Spatial homogeneity of pH in aerosol microdroplets

Acidity plays a key role in atmospheric aerosol formation and strongly influences aerosols’ effects on air quality, climate, and the ecosystem. Fundamental questions about aerosol pH are, however, still under debate: how does the pH of aerosol microdroplets compare to that of the parent bulk solution, and do stable pH gradients exist within these microdroplets? Here, by using advanced single-particle and hyperspectral Raman microscopies, we directly measure the pH distribution in NaH2PO4-Na2HPO4 microdroplets (7–10 μm in radius) produced with the conjugate acid-base pair of H2PO4−-HPO42−. We show that the pH values of the investigated microdroplets are indistinguishable from the parent bulk solutions of the same electrolyte concentration and that the pH is essentially constant across the microdroplets, with standard deviations below 0.2 pH units. Similar results are observed for acidic NaHSO4 microdroplets. Our findings are essential to understanding atmospheric multiphase chemistry and overall reactions and processes occurring in aqueous microdroplets.