Effects of Aerosol Concentration, Relative Humidity, and Non-sphericity on Optical Properties of North African Desert Aerosols: OPAC 4.0 Simulations Validating with MODIS Satellite

Abstract: This study investigates the effects of varying aerosol concentrations, relative humidity (RH), phase functions, effective polarizability, and Ångström exponent (alpha) on North African desert aerosols, comparing OPAC 4.0 simulations with MODIS satellite data. Five model mixtures were analyzed for Mineral Accumulation Non-spherical (MIAN), Mineral Coarse Non-spherical (MICN), Mineral Nucleation Non-spherical (MINN), and water-soluble (WASO) components across concentration ranges. At 0% and 50% RH, OPAC 4.0 simulations showed that RH consistently increases alpha by +0.04 to +0.06 across all components due to hygroscopic growth. MICN and MINN models exhibited excellent internal consistency (R² > 0.99), with alpha values ranging from 0.086 to 0.133 (MICN) and 0.098 to 0.172 (MINN). WASO models produced alpha between 0.097 to 0.189 showing the strongest hygroscopic response. MIAN models showed variable alpha (-0.036 to 0.077), with Model 5 (alpha ≈ 0.04 to 0.05) now consistent with accumulation-mode dust. The turbidity coefficient (beta) remained stable for most OPAC models (0.127 to 0.301). Phase function analysis revealed two distinct angular scattering patterns: strong forward peaks for coarse dust and weaker, broader peaks for finer particles. Effective polarizability increased with concentration and RH. Validation using MODIS Deep Blue data (2000 to 2010) over ten North African locations yielded predominantly negative alpha values at eight sites (mean -0.284 to -0.086), confirming satellite detection of coarse-mode mineral dust. However, MODIS beta was quantitatively unreliable (R² 0.002 to 0.52, unphysical negative and extreme values), highlighting the challenge of bright desert surfaces. Positive alpha at Tamanrasset (+0.195) indicates mixed aerosols (dust + biomass burning). These findings provide a theoretical baseline (OPAC 4.0) and identify specific retrieval limitations (MODIS), improving the interpretation of satellite aerosol products over arid regions.