Single Particle Investigation Supports Interfacial Pathway for SO₂-NO₂ Heterogeneous Reaction

The reaction between SO₂ and NO₂ is a key atmospheric sulfate formation pathway, yet its kinetics and mechanisms remain contentious. Here, we employ an aerosol optical tweezer to probe this reaction on microdroplets at the single-particle level. Two competing mechanistic frameworks were used for data interpretation─an aqueous and an interfacial reaction model. Accounting for intraparticle NO₂ depletion, the aqueous reaction model explains the data given composition-dependent reaction rate constants. However, our experiments show that chloride ions accelerate sulfate production by more than an order of magnitude compared to sulfate ions, which cannot be explained by the aqueous reaction framework. With data extracted from previous nanoparticle experiments and this work, the interfacial reaction model provides a good fit across studies, yielding an interfacial rate constant that exponentially decreases with particle pH. To further probe the effect of organics, we modified the particle composition with ionic surfactants and polyethylene glycol (PEG). The surfactants can slow down or even completely inhibit the reaction, strengthening the hypothesis that the reaction proceeds via an interfacial pathway. Yet for the water-soluble PEG, the reaction rate decreases almost linearly with its mass fraction among solutes. Finally, under atmospherically relevant conditions, a comparison of the rate constants emphasizes using the full particle size distribution to calculate sulfate formation rates. By evaluating the SO₂-NO₂ reaction across a wide particle size range under different frameworks, this work provides a holistic picture of this reaction, underscoring the need to resolve this critical atmospheric process for compositionally complex particles.