Spatial distributions of tropospheric carbon dioxide concentration are examined by analyzing carbon dioxide channel measurements of the High-Resolution Infrared Radiation Sounder (HIRS) onboard NOAA polar-orbiting satellites in conjunction with the mid-tropospheric carbon dioxide concentration retrieved from Atmospheric Infrared Sounder (AIRS) measurements. In association with the increase of atmospheric carbon dioxide concentration, the HIRS radiances show qualitatively homogeneous decadal-scale variations in the troposphere over the entire globe. However, in the mid-troposphere, the extra-tropical regions exhibit more pronounced radiative signatures compared to the tropics, consistent with the geographical distribution of the AIRS-retrieved mid-tropospheric carbon dioxide. Furthermore, distinct inter-channel differences in the HIRS decadal trend are found, implying a latitudinal variation of vertical distribution in the troposphere. Comparisons of the AIRS-retrieved mid-tropospheric carbon dioxide concentrations with surface-based in situ measurements indicate that the HIRS-inferred spatial variations are neither spurious nor transient features.

We also compare the satellite-inferred carbon dioxide distributions with those produced from a data assimilation system (NOAA CarbonTracker). While spatial distributions are generally realistic in the tropics and the northern hemisphere, CarbonTracker fails to reproduce the satellite-derived spatial distributions in the extra-tropical regions of the southern hemisphere. Thus, these results indicate that the satellite observations are useful for improving the understanding of atmospheric carbon dioxide budget as well as supplementing in situ observation system.