Evaluation of Aerosol Optical Depth Products From Multiangular and Polarized Satellite Measurements Over Mountainous Areas

Abstract

This paper aims to determine the factors which introduce more errors to mountainous aerosol optical depth (AOD) in the hope that such results can guide aerosol inversion over mountainous areas. AODs retrieved by the GRASP algorithm from POLDER-3 are compared with those obtained from AERONET over global mountainous areas between 2005 and 2013. Comparation between the AODs from GRASP/POLDER-3 (AOD <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GP</sub> ) and AERONET (AOD <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</sub> ) are used to test the performance of the GRASP algorithm. It was found that the accuracy of AOD <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GP</sub> is significantly different over variable underlying surfaces with different terrain type, and mountainous AOD <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GP</sub> is seriously overestimated. The AOD <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GP</sub> values are much less consistent with AODA over mountainous than urban areas for almost all aerosol types, which suggests that the terrain size has a significant impacts on aerosol remote sensing. Mountainous AOD <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GP</sub> in low latitudes performs better than that in mid-latitudes as the former is less disturbed by shelters and the shadows of mountain bodies. The performance of AOD <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GP</sub> as a function of altitude and slope are similar, where the correlation decreases and the overestimation percentage increases quickly with an increase of slope or altitude. The observation geometry and aerosol profile change as a function of slope and altitude, which impart inaccuracies in the retrieved AOD. Overall, the terrain size is dominant in the multiangular and polarization remote sensing of aerosols and the POLDER measurements are sensitive to mountain terrain size. Mountainous slope and altitude disturb the AOD <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GP</sub> most seriously, followed by latitude and seasonal variation.