This grant supports research by Andrew Ault, Assistant Professor of Environmental Health Sciences and Chemistry at the University of Michigan, to examine the pH of indoor surfaces and answer two related questions: “What are the properties of aqueous films on indoor surfaces?” and, more specifically, “What is the pH of surface water layers indoors?” To this end, Ault will determine the properties of water layers and range of pH values present on the surface of six common building materials—glass, concrete, drywall, latex painted drywall, carpet, and wood—as well as six associated proxy model systems—silicon dioxide, quicklime (cement)/limestone (aggregate), gypsum, synthetic rubber, nylon, and cellulose. Materials will be studied before and after aging for six months in a residential environment. The project will determine the water and water layer properties (including island formation, structured water, and accessible water fraction) as a function of relative humidity (RH) for different materials, model systems, and aged samples. The project also will reveal the intrinsic pH of the samples as a function of RH, as well as the differences in pH for aged samples across spatial scales ranging from nano to macro. Last, Ault and his team will determine the sensitivity of pH to gaseous acids and bases and acidic aerosols and associated kinetics.