Analysis of Pulsed Lidar Measurements of Atmospheric CO2 Column Absorption in the ASCENDS 2011 and 2013 Airborne Campaigns

We have developed a pulsed, wavelength-resolved IPDA lidar technique for measuring the tropospheric CO2 concentrations as a candidate for NASA's ASCENDS mission. The CO2 lidar flies on NASA's DC-8 aircraft and measures the atmospheric backscatter profiles and shape of the 1572.33 nm absorption line using 250 mW average laser power, 30 wavelength samples per scan with 300 scans per second. Our post-flight analysis estimates the lidar range and pulse energies at each wavelength every second. We then solve for the optimum CO2 absorption line shape, and calculated the Differential Optical Depth (DOD) at the line peak and the column average CO2 concentrations. We compared these to radiative transfer calculations based on the HITRAN 2008 database, the atmospheric conditions, and the CO2 concentrations sampled by in-situ sensors on the aircraft. Our team participated in the ASCENDS science flights during July and August 2011. These flights were made over a wide variety of surface and cloud conditions near the US, including over the central valley of California, over several mountain ranges, over both broken and solid stratus cloud deck over the Pacific Ocean, over thin and broken clouds above the US Southwest and Iowa, and over forests near the WLEF tower in Wisconsin. Most flights had 5-6 altitude steps to > 12 km, and clear CO2 absorption line shapes were recorded. Analyses show the retrievals of lidar range and CO2 column absorption, as well as estimates of CO2 mixing ratio worked well when measuring over topography with rapidly changing height and reflectivity, through thin clouds and to stratus cloud tops. For regions where the CO2 concentration was relatively constant, the measured CO2 absorption profile (averaged for 50 sec) matched the predicted profile to better than 1% RMS error for all flight altitudes. For 10 second averaging, the scatter in the retrievals was typically 2-3 ppm and was limited by signal shot noise (i.e. the signal photon count). For flight altitudes above 5 km the biases in retrieved concentrations were 1-2 ppm. Analysis shows the decrease in CO2 due to vegetation when flying over Iowa cropland as well as the sudden increases in CO2 concentration near a coal-fired power plant in New Mexico. Our team also participated in the February 2013 ASCENDS flight campaign, flying over a variety of surfaces in the US, including over Railroad Valley NV, the California Central Valley, desert areas in Arizona, and over cold snow fields in the Rocky Mountains of Colorado and warmer snow in Iowa and Wisconsin. Our post-flight analyses showed that the retrievals of lidar range, lineshape and CO2 column absorption and concentrations worked well when measuring over topography with rapidly changing height and reflectivity, and through thin clouds. As expected, the relative reflectivity of snow surfaces near 1572 nm was small, about 10% of that of the desert, and good line fits and retrievals were made to these as well. Examples from analyzing the 2011 and 2013 measurements will be presented.