Explicit particle kinetics simulations of reactive diffusion at air–water interfaces

Reactive uptake of trace gases by aqueous aerosols reflects a balance between molecular diffusion and chemical reaction, yet separating interfacial from bulk contributions remains challenging. We present an explicit particle-based reactive diffusion framework that directly simulates stochastic transport and reactions at air-water interfaces. Using ozone-nitrite (O3-NO2-) as a model system, we show that concentration profiles reproduce the reacto-diffusive theory, allowing quantitative extraction of the characteristic length. The framework further resolves interfacial vs bulk reactivity, revealing that interfacial chemistry dominates when droplet radii approach or fall below the reacto-diffusive length. The results are presented for the well-established ozone-nitrite chemistry as a test case. Molecular encounters remain within the single-molecule limit, ensuring relevance to atmospheric conditions at lower concentrations without loss of generality. This framework provides a rigorous means to mechanistically connect microscopic kinetics with macroscopic uptake and is readily extendable to multiphase systems. This study further highlights the need for simulation frameworks to resolve nanometer-scale interfacial regions when droplet sizes approach the reacto-diffusive length, where the transition from diffusion-limited to reaction-limited behavior in bulk liquid elevates the dominance of interfacial chemistry.