Heterogeneous Uptake Kinetics of Volatile Organic Compounds on Oxide Surfaces Using a Knudsen Cell Reactor: Adsorption of Acetic Acid, Formaldehyde, and Methanol on α-Fe2O3, α-Al2O3, and SiO2

Abstract

The role of heterogeneous reactions on particulate matter present in the Earth's atmosphere remains an important question in tropospheric chemistry. It has been proposed in several modeling studies that mineral dust may provide reactive surfaces for trace atmospheric gases. Laboratory studies can provide some answers concerning the kinetics of these reactions, so that heterogeneous chemistry can be quantitatively assessed in atmospheric chemistry models. In this study, the heterogeneous uptake kinetics of several volatile organic compounds (VOCs) on oxide surfaces have been measured with a Knudsen cell reactor at 295 K. In particular, the heterogeneous uptake of acetic acid, methanol, and formaldehyde on α-Fe2O3, α-Al2O2, and SiO2 has been investigated. These VOCs are representative of some of the different types of oxygenated organics found in the atmosphere. The oxide particles used in this study are models for mineral dust found in the Earth's atmosphere. Initial uptake coefficients, γ0, have been extracted from the Knudsen cell data. The uptake kinetics have been measured as a function of sample surface area to ensure that realistic surface areas are used in the calculation of the uptake coefficient. Heterogeneous reaction rates are then compared to homogeneous reactions rates for gas-phase reactions involving acetic acid, methanol, and formaldehyde. From this comparison, the possible atmospheric implications of heterogeneous reactions involving these oxygenated organics are discussed.