High-Precision and High-Accuracy Column Dry-Air Mixing Ratio Measurement of Carbon Dioxide Using Pulsed 2- $\mu$ m IPDA Lidar

NASA has been conducting several studies for quantifying and monitoring atmospheric CO
₂
including the Orbiting Carbon Observatory space-based missions. To complement CO
₂
passive sensing, the National Research Council recommended active remote sensing techniques, which are valuable for validating current space-based measurements and potential future missions. Recently, a 2-μm triple-pulse integrated path differential absorption (IPDA) lidar was developed for simultaneous and independent measurements of atmospheric CO
₂
and H
₂
O. This instrument was operated at fixed wavelengths, precisely selected to avoid mutual interferences. Focusing on optimized CO
₂
measurements, this instrument has been updated to operate in double-pulse mode. The objective is to demonstrate high-precision and high-accuracy column CO
₂
measurements using tunable on-line wavelength suitable for adaptive targeting. Statistical analysis of long record CO
₂
field measurement and retrieval results in 1.92-ppm accuracy, equivalent to 0.44% systematic error, and 1.66-ppm precision, equivalent to 0.39% random error. Referring to the R30 CO
₂
absorption line, this was achieved using an on-line laser frequency offset of 1 GHz, a 5.2% target reflectivity, and 10-s average. The record indicated a better than 99% data success rate. Tuning the on-line frequency offset to 0.5 GHz results in 1.11- and 0.33-ppm measurement accuracy and precision, equivalent to 0.26% and 0.08% systematic and random errors, respectively, obtained using 13.6% target reflectivity. Range measurements indicate 0.7-m precision and 0.2-m accuracy. These results demonstrate the reliability of atmospheric CO
₂
measurements with high precision and high accuracy, using the pulsed 2-μm IPDA lidar.