Morphology and Fractal Dimension of Ambient Soot Particles From Vehicular Emissions in Three Contrasting Urban Tunnels

Abstract Vehicular emissions are a major source of soot particles, which significantly impact climate at regional and global scales. However, the morphology of ambient vehicle‐emitted soot particles remains inadequately understood, creating uncertainties in evaluating their warming potential. In this study, soot samples were collected from three urban tunnels with varying vehicle emission standards: Tunnel‐1 and Tunnel‐3 (strict standards) and Tunnel‐2 (less restrictive standards). Notably, sampling in Tunnel‐3 occurred during periods of fog and high humidity. Transmission electron microscopy was employed to analyze the morphological characteristics, fractal dimensions ( D f ), and mixing states of individual particles. Soot particles directly emitted from vehicles showed a higher proportion of externally mixed particles (99%) compared to those collected in Tunnel‐1 (67%), Tunnel‐2 (52%), and Tunnel‐3 (63%), indicating atmospheric aging. Compared to vehicle‐emitted soot ( D f of 1.65), increases in D f of 2%, 6%, and 5% were observed in Tunnel‐1, Tunnel‐2, and Tunnel‐3, respectively, with corresponding D f values of 1.69, 1.75, and 1.74 and geometric mean sizes of 235, 164, and 256 nm. Tunnel‐2, with less restrictive standards, emitted inherently smaller particles along with higher organics, leading to more internal mixing than Tunnels‐1 and ‐3. The study underscores that emission standards and high humidity significantly influence aging and morphological changes of soot particles, addressing a gap in previous laboratory‐based studies. Our findings provide quantitative insights into the morphological evolution of vehicular soot particles, improving the understanding of their aging process and enhancing the accuracy of laboratory‐based emission simulations.