Reacto-Diffusive Length of N₂O₅ in Aqueous Sulfate- and Chloride-Containing Aerosol Particles

Heterogeneous reactions of dinitrogen pentoxide (N₂O₅) on aerosol particles impact air quality and climate, yet aspects of the relevant physical chemistry remain unresolved. One important consideration is the competing effects of diffusion and the rate of chemical reaction within the particle, which determines the length that N₂O₅ travels within a particle before reacting, referred to as the reacto-diffusive length (l). Large values of l imply a dependence of the reactive uptake efficiency of N₂O₅, i.e., γ(N₂O₅), on particle size. We present measurements of the size dependence of γ(N₂O₅) on aqueous sodium chloride, ammonium sulfate, and ammonium bisulfate particles. γ(N₂O₅) on ammonium sulfate and ammonium bisulfate particles ranged from 0.016 ± 0.005 to 0.036 ± 0.001 as the surface-area-weighted particle radius increased from 39 to 127 nm, resulting in an estimated l of 32 ± 6 nm. In contrast, γ(N₂O₅) on sodium chloride particles was independent of particle size, suggesting a near-surface reaction dominated the uptake of N₂O₅. Differences in the reactivity of the N₂O₅ intermediate, NO₂⁺, with water and chloride can explain the observed dependencies. These results allow for parameterizations in atmospheric models to determine a more robust population mean value of γ(N₂O₅) that accounts for the distribution of particle sizes. (Chemical Equation Presented).