Direct observation of the complex S(IV) equilibria at the liquid-vapor interface

Area of Science:

• Atmospheric Chemistry
• Environmental Science
• Physical Chemistry

Background:

• Multi-phase oxidation of sulfur(IV) compounds is a key process in atmospheric chemistry.
• This process contributes to haze formation and severe air pollution episodes.
• Understanding the molecular mechanisms is vital for atmospheric modeling and pollution control.

Purpose of the Study:

• To determine experimentally the pK_a values of sulfur(IV) tautomers.
• To investigate reaction barriers for sulfur dioxide (SO₂) formation pathways.
• To elucidate the molecular-level behavior of sulfur(IV) at aqueous aerosol interfaces.

Main Methods:

• Experimental determination of pK_a values.
• Measurement of reaction barriers for SO₂ formation.
• State-of-the-art molecular-dynamics simulations.
• Analysis of bisulfite depletion and tautomer ratios at the liquid-vapor interface.

Main Results:

• Experimentally determined pK_a values for sulfur(IV) tautomers were reported.
• Reaction barriers for SO₂ formation pathways were identified.
• Molecular dynamics revealed bisulfite depletion at low pH at the liquid-vapor interface.
• A stable contact ion pair between sulfonate and hydronium ions was observed.
• A higher energetic barrier for sulfonic acid dehydration at the interface was found.

Conclusions:

• Interfacial and bulk environments exhibit contrasting physicochemical behavior.
• The pH-dependent tautomer ratio at interfaces significantly impacts SO₂ uptake kinetics.
• This behavior affects reactions involving nitrogen oxides (NO_x) and hydrogen peroxide (H₂O₂) at aerosol interfaces.
• Findings provide molecular-level understanding crucial for atmospheric chemistry and pollution studies.