| Summary: | The gas-phase reaction between <i>trans</i>-2-methyl-2-butenal and chlorine (Cl) atoms has been studied in a simulation chamber at 298 ± 2 K and 760 ± 5 Torr of air under free-NO<sub>x</sub> conditions. The rate coefficient of this reaction was determined as <i>k</i> = (2.45 ± 0.32) × 10<sup>−10</sup> cm<sup>3</sup> molecule<sup>−1</sup> s<sup>−1</sup> by using a relative method and Fourier transform infrared spectroscopy. In addition to this technique, gas chromatography coupled to mass spectrometry and proton transfer time-of-flight mass spectrometry were used to detect and monitor the time evolution of the gas-phase reaction products. The major primary reaction product from the addition of Cl to the C-3 of <i>trans</i>-2-methyl-2-butenal was 3-chloro-2-butanone, with a molar yield (Y<sub>Prod</sub>) of (52.5 ± 7.3)%. Acetaldehyde (Y = (40.8 ± 0.6)%) and HCl were also identified, indicating that the H-abstraction by Cl from the aldehyde group is a reaction pathway as well. Secondary organic aerosol (SOA) formation was investigated by using a fast mobility particle sizer spectrometer. The SOA yield in the Cl + <i>trans</i>-2-methyl-2-butenal reaction is reported to be lower than 2.4%, thus its impact can be considered negligible. The atmospheric importance of the titled reaction is similar to the corresponding OH reaction in areas with high Cl concentration.
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