OH and Cl radicals initiated oxidation of amyl acetate under atmospheric conditions: kinetics, products and mechanisms

Abstract

The relative rate coefficients of the gas-phase reaction of amyl acetate, (AA), CH₃COO(CH₂)₄CH₃ with OH radicals and Cl atoms were determined at (298 ± 2) K and 1000 mbar of pressure. The experiments were developed in two different atmospheric Pyrex chambers coupled with “in situ” Fourier Transform Infrared (FTIR) spectroscopy and Gas Chromatography equipped with flame ionization detection (GC-FID). The rate coefficients obtained from the average of different experiments were (in units of cm³ molecule⁻¹ s⁻¹): k_AA+OH-FTIR = (6.00 ± 0.96) × 10⁻¹²; k_AA+OH-GC-FID = (6.37 ± 1.50) × 10⁻¹² and k_AA+Cl-GC-FID = (1.35 ± 0.14) × 10⁻¹⁰. Additionally, product studies were completed for the Cl-initiated oxidation of AA, in similar conditions of the kinetic experiments by Gas Chromatography coupled with a mass detector (GC-MS) with Solid Phase Micro Extraction (SPME). Acetic acid, formaldehyde, acetaldehyde, propionaldehyde, and butyraldehyde were the main products identified. Complementary Structure Activity Relationships (SAR) were developed to compare with the experimental kinetic results and to clarify the individual reactivity sites of the ester. The atmospheric oxidation pathways of the AA are postulated and discussed taking into account the observed products and the SAR estimations. The initial pathway for the degradation of AA initiated by Cl atoms and OH radicals occurs via H-atom abstraction at –C(O)OCH₂– (C1); –CH₂– (C2); –CH₂– (C3); and –CH₂CH₃– (C4) moieties. The atmospheric implications of the reactions studied were evaluated by the estimation of their tropospheric lifetimes toward OH radicals and Cl atoms to be: τ_OH = 22 and τ_Cl = 62 hours. Consequently, the estimated average ozone production ([O₃] = 2.15) suggests a potential contribution of these compounds emission to the formation of photochemical smog. On the other hand, the Photochemical Ozone Creation Potential (POCP) for AA was calculated to be POCP = 70.2. A moderate risk of photochemical smog production suggests that this ester could be harmful to the health and the biota in urban environments.