluted environments, Cl reactions became comparable with OH and NO3 radicals, with lifetimes of 2.2 h for styrene, 1.8 h for AMS, 2.5 h for TBMS and 2.8 h for indene.