Direct Determination of Aerosol pH: Size-Resolved Measurements of Submicrometer and Supermicrometer Aqueous Particles

Measuring the acidity of atmospheric aerosols is critical, as many key multiphase chemical reactions involving aerosols are highly pH-dependent. These reactions impact processes, such as secondary organic aerosol (SOA) formation, that impact climate and health. However, determining the pH of atmospheric particles, which have minute volumes (10–23–10–18 L), is an analytical challenge due to the nonconservative nature of the hydronium ion, particularly as most chemical aerosol measurements are made offline or under vacuum, where water can be lost and acid–base equilibria shifted. Because of these challenges, there have been no direct methods to probe atmospheric aerosol acidity, and pH has typically been determined by proxy/indirect methods, such as ion balance, or thermodynamic models. Herein, we present a novel and facile method for direct measurement of size-resolved aerosol acidity from pH 0 to 4.5 using quantitative colorimetric image processing of cellular phone images of (NH4)2SO4–H2SO4 aqueous aeros...