Surface Chemistry of α-FeOOH Nanorods and Microrods with Gas-Phase Nitric Acid and Water Vapor: Insights into the Role of Particle Size, Surface Structure, and Surface Hydroxyl Groups in the Adsorption and Reactivity of α-FeOOH with Atmospheric Gases

In this study, heterogeneous interactions of H2O and HNO3on goethite, α-FeOOH, a component of mineral dust aerosol, are investigated with simultaneous QCM measurements and ATR-FTIR spectroscopy. Laboratory synthesized α-FeOOH of varying sizes (microrods and nanorods) when exposed to gas phase H2O and HNO3results in the uptake of these gases. This combined approach of QCM measurements and ATR-FTIR spectroscopy allows for both quantification of the amount of uptake and spectroscopic data that provides information on speciation of adsorbed products. The results show that, in the case of H2O, both microrods and nanorods take up water and that the total amounts of water, when normalized to surface area, are similar. However, for HNO3uptake, the saturation coverage of total and irreversibly bound HNO3on microrods was observed to be higher than that on nanorods, a size effect which is attributed to surface structural changes that occur as a function of particle size. Furthermore, an investigation of the behavior of nitric acid reacted with α-FeOOH in aqueous media was carried out such as to better understand the effects of atmospheric processing upon dispersal within the hydrosphere.