Theoretical study of scattering Angstrom exponent of coated black carbon aerosols: The effect of microphysical configurations. (November 2020) - Zhang, Xiaolin
Highlights: Smaller SAE may not always indicates larger particles, especially thinly coated BC. The importance of particle size distribution on the SAE of coated BC is stressed. The SAE of aged BC is numerically studied with recent observations. Sensitivities of coated BC SAE are numerically investigated. The microphysical factors affecting coated BC SAE are addressed. Abstract: The study numerically assesses the influences of particle microphysics, including shell/core ratio, black carbon (BC) geometry, coated volume fraction of BC, and size distribution, on the scattering Angstrom exponent (SAE) of polydisperse BC aggregates coated by organics based on the multiple-sphere T-matrix method. The dependence of coated BC scattering on shell/core ratio is highest among all microphysical parameters, indicating the significance of ambient measurements of BC shell/core ratio. The SAE of coated BC exhibits wavelength dependence with smaller value at smaller wavelength, whereas the SAE uncertainty due to wavelength selection is limited with differences less than 10%. The SAE values of coated BC between 450 nm and 700 nm have broad variation with a range of 0.5–2.6 for typical coated BC microphysics. The SAE of coated BC is highly sensitive to size distribution, and it decreases with particle becoming larger. Compared to size distribution, the dependence of the SAE on shell/core ratio, BC geometry, coated volume fraction of BC is generally weak, and the SAE of coated BC with larger coated volume fraction of BC becomes less sensitive to BC fractal dimension but more sensitive to shell/core ratio. The popular core-shell Mie model generally shows acceptable SAE results for heavily coated BC with differences within 10%, whereas significant SAE errors can be introduced for thinly coated BC particles. Our work clearly demonstrates the impact of particle size distribution on the SAE of coated BC based on more realistic BC geometries, yet suggesting that smaller SAE of coated BC may not always indicate larger particles, particularly for thinly coated BC aerosols.