  We present an evaluation of airborne Intensity-Modulated Continuous-Wave
(IM-CW) lidar measurements of atmospheric column CO2 mole fractions during the
ACT-America project. This lidar system transmits online and offline wavelengths
simultaneously on the 1.57111-um CO2 absorption line, with each modulated
wavelength using orthogonal swept frequency waveforms. After the spectral
characteristics of this system were calibrated through short-path measurements,
we used the HITRAN spectroscopic database to derive the average-column CO2
mixing ratio (XCO2) from the lidar measured optical depths. Based on in situ
measurements of meteorological parameters and CO2 concentrations for
calibration data, we demonstrate that our lidar CO2 measurements were
consistent from season to season and had an absolute calibration error
(standard deviation) of 0.80 ppm when compared to XCO2 values derived from in
situ measurements. By using a 10-second or longer moving average, a long-term
stability of 1 ppm or better was obtained. The estimated CO2 measurement
precision for 0.1-s, 1-s, 10-s, and 60-s averages were determined to be 3.4 ppm
(0.84%), 1.2 ppm (0.30%), 0.43 ppm (0.10%), and 0.26 ppm (0.063%),
respectively. These correspond to measurement signal-to-noise ratios of 120,
330, 950, and 1600, respectively. The drift in XCO2 over one-hour of flight
time was found to be below our detection limit of about 0.1 ppm. These analyses
demonstrate that the measurement stability, precision and accuracy are all well
below the thresholds needed to study synoptic-scale variations in atmospheric
XCO2.
