Uptake of ammonia by indoor surfaces: dependence on surface permeability, relative humidity, and environmental aging

NH₃ is a common basic gas that sorbs to indoor surfaces, which not only impacts its gas-phase abundance but also the levels of other basic or acidic volatile compounds. Here, we couple a laser spectrometer to a flow chamber to measure NH₃ uptake by paint and glass, which represent indoor permeable and impermeable surfaces, at varying relative humidity. By aging the surfaces in an occupied office, we assess how soiling impacts the NH₃ uptake, and we use a proton transfer reaction mass spectrometer (PTR-MS) to study how NH₃ impacts the gas-surface partitioning of acidic VOCs. We show that NH₃ uptake is surface-dependent, with paint undergoing both adsorptive and absorptive uptake, whereas glass is primarily governed by adsorptive uptake at low RH and interactions with water play a larger role at high RH. On glass, the saturated surface coverage (5.8 × 10¹³ molecules per cm²) is less than a full monolayer coverage, consistent with uptake at a limited number of NH₃ binding sites. For both surfaces, NH₃ uptake decreases with surface soiling, dropping by 22% for paint and 42% for glass after one week of environmental exposure in an occupied office. Lastly, increased gas-to-surface partitioning of organic acids with an aged glass surface arose via NH₃ exposure in a manner consistent with those acids' effective Henry's law constants, indicating that both acid dissociation and polar interactions govern this effect.