Computational Study of the Effect of Mineral Dust on Secondary Organic Aerosol Formation by Accretion Reactions of Closed-Shell Organic Compounds.

Abstract

The effect of dust aerosols on accretion reactions of water, formaldehyde and formic acid was studied in the conditions of Earth s troposphere using at the DLPNO-CCSD(T)/aug-cc-pVTZ//ωB97X-D/6-31++G** level of theory. Detailed analysis of the reaction mechanisms in the gas phase and on the surface of mineral dust, represented by mono- and tri-silicic acid, revealed that mineral dust has the potential for decreasing reaction barrier heights. Specifically, at 0 K, mineral dust can lower the apparent energy barrier of the reaction of formaldehyde with formic acid to zero. However, when the entropic contributions to the reaction free energies were accounted for, mineral dust was found to selectively enhance the reaction of water with formaldehyde, while inhibiting the reaction of formaldehyde and formic acid, in the lower parts of the troposphere (with temperatures around 298 K). In the upper troposphere (with temperatures closer to 198 K), mineral dust catalyzes both reactions, and also the reaction of methanol with formic acid. Despite the intrinsic potential of mineral dust, calculation of the catalytic enhancement parameter for a likely range of dust aerosol concentrations suggested that dust aerosols will not contribute to secondary organic aerosol formation via dimerization of closed-shell organic compounds. The main reason for this is the relatively low absolute concentration of tropospheric dust aerosol, and its inefficiency in increasing the effective reaction rate coefficients.