Surface reflectivity from the Ozone Monitoring Instrument using the Moderate Resolution Imaging Spectroradiometer to eliminate clouds: Effects of snow on ultraviolet and visible trace gas retrievals

摘要

Satellite retrievals of tropospheric composition from measurements of solar backscatter require accurate information about surface reflectivity. We use clear-sky data from the Ozone Monitoring Instrument (OMI) to determine global surface reflectivity under both snow-covered and snow-free conditions at 354 nm. Clear-sky scenes are determined using cloud and aerosol data from the Moderate Resolution Imaging Spectroradiometer/Aqua satellite instrument that flies 12 min ahead of OMI/Aura. The result is a database of OMI-observed Lambertian equivalent reflectivity (LER) that does not rely on statistical methods to eliminate cloud and aerosol contamination. We apply this database to evaluate previous climatologies of surface reflectivity. Except for regions of seasonal snow cover, agreement is best with a climatology from OMI, which selects the surface reflectivity from a histogram of observed LER (mean difference, 0.0002; standard deviation, 0.011). Three other climatologies of surface reflectivity from Total Ozone Mapping Spectrometer, Global Ozone Monitoring Experiment, and OMI, based on minimum observed LER, are less consistent with our cloud- and aerosol-filtered data set (mean difference, -0.008, 0.012, and -0.002; standard deviation, 0.022, 0.026, and 0.033). Snow increases the sensitivity of solar backscatter measurements at ultraviolet and visible wavelengths to trace gases in the lower troposphere. However, all four existing LER climatologies poorly represent seasonal snow. Surface reflectivity over snow-covered lands depends strongly on the vegetation type covering the surface. The monthly variation of snow-covered reflectivity varies by less than 0.1 in fall and winter. Applying our snow-covered surface reflectivity database to OMI NO_2 retrievals could change the retrieved NO_2 column by 20%-50% over large regions with seasonal snow cover.