Recent laboratory and computational chemistry studies have shown that N2O5 reacts at the atmospheric particle surface, with ClNO2 produced from chloride-containing particles. However, the traditional method of simulating multiphase reactions uses mass-based chemical composition, which assumes homogeneous distribution of chemical components across all particles with complete surface availability for reaction. We developed a new approach to parametrize N2O5 uptake and ClNO2 yield that considers the heterogeneity of the aerosol population and applied this in three wintertime environments – Ann Arbor, MI, Kalamazoo, MI, and East Boothbay, ME. Single-particle mass spectrometry and electron microscopy with energy-dispersive X-ray spectroscopy measured single-particle composition and showed that only a fraction of the particulate surface area in each study contained chloride. We assigned N2O5 uptake and ClNO2 yield values to each particle type, based on lab-based aerosol proxy data, and weighted these by particle surface area. This new approach more accurately simulated ClNO2, compared to traditional mass-based methods that frequently overestimate ClNO2 production. In the inland, wintertime environments, road salt aerosol was identified as a dominant chloride aerosol source. This new single-particle-based parameterization is expected to be applicable to other multiphase reactions that occur at the particle surface.

ClNO2 production is controlled by individual particle composition

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