Technical note: A decoupled in situ framework for aerosol multi-wavelength optical properties: application for lidar retrievals

Abstract. Accurate constraints on aerosol complex refractive index (CRI), single scattering albedo (SSA), and lidar ratio (LR) are essential for improving lidar-based aerosol retrievals and reducing radiative-effect uncertainties. However, consistent derivation from in situ observations remains challenging in complex mixed-aerosol environments, where size distribution, absorption, and humidity effects are strongly coupled. Here we present a decoupled inversion framework that integrates SMPS–OPC size distributions, AE33-derived absorption coefficients, and ambient relative humidity observations from the Vipava Valley (Slovenia) in April 2016 to derive dry-state homogeneous-equivalent CRI and RH-corrected lidar-relevant SSA and LR at 355, 532, and 1064 nm. The framework reconstructs a continuous particle size distribution through instrument-response-aware SMPS–OPC geometric alignment and retrieves the reference real refractive index. AE33-derived absorption constraints and a singly subtractive Kramers–Kronig (SSKK) relation are then used to derive a spectrally consistent dry state CRI, after which SSA and LR are recalculated through an RH-corrected wet-state forward optical calculation. The retrieved dry-state mean n values were 1.448, 1.441, and 1.437, and the mean k values were 0.0312, 0.0238, and 0.0192 at 355, 532, and 1064 nm, respectively. The RH-corrected mean SSA values were 0.866, 0.852, and 0.774, and the corresponding mean LR values were 86.4, 58.4, and 39.2 sr. Sensitivity tests showed that the AE33 effective multiple-scattering correction affects absorption-sensitive products, with C_eff = 5 used as a site- and period-specific baseline. The framework provides a practical pathway for linking dry in situ CRI retrievals with RH-corrected multi-wavelength lidar-relevant aerosol optical products.