Elevated near-surface Ångström exponent peaks from dualwavelength CLADS over the North China Plain

Abstract Near-surface aerosol Ångström exponent (AE) profiles are needed for size-related air-quality assessment and to reduce uncertainties in multi-wavelength optical retrievals, yet pulsed lidars typically cannot resolve the lowest few hundred metres and surface in-situ optics lack vertical coverage. Using dual-wavelength (405/532 nm) charge-coupled device–laser aerosol detection system (CLADS)—an imaging lidar with dual laser and dual CCD—we retrieve continuous nocturnal AE and extinction profiles from the ground through the boundary layer over seven nights in November 2018 at Gucheng (North China Plain) during the McFAN campaign. On each of the seven nights, multi-layer AE structure is resolved below ~1.5 km. The central result is a recurrent elevated AE peak: while dual-wavelength extinction is highest at the surface, AE systematically maximizes in the ~110–230 m band (composite median near ~180 m), with a surface-to-peak contrast of ~1.1–1.4 relative to the 10–50 m layer (~1.0–1.3 if the two lowest bins, 10 and 20 m, are excluded and the surface reference is taken as 30–50 m). Aerosol loading and size-sensitive spectral slope are therefore vertically decoupled. Near-surface CLADS extinction at 405 and 532 nm covaries strongly with co-located dry nephelometry (pooled R ≈ 0.88–0.92). By resolving the lower-boundary-layer blind zone, dual-wavelength CLADS indicates that, in these cases, surface-based AE measurements can under-represent the fine-particle-dominated layer aloft, which is relevant to satellite/lidar retrieval constraints and boundary-layer radiative calculations. Nocturnal stable stratification is a plausible but untested explanation, because no concurrent thermodynamic profiles were available.