Remote sounding of trace gases with the EOS/AIRS instrument

Abstract

The AIRS instrument was launched on the Aqua satellite in May of 2002. In addition to the core level 2 products, that include cloud cleared radiances; temperature, moisture, and ozone profiles; surface skin temperature; NDVI (from AIRS visible channels); surface spectral emissivity and reflectivity; and cloud products, the AIRS science team is also developing research algorithms for the retrieval of carbon monoxide (CO), methane (CH₄), and carbon dioxide (CO₂). These algorithms are being tested by the National Oceanographic and Atmosphere Administration (NOAA) National Environmental Satellite, Data, and Information Service (NESDIS) in simulation and applied to real AIRS radiances. The trace gas retrievals require cloud free infrared radiances. In practice, we observe that AIRS measurements without cloud contamination occur less than 5% of the time. A key feature of the AIRS algorithm is the utilization of cloud cleared radiances that removes the effects of clouds and increases the yield of trace gas products to 50-60%. The increased yield should allow a better assessment of sources and sinks of these gases. Determination of sources and sinks of these trace gas requires an unprecedented precision for a remote sounding measurement. In addition, both the variability and errors in the trace gas products tend to be correlated with variability and errors in other products (e.g., clouds, temperature, moisture, and ozone profile). Multi-spectral, high-resolution measurements can minimize the effects of this correlation. Currently, for the AIRS products, we estimate a precision of 15% for CO, 0.5% for CO₂ and 1% for CH₄. The remote sounding methodology for these trace gases is discussed in detail. The METOP IASI and NPOESS CrIS instruments can extend the continuity of these trace gas products over the next two decades. Simulation experiments are used to assess the relative performance of the trace gas retrievals from these sounders.