Study On The Frequency Stabilization And Signal Processing Of The Ground-based Infrared Differential Absorption Lidar System For CO₂ Detection

Greenhouse gases can make the earth's long-wave radiation be re-absorbed and form the greenhouse effect,which has a huge impact on the earth's climate,threatening the human living environment.In response to this global challenge,many countries signed the Paris Agreement at the World Climate Conference on December 12,2015,setting the long-term goal that global temperature would be controlled to rise within 2? compared with the industrialized period and realize the balance of carbon emissions and sequestration by 2050.As the major positive radiative forcing factor,the continued increase of CO₂ concentration has caused a huge impact on the global climate.However,up to now,the temporal and spatial distribution of global carbon sources and sinks has not yet been fully understood,which restricts the consensus of countries on the issue of emission reduction.Therefore,accurate detection of atmospheric CO₂ is a prerequisite for clarifying carbon source and sinks,deepening the understanding of carbon transport,and developing reasonable emission reduction policies.In 1958,the first global atmospheric CO₂ concentration monitoring station was established in Mauna Loa,opening the prelude to global CO₂ monitoring.At present,the atmospheric CO₂ concentration measurement can mainly be divided into passive and active categories.Passive detection mainly includes the ground-based/airborne monitoring by sampling bottle as well as the passive satellite remote sensing.Ground-based/airborne monitoring has high detection precision,which is the main data source to study the carbon cycle.However,high cost and sparse distribution make it unable to be used for the study of global carbon source and sinks.The passive satellite remote sensing has the advantages of wide detection range and fast update speed,and can cover almost all areas.However,due to the limitation of the detection mechanism and distance,the detecting precision of satellite is limited,and can hardly meet the demand for carbon cycle research.The active detection method of CO₂ concentration is dominated by lidar,and differential absorption lidar?DIAL?is recognized as the powerful means of CO₂ monitoring.CO₂-DIAL is less affected by the environment and can obtain the atmospheric CO₂ concentration information with high-precision,high-time and spatial resolution,has attracted major developed countries such as the United States,Germany and Japan competing to develop.Currently,the development of CO₂-DIAL is mainly focused on the Integrated Path Differential Absorption Lidar?IPDA?,which is mature and accurate in the detection of atmospheric CO₂ column concentration.Although the ground-based DIAL can obtain the vertical profile of CO₂ concentration,which is more meaningful in the study of the carbon cycle,its system is complex and difficult to develop.This paper aims at developing a high precision ground-based CO₂-DIAL using infrared laser.The research conducted mainly focuses on the construction of the system as well as the problems encountered in the process of application,including frequency stabilization,signal de-noising and splicing.Progress has been made in the following aspects:1)Built the system of CO₂-DIAL successfully and integrated it into a mobile shelter,enabling field CO₂ detection instead of laboratory fixed measurement.This progress realizes the practicability of this research instrument,laying the foundation to obtain the concentration profile of CO₂ with high precision in the field.2)For the problem of on-line wavelength stability of pulsed laser,a wavelength control unit based on the dual fiber optical path and gas absorption cell is designed,which eliminates the error caused by the energy fluctuation of pulsed laser in the wavelength calibration process.The calibration of on-line wavelength is achieved by the combination of wavelet modulus maxima and the cancellation algorithm of backlash error.In addition,an algorithm of quadrature matching for on-wavelength stabilization is proposed,which enables the wavelength of output laser to be adjusted rapidly when the on-line wavelength drift is detected,realizing the long-term stability of on-line wavelength of pulsed laser.3)For the low signal-to-noise ratio?SNR?caused by the low laser energy,weak echo signal and low detection efficiency,the algorithm of signal de-noising based on the lifting wavelet is proposed.This method selects different predictive and update functions according to the characteristics of the echo signal,which is more targeted to signal denoising,and improvs the detection accuracy of DIAL effectively.4)A piecewise least squares fitting splicing algorithm is proposed creatively based on the traditional signal splicing algorithm.The lidar signal is segmented and the least square fitting with different steps is used according to the signal characteristics of each segment to obtain the best fitting parameters.This method realizes the effective splicing of the analog signal in the near-field and the photon signal in the far-field,improving the detection capability of the lidar system,and achieving continuous concentration measurement of CO₂ with high precision.5)The algorithms of frequency stabilization and signal processing are applied to the experiment.The horizontal,vertical and oblique detection are carried out in Wuhan and Huainan,and a fast lookup table of the absorption cross section is established,which enables the near real-time inversion of CO₂,and the profile distribution information of local atmospheric CO₂ concentration is obtained6)Finally,all the works of this paper are summarized,and the next stage work of CO₂-DIAL is prospected.