Monthly Representations of Mid-Tropospheric Carbon Dioxide from the Atmospheric Infrared Sounder Thomas S. Pagano ∗a , Edward T. Olsen a , Moustafa T. Chahine a , Alexander Ruzmaikin a , Hai Nguyen a , Xun Jiang b a California Institute of Technology, Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA, USA 91109 b Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas, USA ABSTRACT The Atmospheric Infrared Sounder (AIRS) on NASA’s Earth Observing System Aqua spacecraft was launched in May of 2002 and acquires hyperspectral infrared spectra used to generate a wide range of atmospheric products including temperature, water vapor, and trace gas species including carbon dioxide. Here we present monthly representations of global concentrations of mid-tropospheric carbon dioxide produced from 8 years of data obtained by AIRS between the years of 2003 and 2010. We define them as “representations” rather than “climatologies” to reflect that the files are produced over a relatively short time period and represent summaries of the Level 3 data. Finally, they have not yet been independently validated. The representations have a horizontal resolution of 2.0° × 2.5° (Latitude × Longitude) and faithfully reproduce the original 8 years of monthly L3 CO 2 concentrations with a standard deviation of 1.48 ppm and less than 2% outliers. The representations are intended for use in studies of the global general circulation of CO 2 and identification of anomalies in CO 2 typically associated with atmospheric transport. The seasonal variability and trend found in the AIRS CO 2 data are discussed. Keywords: Atmospheric Infrared Sounder, Aqua Spacecraft, Carbon Dioxide 1. INTRODUCTION The Atmospheric Infrared Sounder (AIRS) is a hyperspectral infrared instrument on the EOS Aqua Spacecraft, launched on May 4, 2002. 1 AIRS has 2378 infrared channels ranging from 3.7 μm to 15.4 μm, and a 13.5 km footprint at nadir and scans ±49.5° from an orbit altitude of 705.3 km, covering 95% of the globe every day. AIRS, in conjunction with the Advanced Microwave Sounding Unit (AMSU), produces temperature profiles with 1 K/km accuracy on a global scale, as well as water vapor profiles and trace gas amounts for CO 2 , CO, SO 2 , O 3 , and CH 4 . AIRS data are used for weather forecasting, climate process studies, and validating climate models. Several international groups have successfully retrieved concentrations of mid-tropospheric CO 2 from AIRS. 2 The CO 2 data used in this analysis are produced by the NASA AIRS Project and are unique in a sense of global spatial coverage, i.e., not being limited to only clear sky regions. The data are produced using the method of Vanishing Partial Derivatives (VPD). 3 The VPD method solves for the least squares CO 2 estimates by iteratively minimizing the difference between the observed cloud-cleared radiances and calculated radiances for AIRS using the AIRS Radiative Transfer Algorithm (RTA). The VPD is based on coordinate descent methodology, that is, it applies the minimization independently and sequentially to all geophysical parameters that impact the radiance of a given channel used to retrieve CO 2 , e.g., atmospheric temperature, water vapor, ozone, and carbon dioxide. The process is iterated until the radiance residuals for all parameters are minimized or the change in CO 2 falls below 0.25 ppm. Extensive quality control is applied during the retrieval including: quality of the AIRS geophysical products, monotonically decreasing radiance residuals from one iteration to the next, and spatial homogeneity of a 2 × 2 set of retrievals (clusters) required to be within 2 ppm in an RMS sense. The resulting “Level 2” product achieves a yield of over 15,000 mid-tropospheric CO 2 retrievals per 24-hour period, each with a horizontal footprint of 90 × 90 km centered over the area where acquired. ∗ tpagano@jpl.nasa.gov, (818) 393-3917, http://airs.jpl.nasa.gov Imaging Spectrometry XVI, edited by Sylvia S. Shen, Paul E. Lewis, Proc. of SPIE Vol. 8158, 81580C · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.894960 Proc. of SPIE Vol. 8158 81580C-1 Downloaded from SPIE Digital Library on 04 Jun 2012 to 129.7.16.9. Terms of Use: http://spiedl.org/terms