Expanding Aerosol Acidity Measurements to the Water-Limited Regime: A Coupled Condensational Growth and Colorimetric Approach

Exposure to elevated concentrations of atmospheric aerosols leads to millions of deaths globally per year from respiratory and cardiovascular diseases. Acid-catalyzed multiphase reactions involving semivolatile organic species are a key pathway for forming additional particle-phase mass, as atmospheric aerosols are highly acidic (−1 < pH < 5, average ≈ 2). However, direct aerosol pH measurements are extremely challenging due to small particle sizes (atmospheric mode ∼100 nm), low water content, and the nonconservative nature of the H⁺ ion. This is particularly true in aerosols with pH < 2 where ionic strengths >10 molal are common. Widely used thermodynamic models for predicting aerosol pH have limited measurement-based validation and struggle at low pH, necessitating new experimental approaches. Herein, we present a method for measuring aerosol pH across a wide pH range (−2 to 4), including the difficult-to-access regime where pH < 0. Suspended particles are grown in a precisely controlled, supersaturated environment to a uniform diameter (4.8 μm) prior to impaction on a colorimetric pH indicator, with acidified ammonium sulfate (AAS). This dilution introduces sufficient liquid water for the indicator and increases the pH into the colorimetrically-measurable range. Our results demonstrate that aerosols can be more acidic than the original solution by up to 3 pH units, with increasing deviations from solution at lower pH. Measured aerosols were also more acidic than thermodynamic model predictions. Initial results with this method will enable pH measurement in complex atmospheric chamber experiments and real-world locations where pH values remain highly uncertain and poorly constrained, addressing the critical need for accurate aerosol acidity values to predict secondary aerosol formation.