A ground-based differential absorption lidar for atmospheric vertical CO2 profiling

Abstract

A differential absorption lidar (CO2-VDIAL), which is designed for vertical CO2 profile retrieving by using aerosol-scattered signals, is demonstrated in this paper. To our knowledge, it is the first time that a dye laser has been utilized to realize the wavelength modulation for a CO2-DIAL/IPDA system. Such a design scheme greatly reduces both the threshold and the cost to develop a CO2-DIAL. However, two key problems emerge in this system, i.e., wavelength stability and broad bandwidth. By adopting the CO2-VDIAL, a dual-path gas cell, and the Voigt fitting procedure, the accurate wavelength calibration of infrared pulse laser is achieved. Experimental results show that the error of wavelength calibration can be suppressed under 0.1 pm. And a wavelength stability of ～2 pm is then achieved. For tackling the error introduced by using the laser of a broad bandwidth, simulated experiments are carried out to estimate its influence. On that basis, we propose a method to calculate the correction coefficient and demonstrate the process via experiments by using a gas cell. It is demonstrated that the bandwidth of the output infrared laser is around 600-700 MHz. Hence, the broad bandwidth correction is an indispensable step for this CO2-VDIAL. Finally, horizontal, vertical and continuous detections are carried out to verify the precision and the accuracy of our CO2-VDIAL. The slope method is used to retrieve the XCO2 in the above experiments. In the horizontal detections, an R2 of 0.999 is achieved, suggesting that the precision of the system is excellent. By comparison with the in-situ measurements, a difference is found to be lower than 4 ppm. Consequently, it is concluded that the CO2-VDIAL is capable of providing retrievals with the high precision and accuracy. Moreover, the XCO2 decreases with increasing altitude according to the vertical detection experiment in the midnight on June 19m th 2015 at an urban site, demonstrating that the CO2-VDIAL is capable of providing retrievals of ranged-resolved. Finally, temporal characteristic of XCO2 can be also revealed by the CO2-VDIAL in light of continuous detections. The CO2-VDIAL has already been assembled in a container which is due to be transported to Huainai for further verifications in late 2015. Once we finish the performance optimization, the CO2-VDIAL will be installed in Tibet for long period observation.