Sustained Low‐Acidity of Ammonium Sulfate Particles

Abstract Acidity is a fundamental property of the aqueous phase of aerosol particles. The acidity of aerosol particles has dominantly been investigated using thermodynamic models, yet the validity of the approach has rarely been examined by experiments. Especially, experimental data are scarce for atmospherically important suspending fine‐mode particles. We tackled the issue using the recently developed fluorescence‐based method for quantifying the pH of submicron particles, especially focusing on ammonium sulfate. Particles were exposed to gaseous ammonia (1–2,000 ppb) under a humidity‐controlled environment. Under high (>100 ppb) ammonia concentration, the particles were slightly more acidic (pH = 4–4.5) than the bulk solution of ammonium sulfate (pH = 5.07) due to ammonia evaporation, consistent with thermodynamic models. However, experimentally measured values of pH (3.5–4) were higher than the model predictions (pH = 2–2.5) for the lower ammonia concentration range (<10 ppb), likely due to slow ammonia evaporation. As the atmospheric lifetime of ammonia (hours–day) is shorter than that of sulfate (∼days), exposure of ammonium sulfate particles to a low ammonia condition could also occur in the actual environment. We suggest that the role of the kinetics for gas‐particle partitioning of semi‐volatile chemical species such as ammonia needs to be reexamined for accurately estimating acidity.