Atmospheric aging effects of carbonaceous aerosols in a near-roadside environment: Insights from chemical composition and optical properties.

Traffic-derived carbonaceous aerosols are key drivers of urban climate, yet their evolution from exhaust to ambient air remains unelucidated. This study characterizes the spatiotemporal evolution of near-road carbonaceous aerosols (5-50 m) through chemical and optical analyses across seasons. Total carbonaceous concentrations, especially organic carbon (OC), decreased sharply with distance from the road during daytime observations. The OC/elemental carbon (EC) ratios declined from 3.0 ± 1.7 at 5 m to 1.7 ± 1.5 at 50 m, with the steepest decline (1.3-fold) occurring between 5 and 15 m, identifying 5-15 m as the dominant zone for vehicular OC influence. Additionally, during summer daytime, significant SOC production was presented at a distance of 5 m from the road, primarily existing in the form of OC2. Conversely, brown carbon (BrC) exhibited significantly weaker light absorption than black carbon (BC). While winter vehicular BC absorption was confined within 30 m, summer impacts extended beyond 50 m, potentially linked to additional source superposition in winter. A multiple linear regression model incorporating distance, EC concentration, and char-EC/soot-EC ratio captured variations in BC mass absorption efficiency (MAEBC) within 5-15 m (R2 = 0.82), providing a framework for understanding near-road BC light absorption.