Aerosol retrieval over snow using the RemoTAP algorithm

<p>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 the Polarization &amp; Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar (PARASOL), and a comparison has been made to retrievals with the baseline RemoTAP (without a snow kernel). For retrievals with real PARASOL observations, we use pixels over Aerosol Robotic Network (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 (<span class="inline-formula"><i>τ</i>₅₅₀)</span>, single-scattering albedo (SSA) at 550 nm (<span class="inline-formula"><i>ω</i>₅₅₀)</span> and Ångström exponent (AE) for 440–870 nm (<span class="inline-formula">AE_440−870)</span>. The experiments with both synthetic and real data show that the extended RemoTAP maintains capability for snow-free pixels and has obvious advantages in accuracy and the fraction of successful retrievals for retrieval over snow, especially over surfaces with snow cover <span class="inline-formula">&gt;75</span> %. 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 four-band retrieval (490, 565, 670, 865 nm) with the extended RemoTAP, we obtain a good retrieval result for <span class="inline-formula"><i>τ</i>₅₅₀</span>, <span class="inline-formula"><i>ω</i>₅₅₀</span> and <span class="inline-formula">AE_440−870</span>. Therefore, the four-band retrieval with the extended RemoTAP is recommended for aerosol retrieval over snow.</p>