GeoXO ACX Aerosol Layer Height and Aerosol Optical Depth Retrieval Algorithm

To prepare for NOAA’s GeoXO atmospheric composition instrument (ACX), an ultraviolet-visible spectrometer, we have taken an enterprise approach to the development of aerosol algorithms (optical depth, type, and height) that run seamlessly on different LEO and GEO sensors with observational capabilities in the ultraviolet (UV) to visible (VIS) to near Infrared (IR). As a test case application, we applied NOAA’s approach to derive aerosol optical depth, aerosol type, and aerosol layer height to Tropospheric Monitoring Instrument (TROPOMI) measurements covering the ultraviolet to visible bands including the oxygen bands.

In this algorithm, a database of surface reflectance ratios for different bands such as blue, red, and short-wave infrared that are functions of solar-satellite geometry and location was developed. This database was developed at 0.1°x0.1° spatial resolution using multi-year TROPOMI L1B data. AOD and surface reflectance are varied until a solution is found that gives a minimum residual between derived and prescribed spectral surface reflectance ratio. The surface reflectances at O₂A (764 nm) and O₂B (688 nm) bands are obtained from the retrievals of the other bands and the surface reflectance ratio database. The ALH is retrieved by searching for the ALH that minimizes the differences between measured 688/670 bands and 764/780 bands TOA reflectance ratio with the calculated ratios using the retrieved surface reflectances and AOD in the previous step. The retrieval algorithm is tested using the August-September 2020 data over CONUS region where heavy smoke was observed. The results show good AOD retrieval performance compared to ground-based Aerosol Robotic Network (AERONET) AOD: the retrieved AOD has a correlation of 0.68, a bias of 0.11 and RMSE of 0.37 with respect to AERONET AOD. Comparing to CALIOP ALH, the retrieved ALH has a correlation of 0.68, a bias of 0.21 km and an RMSE of 2.27 km.

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