Evaluation of the Effects of Aerosol Concentrations and Relative Humidity on Polarizability and Phase Function of Urban Aerosols Validated With Modis Satellite Data

This work numerically evaluates urban aerosol optical properties using OPAC 4.0 across 0.4–0.8 µm wavelengths, focusing on insoluble (INSO), soot (SOOT), and water-soluble (WASO) components at 0% and 50% relative humidity (RH). Fifteen aerosol models were developed with five concentration gradients per component. The Ångström exponent ( ) systematically decreases with increasing INSO (1.376-1.273) and increases with SOOT (1.376-1.386) and WASO (1.376-1.433). A critical non-linear hygroscopic response was identified: α increases from 0% to 50% RH at low WASO concentrations but decreases at high concentrations due to hygroscopic saturation and coarse-mode transition. Phase function forward scattering enhances with RH, most pronounced for WASO (1.626-1.785). MODIS validation across ten countries shows OPAC-derived (1.27-1.43) matching anthropogenically influenced regions (Germany =1.27, Ethiopia =1.45), while extreme values (South Africa =2.39; Egypt =0.12) represent aerosol types beyond OPAC's urban scope.