Interfacial electric fields create hyperalkaline shells on fatty acid-coated microdroplet aerosols
Yangyang Liu1,2, Sonali Srivastava1,2, Delicia Gonsalves1,2
1Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA 24061.
Summary
Biomass burning aerosols can form a hyperalkaline surface shell (pH 9-11) on microdroplets, distinct from their acidic core. This "alkaline shell" phenomenon impacts atmospheric chemistry and the environmental fate of emissions.
Area of Science:
- Atmospheric chemistry
- Environmental science
- Physical chemistry
Background:
- Aerosol acidity (pH) is crucial for atmospheric multiphase chemistry, influencing pollutant behavior and aerosol formation.
- Biomass burning aerosols often have core-shell structures with organic coatings, challenging homogeneous models.
- Existing thermodynamic models assume particle homogeneity, not accounting for complex morphologies.
Purpose of the Study:
- To investigate the surface properties of organic-coated aerosols, specifically ammonium sulfate microdroplets coated with stearic acid.
- To determine if aerosol morphology influences surface acidity and its implications for atmospheric chemistry.
- To explore the formation and characteristics of a potential alkaline shell on biomass-burning-relevant aerosols.
Main Methods:
- Single-droplet surface-enhanced Raman spectroscopy
- Confocal fluorescence imaging
- Analysis of microdroplets in a critical size regime (50-150 µm)
Main Results:
- A size-dependent hyperalkaline surface shell (pH 9-11) was observed on stearic acid-coated ammonium sulfate microdroplets.
- Interfacial electric fields (~10^8 V/m) generated by surfactant monolayer alignment drive this phenomenon.
- The alkaline shell is sustained by a feedback loop involving surfactant dipoles, hydroxide confinement, and charge transfer, distinct from bulk equilibria.
Conclusions:
- Organic-coated aerosols can possess a hyperalkaline surface, acting as unique microreactors.
- This surface alkalinity can accelerate base-catalyzed reactions in the atmosphere.
- Findings challenge homogeneous thermodynamic assumptions and highlight the altered environmental fate of biomass burning emissions.
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