Validation of airborne CO2 laser measurements

This paper discusses the flight test validation of a unique, multi-frequency, intensity-modulated, single-beam laser absorption spectrometer (LAS) that operates near 1.57 mm for remote column CO₂ measurements.  This laser system is under development for a future space-based mission to determine the global distribution of regional-scale CO₂ sources and sinks, which is the objective of the NASA Active Sensing of CO₂ Emissions during Nights, Days, and Seasons (ASCENDS) mission.  A prototype of this LAS system, called the Multi-frequency Fiber Laser Lidar (MFLL), was developed by ITT, and it has been flight tested in nine airborne campaigns since May 2005.  This paper focuses on the most recent results obtained over the last 1½ years of flight testing where the MFLL remote CO₂ column measurements were evaluated against airborne in situ CO₂ profile measurements traceable to World Meteorological Organization standards.

A comprehensive multiple-aircraft flight test program was conducted over Oklahoma and Virginia in July-August 2009.  Examples of the MFLL-derived surface reflectance and average CO₂ column variations along the 50-km flight legs over the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Central Facility (CF) in Lamont, Oklahoma; over rural Virginia and North Carolina; and over the Chesapeake Bay are shown in this paper.  Over land the surface reflectance varies by a factor of 2 to 3 over very short distances (meters), but due to the simultaneous transmission of the "on" and "off" wavelengths, these surface reflectance features are identically eliminated upon taking the ratio of these signals.  For a flight altitude of 4.6 km, the average signal to noise ratio (SNR) for a 1-s CO₂ column measurement was found to be 760, which is the equivalent of a CO₂ mixing ratio precision of 0.60 ppmv, and for a 10-s average the SNR was found to be 2002 or 0.20 ppmv.  On a subsequent flight over the Chesapeake Bay, where the water reflectance is generally lower by a factor of 2.5 to 3 compared to land surfaces, the 10-s average CO₂ SNR was found to be 1300 or 0.30 ppmv.  Absolute comparisons of MFLL-derived and in situ-derived CO₂ column measurements were made for all six daytime flights conducted over Oklahoma and Virginia in 2009.  Of the 32 independent comparisons of MFLL and in situ CO₂ measurements obtained in over six flights, all but one were within ±4 ppmv, and the average difference for all the comparisons (MFLL - in situ) was +0.32 ppmv with a standard deviation of 2.4 ppmv.

A major ASCENDS flight test campaign was conducted using the NASA DC-8 during 6-18 July 2010.  The MFLL system and associated in situ CO₂ instrumentation were operated on DC-8 flights over the Central Valley of California, the desert of southeastern California/Nevada, the Pacific Ocean off of the Baja Peninsula, Railroad Valley, Nevada, and the DOE ARM CF in Lamont, Oklahoma.  Remote CO₂ column measurements were made from altitudes of 2.5 to 13 km, and in situ CO₂ profiles were obtained on spirals from the highest altitude on each flight to as low as 30 m at the center of the flight track.  Radiosondes were also launched in conjunction with these flights to constrain the meteorological conditions for the validation of the MFLL CO₂ column measurements.  Results from these DC-8 flight tests are also discussed in this paper.

The high-precision, high-accuracy remote CO₂ measurements obtained by the MFLL system represent a major step towards the realization of the needed capability for future space-based laser measurements of the global distribution of CO₂.