Uncovering the degradation kinetics and mechanisms of difluoroacetone in atmosphere and at air-water interface by OH radical and Cl atom: A theoretical investigation

The environmental and health risks of fluorinated compounds have attracted more and more attention due to their essential roles in the human body and potential contributions to greenhouse effects. Herein, the degradation mechanisms, kinetics properties, subsequent transformations, and atmospheric lifetimes of difluoroacetone (CF2HC(O)CH3) initiated by the Cl atom and OH radical were investigated in the atmosphere and at the air-water interface. The reaction coefficients and product branching ratios (or regioselectivity) for H-abstraction channels were calculated and analyzed within the range of 200-800 K by using multi-structural canonical variational transition state theory with small curvature tunneling (MS-CVT/SCT). At 297 K, the total rate coefficients of CF2HC(O)CH3 with the OH radical and Cl atom are, respectively, 1.39 × 10-14 and 8.04 × 10-14 cm3 molecule-1 s-1, which are consistent with the existing experimental data. In the presence of HO2, O2, and NO, CF2HC(O)CH3 can convert into COF2 and CO2 as end products. Our findings indicate that the OH radical plays a more significant role in determining the atmospheric lifetime of CF2HC(O)CH3 than the Cl atom. At the air-water interface, the H-abstraction reaction of CF2HC(O)CH3 induced by the OH radical occurs more rapidly at the -CH3 site (0.50 ps) than at the -CF2H site (2.50 ps), which is opposite to the selectivity of the gas-phase reaction. This study contributes to understanding the evolution mechanism of fluorinated acetone in a complex environment and improves our understanding of atmospheric chemical effects on aerosol surfaces.