Side-line tunable laser transmitter for differential absorption lidar measurements of CO₂: Design and application to atmospheric measurements

A 2 μm wavelength, 90mJ, 5 Hz pulsed Ho laser is described with wavelength control to precisely tune and lock the wavelength at a desired offset up to 2.9 GHz from the center of a CO₂ absorption line. Once detuned from the line center the laser wavelength is actively locked to keep the wavelength within 1.9MHz standard deviation about the setpoint. This wavelength control allows optimization of the optical depth for a differential absorption lidar (DIAL) measuring atmospheric CO₂ concentrations. The laser transmitter has been coupled with a coherent heterodyne receiver for measurements of CO₂ concentration using aerosol backscatter; wind and aerosols are also measured with the same lidar and provide useful additional information on atmospheric structure. Range-resolved CO₂ measurements were made with < 2.4% standard deviation using 500 m range bins and 6.7 min (1000 pulse pairs) integration time. Measurement of a horizontal column showed a precision of the CO₂ concentration to < 0.7% standard deviation using a 30 min (4500 pulse pairs) integration time, and comparison with a collocated in situ sensor showed the DIAL to measure the same trend of a diurnal variation and to detect shorter time scale CO₂ perturbations. For vertical column measurements the lidar was setup at the WLEF tall tower site in Wisconsin to provide meteorological profiles and to compare the DIAL measurements with the in situ sensors distributed on the tower up to 396 m height. Assuming the DIAL column measurement extending from 153 m altitude to 1353 m altitude should agree with the tower in situ sensor at 396 m altitude, there was a 7.9 ppm rms difference between the DIAL and the in situ sensor using a 30 min rolling average on the DIAL measurement.