Research on Cloud Fraction Inversion Algorithm of Environmental Trace Gas Monitoring Instrument

The trace gas retrieval algorithms rely on information on cloud properties for each ground pixel. Such information is important for the development of EMI cloud products because clouds have a significant impact on the photon path. The photon path strongly affects the information on trace gases contained in the satellite observations. In this study we discuss a cloud algorithm based on O2-O2 absorption at 477 nm using EMI measurements. Firstly, the O2-O2 slant column densities (SCDs) were retrieved from the range of 460-490nm using the differential optical absorption spectroscopy (DOAS) technique, meanwhile the stripes of O2-O2 SCDs were corrected. Secondly, the different values of solar zenith angle (SZA), viewing zenith angle (VZA), relative azimuth angle (RAA), surface altitude and surface albedo were set as input parameters to the radiative transfer model SCIATRAN to establish lookup tables of O2-O2 SCDs and continuum reflectance. To acquire lookup tables of cloud fraction with O2-O2 SCDs and continuum as nodes we inverted the above-mentioned lookup tables. Finally, the EMI measured values of O2-O2 SCDs and continuum reflectance used to obtain effective cloud fraction. The results with the OMI cloud products were compared to validate EMI cloud algorithm. The high and low values of cloud fraction distribute at the same locations. What’s more, the trends of EMI and OMI cloud fraction frequency percent are in good agreement, the cloud frequency distribution shows a decreasing trend from low to high cloud fraction, and when cloud fraction at 0 and 1, both of their frequency percent are very high. The OMI and EMI cloud fraction agree well with each other and have a high correlation coefficient (R) of 0.82.