Aerosol retrieval over snow using RemoTAP

Abstract. In order to conduct accurate aerosol retrieval over snow, the Remote Sensing of Trace Gases and Aerosol Products (RemoTAP) algorithm developed by SRON Netherlands Institute for Space Research is extended with a Bi-directional Reflection Distribution Function (BRDF) for snow surfaces. The capability of the extended algorithm is validated with both synthetic measurements and real satellite measurements from PARASOL and a comparison has been made to retrievals with the baseline RemoTAP (without snow kernel). For retrievals on real PARASOL observations, we use pixels over AERONET stations for validation and we use the MODIS snow cover products to identify pixels over snow. We evaluate the retrieved aerosol optical thickness (AOT) at 550 nm (𝜏₅₅₀), single-scattering albedo (SSA) at 550 nm (𝜔₅₅₀), and Angstrom exponent (AE) for 440 nm – 870 nm (AE_440−870). Both the experiments with synthetic- and real data show that the extended RemoTAP maintains capability on snow-free pixels and has obvious advantages on accuracy and fraction of successful retrievals for retrieval over snow, especially over surfaces with snow cover > 75 %. According to the real data experiment, we find that the retrieval algorithm has difficulty in fitting the PARASOL 1020 nm band, where snow reflectance is significantly lower than that for the visible bands. When we perform a 4-band retrieval (490 nm, 565 nm, 670 nm, 865 nm) with the extended RemoTAP, we obtain a good retrieval result for 𝜏₅₅₀, 𝜔₅₅₀ and AE_440−870. Therefore, the 4-band retrieval with the extended RemoTAP is recommended for aerosol retrieval over snow.