Photochemical ultrafine particles pose potential inflammatory risk despite low PM2.5 mass

BACKGROUND: Air quality improvements have reduced PM2.5 mass, but particle surface area, which primarily contributes to inflammatory responses, may not follow the same trend, potentially creating health-relevant exposure. RESULTS: and secondary organic carbon importance) contributes to these periods by producing abundant 50-100 nm particles during summer-autumn afternoons under warm, high-radiation conditions. CONCLUSIONS: These photochemical ultrafine particle events represent a mechanistically distinct pollution regime where mass-based frameworks may underestimate potential inflammatory effects, a gap that may widen as emission controls reduce primary sources and climate warming enhances photochemistry.