MOPITT multispectral CO observations
The MOPITT team has made significant advances in demonstrating multispectral retrievals of CO (carbon monoxide) with enhanced sensitivity to near-surface CO. MOPITT, (Measurements Of Pollution In The Troposphere), on EOS-Terra, has been measuring CO since March 2000, and is the only satellite instrument with both thermal infrared (TIR) and near infrared (NIR) CO channels. The standard MOPITT V4 product uses TIR-only radiances to produce CO distributions for day/night, ocean/land observations. Here we show results from combining the TIR and NIR channels, which is only possible for day/land cases. Exploiting both TIR and NIR channels has been possible due to recent progress in characterizing NIR radiance errors. This has allowed us to trade off sensitivity to near surface CO for larger random errors in the combined retrieval. In order to isolate the spatial effects of adding the NIR channels, we use a single a priori profile and covariance for all these retrievals. By examining retrieval diagnostics such as DFS (degrees of freedom for signal) and averaging kernels (AK) for the multispectral retrieval (TIR+NIR) as compared to the TIR-only retrieval, we find that adding the NIR channel to the retrieval significantly increases sensitivity to CO, especially near the surface, but with high spatial variability. The cases with the largest increases in DFS are over regions of high CO sources and areas that have low thermal contrast, as seen in the figure below. Enhanced sensitivity to near surface CO is especially evident in central/east Asia where source regions for urban areas with high population density are clearly identifiable in the MOPITT multispectral CO retrievals.
Figure 1. Surface layer degrees of freedom for signal (DFS) for 2005-2008 September/October/November (SON) average in 1°x1° bins. Left panel shows MOPITT TIR-only retrievals and right panel shows MOPITT TIR+NIR (multispectral) retreivals. Surface layer DFS is defined as the trace of the averaging kernel (AK) from 800 hPa to the surface. Gray areas indicate surfaces with pressure < 800 hPa.
Figure 2.Comparison of TIR-only (left panel) and TIR+NIR (right panel) retrievals of surface CO. Both panels are seasonal averages for 2005-2008, September/October/November (SON) in central/east Asia.
