Boundary Layer CO2 mixing ratio measurements by an airborne pulsed IPDA lidar

Abstract:

Since the primary signature of CO₂ fluxes at the surface occurs in the planetary boundary layer (PBL), remote sensing measurements of CO₂ that can resolve the CO₂ absorption in the PBL separate from the total column are more sensitive to fluxes than those that can only measure a total column. The NASA Goddard CO₂ sounder is a pulsed, range-resolved lidar that samples multiple (presently 30) wavelengths across the 1572.335 nm CO₂ absorption line. The range resolution and line shape measurement enable CO₂ mixing ratio measurements to be made in two or more altitude layers including the PBL via lidar cloud-slicing and multi-layer retrievals techniques.

The pulsed lidar approach allows range-resolved backscatter of scattering from ground and cloud tops. Post flight data analysis can be used split the vertical CO₂ column into layers (lidar cloud-slicing) and solve for the CO₂ mixing ratio in each layer. We have demonstrated lidar cloud slicing with lidar measurements from a flight over Iowa, USA in August 2011 during the corn-growing season, remotely measuring a ≈15 ppm drawdown in the PBL CO₂. We will present results using an improved lidar cloud slicing retrieval algorithm as well as preliminary measurements from the upcoming ASCENDS 2014 flight campaign.

The CO₂ absorption line is also more pressure broadened at lower altitudes. Analyzing the line shape also allows solving for some vertical resolution in the CO₂ distribution. By allowing the retrieval process to independently vary the column concentrations in two or more altitude layers, one can perform a best-fit retrieval to obtain the CO₂ mixing ratios in each of the layers. Analysis of airborne lidar measurements (in 2011) over Iowa, USA and Four Corners, New Mexico, USA show that for altitudes above 8 km, the CO₂ sounder can detect and measure enhanced or diminished CO₂ mixing ratios in the PBL even in the absence of clouds. We will present these results as well as preliminary measurements from the upcoming ASCENDS 2014 flight campaign, where the CO₂ sounder instrument will have a more sensitive detector. We will compare results from the two techniques and discuss the potential of such measurements from space.