Title: Airborne Pulsed 2-µm Direct Detection Lidar for CO2 Column Measurement
Presenting Author: Upendra Singh
Organization: NASA LaRC
Co-Author(s): Jirong Yu and Mulugeta Petros, NASA LaRC; Tamer Refaat, Applied Research Center, Old Dominion University; Karl Reithmaier, Science Systems & Applications, Inc.; Ruben Remus, NASA LaRC

Abstract:
Double-pulse 2-µm lasers have been demonstrated with energy as high as 600 mJ and up to 10 Hz repetition rate at NASA Langley Research Center. The two laser pulses are separated by 200 µs and can be tuned and locked separately. Applying double-pulse laser in Differential Absorption Lidar (DIAL) system enhances the CO2 measurement capability by increasing the overlap of the sampled volume between the on-line and off-line. To avoid detection complicity, integrated path differential absorption (IPDA) lidar provides higher signal-to-noise ratio measurement compared to conventional range-resolved DIAL. Rather than weak atmospheric scattering returns, IPDA rely on the much stronger hard target returns that is best suited for airborne platforms. In addition, the IPDA technique measures the total integrated column content from the instrument to the hard target but with weighting that can be tuned by the transmitter. Therefore, the transmitter could be tuned to weight the column measurement to the surface for optimum CO2 interaction studies or up to the free troposphere for optimum transport studies. Recently, with support of Earth Science Technology Office (ESTO), NASA LaRC 2-µm IPDA team developed, integrated and conducted an extensive series of CO2 column measurement using a double-Pulsed 2-µm direct detection IPDA lidar for from an airborne platform. This compact IPDA lidar aims to provide accurate, high-resolution atmospheric CO2 column measurements. The weighting function at 2-µm is favorable for air/space-borne measurements in the lower troposphere, including the boundary layer, where the CO2 sources and sinks reside. The IPDA wavelength control agility feature allows, while in flight, optimizing the weighting function for specific measurement condition. Pulsed IPDA provides an opportunity to make scattering surface elevation (SSE) measurement with sufficient accuracy and ability to filter out thin cloud and aerosol backscatter to reduce measurement biases. Over 21 days in March/April 2014, the pulsed 2-µm direct detection IPDA lidar completed its engineering flights onboard the NASA B-200 aircraft. The flights occurred over diverse terrains of different reflectivity (soil, vegetation, ocean, snow, sand, etc.). Preliminary analysis shows the measured CO2 optical depth for different column lengths matched modeled values within few percent, with high signal-to-noise ratio. Although post-flight data analysis is ongoing, initial results are encouraging and the instrument has demonstrated the first proof-of-principle of an airborne, direct-detection, double-pulsed, 2-µm CO2 measurement. The presentation will describe the development of the 2-µm IPDA lidar system and present the airborne measurement of column CO2 by IPDA and its comparison with model and in-situ instrument.