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 because of their essential roles in the human body and potential contributions to greenhouse effects. Herein, the degradation mechanism, kinetic properties, subsequent transformation, and atmospheric lifetime of difluoroacetone (CF2HC(O)CH3) initiated by 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 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 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 the 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 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.