现行POL DER陆地气溶胶偏振反演算法对粒子尺度的敏感性分析

The Sensitivity of the Current POLDER Land Aerosol Polarized Retrieval Algorithm to Particle Size

目前,偏振遥感已被用于气溶胶卫星和地基遥感中,其仅对小粒子敏感。本研究以2007-2008年间的POLDER(Polarization and Directionality of the Earth’s Reflectances)和AERONET(Aerosol Robotic Network)合肥站的气溶胶数据为研究对象,探讨了偏振遥感对气溶胶粒子尺度的敏感性。利用AERONET合肥站的尺度分布数据和折射指数数据,计算了不同尺度范围内气溶胶粒子的光学厚度,并与对应的POLDER反演结果作拟合,通过寻找最佳拟合对应的尺度范围确定偏振遥感敏感的粒子尺度。结果表明,偏振遥感并非对所有尺度的小模式气溶胶粒子都敏感,在865 nm波段,其敏感的气溶胶粒子上限半径约为0.3μm左右。这一结果对指导气溶胶偏振遥感,以及理解和应用偏振遥感气溶胶产品等具有重要意义。

One of the polarized remote sensing advantages is its insensitivity to the land surface reflection, and now it has been widely used for aerosol properties retrieval from satellite and ground-based remote sensing. As to aerosols, the polarized remote sensing is only sensitive to small particles; however, there are different understandings of the small particle size to which extend the polarized remote sensing is sensitive. For example, some studies take fine mode aerosols as small particles, but the maximum radius of fine mode aerosol varies between 0.4～1.0μm. What＇ s the upper threshold of the particle size to which the polarized remote sensing is sensitive？ In order to determine the sensitivity of the polarized remote sensing to aerosol particle size, three steps are conducted as follows： firstly, the POLDER （Polarization and Directionality of the Earth＇ s Reflectances, which is equipped on Polarization and Anisotropy of Reflectances for Atmospheric Science coupled with Observations from a Lidar） and AERONET （Aerosol Robotic Network） data in Hefei between 2007 and 2008 are collectedand processed, i.e. the AERONET data within a 1-hour range window （~30min） around the POLDER during ze- nith pass and the POLDER data in a 3~3 pixel box centered on the Hefei site are averaged in time and space re- spectively. Secondly, the aerosol optical depths in different particle size range are calculated using the refractive index and size distribution data from Hefei site according to Mie theory. Finally, the particle size to which the po- larized remote sensing is sensitive is determined by finding the best fit between the POLDER aerosol optical depth and the AERONET aerosol optical depth calculated above. The results show that the polarized remote sens- ing is not sensitive to all the fine mode particles, but sensitive to particles with radius approximately less than 0.3μm at 865nm band. The results will not only help us to understand and utilize the aerosol products obtained from the polarized remote sensing, but also help us in polarized remote sensing application.