Research On Performance Simulation And Retrieval Algorithm Of Space-borne Lidar For Measuring CO₂ Concentration

The atmospheric carbon dioxide is one of the most important greenhouse gases that leads to increasing atmospheric temperature. To have a precise knowledge of sources and sinks of global CO2 and its concentration, a useful and effective method to detect CO2 concentration is urgently needed.The current global CO2 concentration data are partly achieved from the network of about 100 observation stations around the world. However, due to limited spatial resolution, these data are not sufficient for accurate analysis. To increase the spatial resolution, many institutions from different countries have launched satellite equipped with passive detection instruments. However, these instruments have many disadvantages such as low detection accuracies and low coverage ratio in global scale.To close these gaps, there are two missions based on active detection method, A-SCOPE and ASCENDS, are proposed by ESA and NASA, respectively. Both satellites will carry an Integrated Path Differential Absorption (IPDA) Lidar, which emits two laser beams alternatively, of which the wavelengths are in the vicinity of a CO2 absorption line. One is in the place sensitive to CO2 absorption, called on-line while another is insensitive, called off-line. The information of CO2 concentration can be extracted by comparing the return signals of the two laser beams.The IPDA Lidar can not only make up the deficiency of passive detectors stated above, but also has a high detection accuracy which is up to 1ppmv. Up to now, the above two spaceborne IPDA Lidars are both in pre-research process and not ready for performing routine measurements. In this background, this paper studies on the performance simulation and retrieval algorithm of CO2 concentration based on IPDA Lidar.Firstly, the paper briefly introduces the theories and process of CO2 volume weighted mixing ratio, CO2 column concentration of the spaceborne IPDA Lidar. According to typical parameters of optical system, atmosphere and platform, the paper shows the simulation of ground return signals,APD output voltage, APD output Signal to Noise Ratio (SNR) and random error induced from SNR. A global map of RRE (Relative Random Error) is made out according to MODIS data of surface reflectance and aerosol, of which the result shows that the IPDA has a good detection performance worldwide under given parameters including system optical parameters, atmospheric parameters, platform parameters and APD parameters. Besides, the paper also simulates the effect of the some atmospheric parameters, including temperature, pressure, water vapor mixing ratio, and some system optical parameters, including laser linewidth, laser frequency jitter, laser spectral purity, laser energy fluctuation and accuracy of energy detection, on the measurement error of CO2 column concentration. The result shows, when the accuracy of atmospheric temperature is 0.5K, the relative error of retrieved CO2 concentration is 0.0212%, corresponding absolute error of about 0.081ppmv (with assumed average column concentration of 380ppmv); when the accuracy of atmospheric pressure is 0.5hPa, the error is 0.027%(0.103ppmv); when the accuracy of relative humidity is 5%, the error is 0.012%(0.046ppmv); when the relative pulse energy calibration is 0.05%, the error is 0.028%(0.105ppmv);when the Lidar line-width is 30MHz, the error is 0.0036%(0.0137ppmv); when the laser frequency jitter is 0.3MHz, the error is 0.026%(0.1ppmv); when the laser spectral purity is 99.9% and the bandwidth of optical filter is 6GHz, the absolute error is about 0.109ppmv. According to the results above, the paper proposes the error budget of parameters. The results show, when the atmospheric temperature offset is 1K, atmospheric pressure offset is 1hPa, relative humidity offset is 10%, laser energy fluctuation is 0.2%, Lidar line-width is 70 MHz, the laser frequency jitter is 0.9 MHz, and the laser spectral purity is 99.9%, the optical filter bandwidth is 42GHz, the total error is 0.236%(0.8988ppmv), which can almost meet the accuracy of 1 ppmv.Secondly, this paper studies the correction method of the retrieval algorithm using the simulated return signals base on system optical parameters and atmospheric parameters, and then, this paper verifies the accuracy of CO2 concentration numerical calculation, of which the necessity is that though the retrieval algorithm is correct theoretically, the uncertainty of accuracy of numerical calculation has a huge effect on the result in practice because the magnitude of the retrieval result is ppm. Besides, the paper also refers to two key problems in the process of numerical verification includes the calculation of CO2 absorption cross section and the optimization of height resolution.In conclusion, the paper researches on the performance simulation of the IPDA Lidar and the retrieval algorithm for the CO2 column concentration based on system optical parameters and atmospheric parameters. This is an important part in the process of pre-research for spaceborne IPDA. The results of simulation have very important guiding meaning for design of hardware system and research of retrieval algorithm is pre-condition of accurately achieving CO2 column concentration.