Heterogeneous Uptake and Adsorption of Gas-Phase Formic Acid on Oxide and Clay Particle Surfaces: The Roles of Surface Hydroxyl Groups and Adsorbed Water in Formic Acid Adsorption and the Impact of Formic Acid Adsorption on Water Uptake.

Org. acids in the atm. are ubiquitous and are often correlated with mineral dust aerosol. Heterogeneous chem. and the uptake of org. acids on mineral dust particles can potentially alter the properties of the particle. In this study, heterogeneous uptake and reaction of formic acid, HCOOH, the most abundant carboxylic acid present in the atm., on oxide and clays of the most abundant elements, Si and Al, present in the Earth's crust are investigated under dry and humid conditions. In particular, quant. adsorption measurements using a Quartz Crystal Microbalance (QCM) coupled with spectroscopic studies using Attenuated Total Reflection Fourier Transform IR (ATR-FTIR) spectroscopy are combined to allow for both quantification of the amt. of uptake and identification of distinct adsorbed species formed on silica, alumina, and kaolinite particle surfaces at 298 K. These oxides and clay particles show significant differences in the extent and speciation of adsorbed HCOOH due to inherent differences in surface -OH group reactivity. Adsorbed water, controlled by relative humidity, can increase the irreversible uptake of formic acid. Interestingly, the resulting layer of adsorbed formate on the particle surface decreases the particle hydrophilicity thereby decreasing the amt. of water taken up by the surface as measured by QCM. Atm. implications of this study are discussed. [on SciFinder(R)]