Interfacial electric fields create hyperalkaline shells on fatty acid-coated microdroplet aerosols.

Aerosol acidity (pH) is a fundamental property governing atmospheric multiphase chemistry, as it influences pollutant partitioning, secondary organic aerosol formation, and trace metal solubility. Thermodynamic models typically assume the internal homogeneity of submicron particles; however, fresh aerosols produced via biomass burning often possess complex core-shell morphologies wherein an aqueous core is coated by organic surfactants. Here, we report that a size-dependent "alkaline shell" exists for aqueous ammonium sulfate microdroplets coated with stearic acid, a ubiquitous constituent of biomass burning. Using single-droplet surface-enhanced Raman spectroscopy and confocal fluorescence imaging, we identify a critical size regime (50 to 150 µm) where alignment of the surfactant monolayer generates strong interfacial electric fields (~108 V/m). This field drives local partitioning of protons and hydroxide ions, sustaining a hyperalkaline surface shell (pH ~9 to 11). We attribute this phenomenon to a feedback loop, distinct from bulk equilibria, involving surfactant dipole alignment, hydrophobic confinement of hydroxide, and interfacial charge transfer. These findings add critical nuance to bulk thermodynamic predictions, demonstrating that while the aerosol core remains acidic, the interface of organic-coated aerosols can act as a unique, high-pH microreactor in the atmosphere, potentially accelerating base-catalyzed reactions and altering the environmental fate of biomass emissions.