On the negative polarization's effects at top of atmosphere

Abstract

Polarized signal has been proven to be sensitive to the aerosol optical depth and aerosol microphysical parameters. However, in most current applications, the directions of polarization, which are expressed by the sign of polarization, have long been regarded as minor factor. In most remote sensing retrievals, the absolute values of negative polarizations are considered. Therefore, whether this neglection will introduce uncertainty in remote sensing has not been investigated. In this study, TOA polarized reflectances are calculated with the measured surface (with sign) and modeled (without sign) polarized reflectance to obtain the TOA polarized reflectance dataset. Our results suggest that 7% of the surface measured polarized reflectance is negative, while after the vector radiative transfer calculation, this proportion increases to 20–50%. Although correlations between absolute values of measured and modeled data are high (>0.99), when including the effects of the sign, the correlations will decrease to less than 0.5 or even 0.2, and RMSE will increase 10 times. These discrepancies will introduce a lot of uncertainty in polarized remote sensing retrieval when mixed pixels with opposite polarizations for subpixels occur. Furthermore, the same polarization for different objects is another problem when there are both negative and positive polarizations. Our study is particularly important for future applications of polarized signals to obtain accurate and reliable results.