Lidar measurements of the column CO2 mixing ratio made by NASA Goddard’s CO2 Sounder during the NASA ASCENDS 2014 Airborne campaign.

Abstract:

Remote sensing measurements of CO₂ from space can help improve our understanding of the carbon cycle and help constrain the global carbon budget. However, such measurements need to be sufficiently accurate to detect small (1 ppm) changes in the CO₂ mixing ratio (XCO₂) against a large background (~ 400 ppm). Satellite measurements of XCO₂ using passive spectrometers, such as those from the Japanese GOSAT (Greenhouse gas Observing Satellite) and the NASA OCO-2 (Orbiting Carbon Observatory-2) are limited to daytime sunlit portions of the Earth and are susceptible to biases from clouds and aerosols. For this reason, NASA commissioned the formulation study of ASCENDS a space-based lidar mission. NASA Goddard Space Flight Center’s CO2 Sounder lidar is one candidate approach for the ASCENDS mission.

The NASA GSFC CO₂ Sounder measures the CO₂ mixing ratio using a pulsed multi-wavelength integrated path differential absorption (IPDA) approach. The CO₂ Sounder has flown in the 2011, 2013 and 2014 ASCENDS airborne campaigns over the continental US, and has produced measurements in close agreement with in situ measurements of the CO₂ column. In 2014, the CO₂ Sounder upgraded its laser with a precision step-locked diode laser source to improve the lidar wavelength position accuracy. It also improved its optical receiver with a low-noise, high efficiency, HgCdTe avalanche photo diode detector. The combination of these two technologies enabled lidar XCO₂ measurements with unprecedented accuracy.

In this presentation, we show analysis from the ASCENDS 2014 field campaign, exploring: (1) Horizontal XCO₂ gradients measured by the lidar, (2) Comparisons of lidar XCO₂ measurements against the Parameterized Chemistry Transport Model (PCTM), and (3) Lidar column water vapor measurements using a HDO absorption line that occurs next to the CO₂ absorption line. This can reduce the uncertainty in the dry air column used in XCO₂ retrievals.