Testing of a Two-Micron Lidar System for Carbon Dioxide Profiling in the Atmosphere

Utilizing a tunable two-micron pulse laser transmitter, a two-micron lidar system has been developed at NASA Langley Research Center (LaRC) for profiling atmospheric carbon dioxide. Owing to the unavailability of conventional detectors that meet the carbon dioxide measurement requirements at this wavelength, this system employs an advanced two-micron heterojunction phototransistor (HPT) at the receiver end. This application of HPT in lidar results in a systematic effect in the form of reduction in the resolution of the detected backscattered signals. Fortunately, the resolution could be recovered through signal processing by deconvolution algorithms with a minimal reduction in the signal-to-noise ratio. Two series of atmospheric tests were conducted during April-May, 2008 at NASA LaRC. First, the system was operated in the backscatter lidar mode and the atmospheric backscatter returns were compared to the data obtained from the Compact Aerosol Lidar (CAL) operating at 532 and 1064 nm wavelengths. An analysis was conducted to compare the performance of the HPT against the performance of the mature Si-APD and PMT detectors operating at different wavelengths. Similar atmospheric backscattering profiles were obtained from the three systems, validating the HPT for lidar applications. Second, operating for the first time in the differential absorption lidar (DIAL) mode at NASA Langley and targeting the R22 carbon dioxide line in the 2.05-µm band, the system was able to measure the gas mixing ratio in the boundary layer. Such measurements were achieved after obtaining recent characterization of the R22 line parameters and using Wallops Island metrological data for carbon dioxide cross section calculation and water vapor correction that resulted in the retrieval of gas mixing ratio. The DIAL measurements were compared with simultaneous in-situ carbon dioxide sensor measurements. Carbon dioxide mixing ratios of 390.8 and 386.4 ppm with 2.7% and 0.1% uncertainties resulted from the initial DIAL and in-situ measurements, respectively. The details of the two-micron DIAL system and results of these measurements will be presented.