Atmospheric Backscatter Profiles at 1572nm from Pulsed Lidar Measurments of CO2 Column Absorption from the 2011 ASCENDS Flight Campaign

We present height-resolved backscatter profiles from the NASA Goddard Space Flight Center's CO2 sounder lidar, rich in detail, which shows clear evidence of multiple backscatter layers, clouds, and aerosols allowing for the identification of the Planetary Boundary Layer (PBL). This data is recorded as a consequence of our pulsed lidar measurements of the CO2 column absorption. The CO2 Sounder is a pulsed lidar for active remote measurements of CO2 abundance from an airborne platform and is one candidate for the lidar on the NASA ASCENDS mission. The lidar uses a scanning, pulsed laser and fiber amplifier in a Master Oscillator Power Amplifier (MOPA) configuration to measure CO2 absorption at 1572.335 nm, lineshape, range to scattering surface and backscatter profiles. The laser is scanned across the absorption feature measuring at 30 discrete wavelengths/scan and ~300 scans/sec. The time-resolved return signal, with a temporal resolution of 8ns, is detected by a photon-counting PMT fiber coupled to a modified commercial, 2m focal length f10 Schmidt-Cassegrain telescope. The column density for CO2 is estimated from the differential optical depth (DOD) of the scanned absorption line using an integrated-path differential absorption (IPDA) technique and the optical path from the time of flight. A backscatter profile of the measured column is recorded for every pulse of every scan and integrated for 1 second. The backscatter profiles we will show are determined from the receivers photon counting record using a cross-correaltion technique (sliding inner product) with a vertical resolution of better than 300m, set by the 1μs pulse width from the MOPA. The range to the surface can be determined to a few meters. Major benefits of a pulsed technique using time-resolved detection to measure lineshape, is the unambiguous detection of the ground return, intervening clouds, aerosols and information on the vertical distribution of CO2. This technique can uniquely identify the ground-return, removing scatter due to intervening aerosols and clouds, notably sub-visible cirrus, which improves the accuracy of the retrieval. Further analyses of the CO2 column incorporating the backscatter profiles allows for the identification of the PBL. Identification of the height above ground of the PBL in which the CO2 is greatly varying, both spatially and temporally, improves the quality of the retrieval from the knowledge of both the lineshape and depth of the PBL. We will present backscatter data from the 2011 ASCENDS campaign from predominantly the central and western USA. The data, rich in detail, shows clear evidence of multiple backscatter layers, clouds, the PBL, surface aerosols and smoke plumes from forest fires to clear air column over the Pacific Ocean.