Atmospheric photo-oxidation of acetic anhydride: Kinetic study and reaction mechanism. Products distribution and fate of CH3C(O)OC(O)CH2O· radical

Abstract

The rate coefficient for the gas-phase reaction of acetic anhydride (Ac₂O) with chlorine atoms at 298 K and atmospheric pressure was experimentally determined (k_Ac2O+Cl = (1.3 ± 0.4) × 10^-12 cm³ molec⁻¹ s⁻¹), while the rate coefficient for the reaction with the hydroxyl radical was estimated (k_Ac2O+OH=1.9 x 10^-13 cm³ molec⁻¹ s⁻¹). For the Structure-Activity Relationship method, a value of 0.02 was determined for the −C(O)OC(O) group. The mechanism of photo-oxidation of acetic anhydride initiated by chlorine atoms was determined and CO, CO₂, CH₃C(O)OH (32 %), CH₃C(O)OC(O)C(O)H, and 3-hydroxy-1,4-dioxane-2,6-dione (20 %) were identified as products by infrared spectroscopy. Here we determined for the first time the relative energies of the primary reaction pathways for the CH₃C(O)OC(O)CH₂O· radical using computational methods, which confirmed our experimental data. Finally, the environmental implications of acetic anhydride emissions were calculated, showing an atmospheric lifetime between 31 and 220 days for the reaction with atmospheric radicals, while its wet deposition lifetime is 1.5 years.