Mechanism of Nitrous Acid Uptake onto Soil Particles Coated with Thin-Films of Water

Recent field studies have greatly enhanced our understanding of the sources of nitrous acid (HONO), indicating that this important OH radical source is formed on both ground and aerosol surfaces in the boundary layer. However, large uncertainties remain in quantifying HONO sinks and determining the mechanism of HONO uptake onto surfaces coated in thin films of water, which also controls the HONO budget. We report here the first laboratory study of HONO uptake onto soil particles under atmospheric conditions using a coated-wall flow tube coupled to a highly sensitive chemical ionization mass spectrometer (CIMS). A detailed study of the soil composition and interaction of water with the internal and external mineral surface area provides unique insights into the role of water in controlling the multiphase chemistry of HONO on these surfaces. A chemical kinetics model based on the competitive adsorption of HONO and water onto the particle surfaces explains the dependence of the HONO uptake coefficients on the initial HONO concentration and relative humidity. Results will be compared to recent field observations of HONO in the troposphere.