Heterogeneous chemistry of monocarboxylic acids on α-Al₂O₃ at different relative humidities

A study of the atmospheric heterogeneous reactions of formic acid, acetic acid, and propionic acid on alpha-Al2O3 was performed at ambient condition by using a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) reactor. From the analysis of the spectral features, observations of carboxylates formation provide strong evidence for an efficient reactive uptake process. Comparison of the calculated and experimental vibrational frequencies of adsorbed carboxylates establishes the bridging coordinated structures on the surface. The uptake coefficients of formic acid, acetic acid, and propionic acid on alpha-Al2O3 particles are (2.07 +/- 0.26)x10(-3) or (2.37 +/- 0.30) x10(-7), (5.00 +/- 0.69)x10(-3) or (5.99 +/- 0.78)x10(-7), and (3.04 +/- 0.63)x10(-3) or (3.03 +/- 0.52)x10(-7), respectively (using geometric or BET surface area). Furthermore, the effect of varying relative humidity (RH) on these heterogeneous reactions was studied. The uptake coefficients of monocarboxylic acids on alpha-Al2O3 particles increase initially (RH < 20%) and then decrease with the increased RH (RH > 20%) which was due to the effect of water on carboxylic acid solvation, particle surface hydroxylation, and competition for reactive sites. On the basis of the results of experimental simulation, the mechanism of heterogeneous reaction of alpha-Al2O3 with carboxylic acids at ambient RH was discussed. The loss of atmospheric monocarboxylic acids due to reactive uptake on available mineral dust particles may be competitive with homogeneous loss pathways, especially in dusty urban and desertified environments.