Wildfires across North America are intensifying, releasing fine particulate matter (PM2.5) rich in organic matter (OM) that strongly affects air quality, cloud formation, and climate. Yet, the chemical characterization of wildfire aerosol hygroscopicity remains poorly understood. We introduce a novel approach that simultaneously quantifies the chemical composition and hygroscopicity of PM2.5 and OM on a single Teflon filter, directly linking composition to water uptake. PM2.5 samples were collected from three Interagency Monitoring of Protected Visual Environments (IMPROVE) monitoring sites: Yosemite and Sequoia, impacted by local wildfires, and the Proctor Maple Research Facility (PMRF, Vermont), influenced by long-range transported smoke from Canada. OM composition was determined using a nondestructive Fourier transform infrared (FTIR) spectroscopy technique, while other PM2.5 components followed the IMPROVE protocol. The measured hygroscopicity varied between local and long-range smoke and even within each type, reflecting compositional and oxidative differences that led to distinct critical supersaturations for cloud activation. The measured PM2.5 hygroscopicity reflects a delicate compositional interplay: promoted by ammonium sulfate and oxygenated OM, yet constrained by nonoxygenated OM. This study provides the first composition-resolved quantification of hygroscopicity for local and long-range wildfire smoke at near-saturation RH, helping to reduce uncertainties in cloud formation, climate forcing, and air-quality modeling.
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