A Multi Constraint Absorbing Aerosol Microphysics and Optics Framework for Radiative Forcing Uncertainty and Methane Retrieval Biases in Industrial Regions

Absorbing aerosols in industrial regions exhibit rapidly evolving particle size distributions and mixing states that are poorly represented by common fixed-parameter assumptions, introducing coupled uncertainties in both aerosol radiative forcing and shortwave infrared trace-gas retrievals, particularly for methane (CH₄) in the 2.3-2.4 μm window. Here we develop a multi-constraint framework that combines column optical observations (multi-band AOD and SSA) with in situ size-resolved measurements and multi-wavelength black carbon mass to jointly constrain aerosol microphysics and optical behavior. A physically constrained core-shell Mie model is used to generate microphysically plausible solution ensembles, filtered by multi-waveband optical consistency and probability-density overlap with observed size spectra, thereby reducing inversion non-uniqueness and suppressing biases toward coarse-mode dominance and overly strong internal mixing.

The resulting constrained aerosol optical properties are propagated through radiative transfer modeling to quantify top-of-atmosphere and atmospheric forcing sensitivities to microphysical variability in industrial environments. Finally, the same observation-constrained absorption spectra are extended across 0.25-4 μm (with enhanced spectral resolution in methane-sensitive bands similar to TROPOMI) to diagnose wavelength-dependent aerosol-CH₄ spectral coupling: we show that even when absolute SWIR absorption is modest, aerosol-induced transmittance perturbations can partially overlap CH₄ absorption troughs and destabilize continuum/baseline fitting, such that broadband retrieval windows may accumulate small spectral mismatches into substantial interference. This behavior is further amplified by time-varying spectral slopes (e.g., non-stationary AAE), implying that fixed aerosol parameterizations are insufficient for robust CH₄ retrieval correction in complex emission regions. We note that we have direct radiative forcings ranging from -4 to -33 W/m², which at the lower end of the range may allow a way to bias-correct retrievals of CH₄, while at the higher end of the range implies that any signals retrieved for CH₄ are significantly caused by BC. Overall, this integrated approach provides a transferable pathway to simultaneously improve absorbing-aerosol forcing estimates and reduce aerosol-induced biases in satellite methane retrievals via band-optimized, aerosol-aware retrieval strategies.