Impact of Aerosol Property on the Accuracy of a CO 2 Retrieval Algorithm from Satellite Remote Sensing

摘要

Based on an optimal estimation method, an algorithm was developed to retrieve the column-averaged dry-air mole fraction of carbon dioxide (XCO 2 ) using Shortwave Infrared (SWIR) channels, referred to as the Yonsei CArbon Retrieval (YCAR) algorithm. The performance of the YCAR algorithm is here examined using simulated radiance spectra, with simulations conducted using different Aerosol Optical Depths (AODs), Solar Zenith Angles (SZAs) and aerosol types over various surface types. To characterize the XCO 2 retrieval algorithm, reference tests using simulated spectra were analysed through a posteriori XCO 2 retrieval errors and averaging kernels. The a posteriori XCO 2 retrieval errors generally increase with increasing SZA. However, errors were found to be small (<1.3 ppm) over vegetation surfaces. Column averaging kernels are generally close to unity near the surface and decrease with increasing altitude. For dust aerosol with an AOD of 0.3, the retrieval loses its sensitivity near the surface due to the influence of atmospheric scattering, with the peak of column averaging kernels at ~800 hPa. In addition, we performed a sensitivity analysis of the principal state vector elements with respect to XCO 2 retrievals. The reference tests with the inherent error of the algorithm showed that overall XCO 2 retrievals work reasonably well. The XCO 2 retrieval errors with respect to state vector elements are shown to be <0.3 ppm. Information on aerosol optical properties is the most important factor affecting the XCO 2 retrieval algorithm. Incorrect information on the aerosol type can lead to significant errors in XCO 2 retrievals of up to 2.5 ppm. The XCO 2 retrievals using the Thermal and Near-infrared Sensor for carbon Observation (TANSO)-Fourier Transform Spectrometer (FTS) L1B spectra were biased by 2.78 ± 1.46 ppm and 1.06 ± 0.85 ppm at the Saga and Tsukuba sites, respectively. This study provides important information regarding estimations of the effects of aerosol properties on the CO 2 retrieval algorithm. An understanding of these effects can contribute to improvements in the accuracy of XCO 2 retrievals, especially combined with an aerosol retrieval algorithm.