Side-line tunable laser transmitter for differential absorption lidar measurements of CO_2: design and application to atmospheric measurements

Abstract

A $2\mathrm{\mu m}$ wavelength, $90\mathrm{mJ}$, $5\mathrm{Hz}$ pulsed Ho laser is described with wavelength control to precisely tune and lock the wavelength at a desired offset up to $2.9\mathrm{GHz}$ from the center of a ${\mathrm{CO}}_2$ absorption line. Once detuned from the line center the laser wavelength is actively locked to keep the wavelength within $1.9\mathrm{MHz}$ standard deviation about the setpoint. This wavelength control allows optimization of the optical depth for a differential absorption lidar (DIAL) measuring atmospheric ${\mathrm{CO}}_2$ concentrations. The laser transmitter has been coupled with a coherent heterodyne receiver for measurements of ${\mathrm{CO}}_2$ concentration using aerosol backscatter; wind and aerosols are also measured with the same lidar and provide useful additional information on atmospheric structure. Range-resolved ${\mathrm{CO}}_2$ measurements were made with $<2.4\%$ standard deviation using $500\mathrm{m}$ range bins and $6.7\min$ ($1000\mathrm{pulse}$ pairs) integration time. Measurement of a horizontal column showed a precision of the ${\mathrm{CO}}_2$ concentration to $<0.7\%$ standard deviation using a $30\min$ ($4500\mathrm{pulse}$ 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 ${\mathrm{CO}}_2$ 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 $396\mathrm{m}$ height. Assuming the DIAL column measurement extending from $153\mathrm{m}$ altitude to $1353\mathrm{m}$ altitude should agree with the tower in situ sensor at $396\mathrm{m}$ altitude, there was a $7.9\mathrm{ppm}$ rms difference between the DIAL and the in situ sensor using a $30\min$ rolling average on the DIAL measurement.