Airborne lidar measurements of atmospheric CO₂ column concentrations to cloud tops made during the 2017 ASCENDS/ABoVE campaign

<p>We measured the column-averaged atmospheric <span class="inline-formula">CO₂</span> mixing ratio (<span class="inline-formula">XCO₂</span>) to a variety of cloud tops with an airborne pulsed multi-wavelength integrated path differential absorption (IPDA) lidar during NASA's 2017 ASCENDS/ABoVE airborne campaign. Measurements of height-resolved atmospheric backscatter profiles allow this lidar to retrieve <span class="inline-formula">XCO₂</span> to cloud tops, as well as to the ground, with accurate knowledge of the photon path length. We validated these measurements with those from an onboard in situ <span class="inline-formula">CO₂</span> sensor during spiral-down maneuvers. These lidar measurements were 2–3 times better than those from previous airborne campaigns due to our using a wavelength step-locked laser transmitter and a high-efficiency detector for this campaign. Precisions of 0.6 parts per million (<span class="inline-formula">ppm</span>) were achieved for 10 <span class="inline-formula">s</span> average measurements to mid-level clouds and 0.9 <span class="inline-formula">ppm</span> to low-level clouds at the top of the planetary boundary layer. This study demonstrates the lidar's capability to fill in <span class="inline-formula">XCO₂</span> measurement gaps in cloudy regions and to help resolve the vertical and horizontal distributions of atmospheric <span class="inline-formula">CO₂</span>. Future airborne campaigns and spaceborne missions with this capability can be used to improve atmospheric transport modeling, flux estimation and carbon data assimilation.</p>