Size Dependent Uncatalyzed Sulfite Oxidation in Aqueous Microdroplets

Area of Science:

• Atmospheric Chemistry
• Environmental Science
• Physical Chemistry

Background:

• Aqueous aerosols and microdroplets have distinct chemical kinetics compared to bulk phases.
• These differences significantly influence air quality and climate through multiphase atmospheric processes.
• Understanding microdroplet chemistry is vital for accurate atmospheric modeling.

Purpose of the Study:

• To investigate the uncatalyzed oxidation of sulfite to sulfate by oxygen (O2) in aqueous microdroplets.
• To determine the influence of microdroplet size, gas-phase composition, and temperature on reaction kinetics.
• To elucidate the role of interfacial reactions versus bulk kinetics in microdroplet chemical transformations.

Main Methods:

• Utilized *in situ* micro-Raman spectroscopy to monitor reactions in deposited microdroplets.
• Varied microdroplet size, O2 concentration, and temperature during experiments.
• Employed a resistor-based model to analyze multiphase mass transfer and reaction kinetics.

Main Results:

• The uncatalyzed sulfite oxidation rate is dependent on microdroplet size, scaling with the surface-area to volume ratio (1/radius).
• Reaction rates increase significantly in smaller, atmospherically relevant droplet sizes due to enhanced interfacial reaction efficiency.
• Quantified kinetic parameters: *k*₂ = 9.43 × 10⁻³ M⁻¹ s⁻¹, *γ*_s,0 = 9.27 × 10⁻¹⁰ at 298 K and 21% O2.

Conclusions:

• Microdroplet size critically affects sulfite oxidation rates, with smaller sizes exhibiting accelerated kinetics.
• The observed size-dependence arises from a combination of slow bulk kinetics and efficient interfacial reactions.
• Findings necessitate improved representations of sulfate formation in atmospheric models, particularly for microscale phenomena.