ClNO2 production is controlled by individual particle composition

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.