Wavelength Scanning Employing the Return to Zero Modulation Technique to Retrieve Atmospheric CO2 Profiles

Abstract:

The ability to make range resolved measurements of CO2 in the lower atmosphere is critical to helping understand regional scale sources and sinks while providing data essential for validating the future ASCENDS mission. The Return-to-Zero Pseudo Noise modulation technique has been explored for making range resolved retrievals of CO2 within the lower troposphere using commercial off the shelf technology. It generates multiple laser pulses within the detector’s field of view while still enabling range resolved measurements. This permits a much higher duty cycle to be achieved than if using a normal pulsed laser. Because of issues relating to how this technique treats background counts a different approach to the conventional DIAL online/offline has been explored. This involves stepping over an absorption line using a number of discrete wavelengths to map out the line. The data is then fit to a theoretical curve which permits the accurate determination of CO2 over the path. This approach is similar in implementation to that employed by the Goddard ASCENDS instrument except that the laser is now modulated with the RZPN technique. An assessment of this approach with particular emphasis on how it treats background counts will be discussed.