Fig. 1. China's regional annual average distribution of CO columns from March 2000 to February 2009^［20］

Fig. 2. Channel sensitivities to CO₂， H₂O， N₂O， O₃， CO， CH₄， NO， NO₂， and SO₂ at the 4.67 μm band

Fig. 3. The distributions of the 300 channels in the spectrum of CO brightness temperature change selected by analyzing the STI ratio（STI>1） for the air masses of Beijing-Tianjin area in winter

Fig. 4. Jacobians of the initial channel group for the air masses of Beijing-Tianjin area in winter

Fig. 5. Jacobians of the channel at the top and bottom of characteristic peak of the CO brightness temperature change spectrum among the initial channel group for the air masses of Beijing-Tianjin area in winter

Fig. 6. Schematic diagram of channel selection from the initial channel group at the peak of the CO brightness temperature change spectrum（a）three channels（b）four channels

Fig. 7. The distribution in the CO brightness temperature change spectrum of the channels selected by the Peak sampling channel selection method for the air masses of four regions in（a）winter and（b）summer.

Fig. 8. The RMSEs of the retrieved CO profiles by the Peak sampling channel set, OSP channel set and all 4001-channel set for the winter air masses in（a）Beijing-Tianjin（b）Alxa（c）Yangtze River Delta（d）Pearl River Delta

Fig. 9. The RMSEs of the retrieved CO profiles by the Peak sampling channel set, OSP channel set and all 4001-channel set for the summer air masses in（a）Beijing-Tianjin（b）Alxa（c）Yangtze River Delta（d）Pearl River Delta

Fig. 10. The biases of the retrieved CO profiles by the Peak sampling channel set and OSP channel set for the winter air masses in（a）Beijing-Tianjin（b）Alxa（c）Yangtze River Delta（d）Pearl River Delta