Recently, SONG Zigeng, a doctoral student in our laboratory, published a research paper titled "Effect of the Vertical Distribution of Absorbing Aerosols on the Atmospheric Correction for Satellite Ocean Color Remote Sensing" in IEEE Transactions on Geoscience and Remote Sensing. The corresponding author is researcher HE Xianqiang in our laboratory.
Absorbing aerosols have a non-negligible impact on the atmospheric correction of water color satellite remote sensing, especially causing large errors in the retrieval of water-leaving radiance in the blue light band and ultraviolet band. Existing absorptive aerosol correction models usually do not consider vertical distribution changes and lack quantitative understanding of their impact. Based on radiative transfer simulation, this paper studies the impact of the vertical distribution of absorptive aerosols on the retrieval (atmospheric correction) of satellite-observed top-of-atmosphere (TOA) radiance and water-leaving radiance. First, the occurrence frequency of global absorptive aerosols (sand aerosols, smoke aerosols) was obtained based on the long-term lidar satellite CALIPSO observation data statistics, and it was found that the occurrence frequency of the Sahara and Arabian Deserts, China, central and southern Africa, and the coastal areas of the Indian Peninsula The average annual occurrence frequency exceeds 30% (Figure 1). Secondly, based on the AERONET ground-based observation aerosol optical property data, a new aerosol classification algorithm was established to obtain the optical properties of absorptive aerosols over the ocean (Figure 2).
Figure 1. Spatial distribution of the average annual occurrence frequency of various aerosols in global oceans. (a) Marine aerosol; (b) Sand aerosol; (c) Smoke aerosol; (d) Mixed aerosol
Figure 2 (a) Spatial distribution of aerosol types obtained based on AERONET site data; (b) real part of complex refractive index; (c) imaginary part of complex refractive index; (d) asymmetry factor; (e) particle size spectrum distribution.
On the basis of obtaining the optical properties of absorptive aerosols and based on radiative transfer simulation, the influence of the vertical distribution of absorptive aerosols on the reflectivity of the top of atmosphere (TOA) at 412 nm was studied. The results show that changes in the vertical distribution of dust aerosols can affect TOA reflectance by up to 8% (Figure 3), which is equivalent to the impact of changes in aerosol optical thickness. Under the assumption of Gaussian vertical distribution, if the influence of absorptive aerosols is not considered, the 412 nm water-leaving radiance may have an error of about 4%-10% (Figure 4), using exponential vertical distribution and simplified double layer The errors produced by the atmospheric model will reach 12% ~ 15% and 30% ~ 40%. Overall, in the case of absorptive aerosols, inaccurate vertical distribution information and aerosol mode selection may result in retrieval uncertainties of water-leaving radiance of 10% to 80%.
The aerosol classification algorithm, absorptive aerosol optical property model and vertical distribution radiation simulation results established in this study can lay the foundation for the further development of strongly absorptive aerosol atmospheric correction models.
Figure 3 At the same optical thickness (AOD = 0.3), the difference in reflectivity at the top of the atmosphere at 412 nm caused by different mean heights of dust aerosols. The circumferential direction in the figure represents the satellite observation-solar relative azimuth angle (0°~180°), and the radial direction is the mean altitude. The values of the solar zenith angle (SZA) are 0°, 30°, and 60°, and the values of the observation zenith angle (VZA) are 0°, 20°, 40°, and 60°.
Figure 4 Under the same optical thickness (AOD = 0.2), 412 nm caused by different observation zenith angles of dust aerosols (a~d), urban aerosols (e~h), and oceanic aerosols (i~l) Difference in radiance out of water (after atmospheric correction). The circumferential direction in the figure represents the satellite observation-sun relative azimuth angle (0°~180°), and the radial direction is the observation zenith angle. The solar zenith angle (SZA) values are 20° and 60°, and the three aerosol vertical distribution models all adopt Gaussian distribution. Case I is a medium turbid water body, and Case II is a highly turbid water body.
Song, Z., He, X., Bai, Y., Wang, D., Gong, F., Zhu, Q., Li T., & Li H. Effect of the Vertical Distribution of Absorbing Aerosols on the Atmospheric Correction for Satellite Ocean Color Remote Sensing. IEEE Transactions on Geoscience and Remote Sensing, doi: 10.1109/TGRS.2022.3151219.
