Evolution of the Composition and pH of Single Aqueous Aerosols Produced from Bulk Aqueous Sulfite Solutions

Sulfur speciation and pH are important parameters in aerosol chemistry that influence the oxidation of S(IV) to S(VI), a critical process that contributes to haze pollution. Here, we monitor the evolution of the composition and pH of aqueous aerosols (radii ∼ 4 μm) prepared from bulk solutions containing sulfite/bisulfite through quantitative spectroscopic measurements utilizing an aerosol optical tweezer coupled to cavity-enhanced Raman spectroscopy. Under N₂ at ∼80% relative humidity, individual, trapped aqueous aerosols are generated from solutions ranging from ca. pH 2-6 with a vibrating mesh nebulizer. Aerosols show a significant loss of sulfur relative to bulk solutions due to SO₂ evaporation with a concomitant increase in the aerosol pH. An ambient pressure mass spectrometer confirms SO₂ partitioning from the aerosol into the gas phase. Most interesting is that the loss of sulfur from the aerosol due to the partitioning of gas-phase SO₂ occurs at a higher solution pH compared to bulk solutions, suggesting an enrichment of protons at the aerosol surface that plays a role in the protonation of bisulfite, a precursor to the formation of gas-phase SO₂. An electrical low-pressure impactor was used to determine whether the aerosols have a slight positive charge. In air (21% O₂ in N₂), S(IV) is completely oxidized to S(VI), as sulfate, on the time scale of minutes and occurs several hundred times faster than for bulk solutions. In addition to soluble S(IV) oxidation, we also establish the time scales for gas-phase partitioning and aerosol pH changes. Overall, this study shows the need to quantify aqueous aerosol composition, pH, and charge following production from bulk solutions and the role of aqueous surfaces in driving compositional changes, highlighting the role of surfaces in multiphase sulfur chemistry for accurate representation in air quality models.