OH Radical and Chlorine Atom Kinetics of Substituted Aromatic Compounds: 4-chlorobenzotrifluoride ( p -ClC 6 H 4 CF 3 ).

The mechanisms for the OH radical and Cl atom gas-phase reaction kinetics of substituted aromatic compounds remain a topic of atmospheric and combustion chemistry research. 4-Chlorobenzotrifluoride ( p -chlorobenzotrifluoride, p -ClC 6 H 4 CF 3 , PCBTF) is a commonly used substituted aromatic volatile organic compound (VOC) in solvent-based coatings. As such, PCBTF is classified as a volatile chemical product (VCP) whose release into the atmosphere potentially impacts air quality. In this study, rate coefficients, k 1 , for the OH + PCBTF reaction were measured over the temperature ranges 275-340 and 385-940 K using low-pressure discharge flow-tube reactors coupled with a mass spectrometer detector in the ICARE/CNRS (Orléans, France) laboratory. k 1 (298-353 K) was also measured using a relative rate method in the thermally regulated atmospheric simulation chamber (THALAMOS; Douai, France). k 1 ( T ) displayed a non-Arrhenius temperature dependence with a negative temperature dependence between 275 and 385 K given by k 1 (275-385 K) = (1.50 ± 0.15) × 10 -14 exp((705 ± 30)/T) cm 3 molecule -1 s -1 , where k 1 (298 K) = (1.63 ± 0.03) × 10 -13 cm 3 molecule -1 s -1 and a positive temperature dependence at elevated temperatures given by k 1 (470-950 K) = (5.42 ± 0.40) × 10 -12 exp(-(2507 ± 45) /T) cm 3 molecule -1 s -1 . The present k 1 (298 K) results are in reasonable agreement with two previous 296 K (760 Torr, syn. air) relative rate measurements. The rate coefficient for the Cl-atom + PCBTF reaction, k 2 , was also measured in THALAMOS using a relative rate technique that yielded k 2 (298 K) = (7.8 ± 2) × 10 -16 cm 3 molecule -1 s -1 . As part of this work, the UV and infrared absorption spectra of PCBTF were measured (NOAA; Boulder, CO, USA). On the basis of the UV absorption spectrum, the atmospheric instantaneous UV photolysis lifetime of PCBTF (ground level, midlatitude, Summer) was estimated to be 3-4 days, assuming a unit photolysis quantum yield. The non-Arrhenius behavior of the OH + PCBTF reaction over the temperature range 275 to 950 K is interpreted using a mechanism for the formation of an OH-PCBTF adduct and its thermochemical stability. The results from this study are included in a discussion of the OH radical and Cl atom kinetics of halogen substituted aromatic compounds for which only limited temperature-dependent kinetic data are available.