On the role of particle inorganic mixing state in the reactive uptake of N₂O₅ to ambient aerosol particles

The rates of heterogeneous reactions of trace gases with aerosol particles are complex functions of particle chemical composition, morphology, and phase state. Currently, the majority of model parametrizations of heterogeneous reaction kinetics focus on the population average of aerosol particle mass, assuming that individual particles have the same chemical composition as the average state. Here we assess the impact of particle mixing state on heterogeneous reaction kinetics using the N₂O₅ reactive uptake coefficient, γ(N₂O₅), and dependence on the particulate chloride-to-nitrate ratio (nCl^-/nNO₃ ^-). We describe the first simultaneous ambient observations of single particle chemical composition and in situ determinations of γ(N ₂O₅). When accounting for particulate nCl ^-/nNO₃^- mixing state, model parametrizations of γ(N₂O₅) continue to overpredict γ(N ₂O₅) by more than a factor of 2 in polluted coastal regions, suggesting that chemical composition and physical phase state of particulate organics likely control γ(N₂O₅) in these air masses. In contrast, direct measurement of γ(N₂O ₅) in air masses of marine origin are well captured by model parametrizations and reveal limited suppression of γ(N₂O ₅), indicating that the organic mass fraction of fresh sea spray aerosol at this location does not suppress γ(N₂O₅). We provide an observation-based framework for assessing the impact of particle mixing state on gas-particle interactions.