Constraining the Imaginary Refractive Index of Brown Carbon via an Observation-Informed Inversion Framework

Brown carbon (BrC) is a significant component of absorbing aerosols, yet its wavelength-dependent complex refractive index (CRI) remains one of the least constrained parameters in aerosol optical modeling. This study aims to constrain the imaginary part of the BrC CRI using an observation-informed physical inversion framework. We utilized in-situ absorption measurements collected in Ansan, South Korea, representing seasonal variations in 2024, alongside aerosol mass concentrations simulated by the A Global/Regional Integrated Model System-Chemistry Climate Model (GRIMs-CCM) and Weather Research & Forecasting Model (WRF) coupled with GEOS-Chem chemistry (WRF-GC) models. Optical properties were computed using the Flexible Aerosol Optical Depth (FlexAOD) system based on Mie theory. In our framework, organic carbon was partitioned into water-soluble and water-insoluble components to account for hygroscopic and compositional differences. The imaginary refractive index was parameterized as a power-law function of wavelength. By iteratively adjusting the spectral exponent to minimize discrepancies between observed and simulated Absorption Ångström Exponent (AAE) values (365–500 nm), we derived optimized CRI values. The results show that the optimized imaginary refractive index decreases monotonically with increasing wavelength, with the strongest spectral gradient observed in winter, indicative of enhanced shortwave absorption by BrC. The retrieved values align with reported ranges for strongly absorbing BrC. This study presents a physically consistent framework for improving the representation of BrC optical properties in radiative forcing assessments. Acknowledgment: This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government (MSIT) (No. RS-2025-16070879).