Heterogeneous reaction of acetic acid on MgO, α-Al2O3, and CaCO3 and the effect on the hygroscopic behaviour of these particles

Mixtures of organic compounds with mineral dust are ubiquitous in the atmosphere, whereas the formation pathways and hygroscopic behavior of these mixtures are not well understood. In this study, in situ DRIFTS, XRD, and a vapor sorption analyzer were used to investigate the heterogeneous reaction of acetic acid on alpha-Al2O3, MgO, and CaCO3 particles under both dry and humid conditions while the effect of reactions on the hygroscopic behavior of these particles was also measured. In all cases, formation of acetate is significantly enhanced in the presence of surface water. However, the reaction extent varied with the mineral phase of these particles. For alpha-Al2O3, the reaction is limited to the surface with the formation of surface coordinated acetate under both dry and humid conditions. For MgO, the bulk of the particle is involved in the reaction and Mg(CH3COO)(2) is formed under both dry and humid conditions, although it exhibits a saturation effect under dry conditions. In the case of CaCO3, acetic acid uptake is limited to the surface under dry conditions while it leads to the decomposition of the bulk of CaCO3 under humid conditions. While coordinated surface acetate species increased the water adsorption capacity slightly, the formation of bulk acetate promoted the water absorption capacity greatly. This study demonstrated that heterogeneous reaction between CH3COOH and mineral dust is not only an important sink for CH3COOH, but also has a significant effect on the hygroscopic behavior of mineral dust.