Ailin Liang

Method for wavelength stabilization of pulsed difference frequency laser at 1572 nm for CO(2) detection lidar.

High-accuracy on-line wavelength stabilization is required for differential absorption lidar (DIAL), which is ideal for precisely measuring atmospheric CO(2) concentration. Using a difference-frequency laser, we developed a ground-based 1.57-μm pulsed DIAL for performing atmospheric CO(2) measurements. Owing to the system complexity, lacking phase, and intensity instability, the stabilization method was divided into two parts-wavelength calibration and locking-based on saturated absorption. After obtaining the on-line laser position, accuracy verification using statistical theory and locking stabilization using a one-dimensional template matching method, namely least-squares matching (LSM), were adopted to achieve wavelength locking. The resulting system is capable of generating a stable wavelength.

Performance Evaluation for China’s Planned CO2-IPDA

Active remote sensing of atmospheric XCO2 has several advantages over existing passive remote sensors, including global coverage, a smaller footprint, improved penetration of aerosols, and night observation capabilities. China is planning to launch a multi-functional atmospheric observation satellite equipped with a CO2-IPDA (integrated path differential absorption Lidar) to measure columnar concentrations of atmospheric CO2 globally. As space and power are limited on the satellite, compromises have been made to accommodate other passive sensors. In this study, we evaluated the sensitivity of the system’s retrieval accuracy and precision to some critical parameters to determine whether the current configuration is adequate to obtain the desired results and whether any further compromises are possible. We then mapped the distribution of random errors across China and surrounding regions using pseudo-observations to explore the performance of the planned CO2-IPDA over these regions. We found that random errors of less than 0.3% can be expected for most regions of our study area, which will allow the provision of valuable data that will help researchers gain a deeper insight into carbon cycle processes and accurately estimate carbon uptake and emissions. However, in the areas where major anthropogenic carbon sources are located, and in coastal seas, random errors as high as 0.5% are predicted. This is predominantly due to the high concentrations of aerosols, which cause serious attenuation of returned signals. Novel retrieving methods must, therefore, be developed in the future to suppress interference from low surface reflectance and high aerosol loading.

Comparison of Satellite-Observed XCO2 from GOSAT, OCO-2, and Ground-Based TCCON

CO2 is one of the most important greenhouse gases. Its concentration and distribution in the atmosphere have always been important in studying the carbon cycle and the greenhouse effect. This study is the first to validate the XCO2 of satellite observations with total carbon column observing network (TCCON) data and to compare the global XCO2 distribution for the passive satellites Orbiting Carbon Observatory-2 (OCO-2) and Greenhouse Gases Observing Satellite (GOSAT), which are on-orbit greenhouse gas satellites. Results show that since GOSAT was launched in 2009, its mean measurement accuracy was −0.4107 ppm with an error standard deviation of 2.216 ppm since 2009, and has since decreased to −0.62 ppm with an error standard deviation of 2.3 ppm during the past two more years (2014–2016), while the mean measurement accuracy of the OCO-2 was 0.2671 ppm with an error standard deviation of 1.56 ppm from September 2014 to December 2016. GOSAT observations have recently decreased and lagged behind OCO-2 on the ability to monitor the global distribution and monthly detection of XCO2. Furthermore, the XCO2 values gathered by OCO-2 are higher by an average of 1.765 ppm than those by GOSAT. Comparison of the latitude gradient characteristics, seasonal fluctuation amplitude, and annual growth trend of the monthly mean XCO2 distribution also showed differences in values but similar line shapes between OCO-2 and GOSAT. When compared with the NOAA statistics, both satellites’ measurements reflect the growth trend of the global XCO2 at a low and smooth level, and reflect the seasonal fluctuation with an absolutely different line shape.

Comparison of Global XCO2 Concentrations From OCO-2 With TCCON Data in Terms of Latitude Zones

This work evaluated the performance of the orbiting carbon observatory 2 (OCO-2) in terms of global atmospheric CO 2 observations for 20 months (September 2014 to April 2016). Three versions of data on CO2 are currently available, namely, version 7, version 7r, and Lite File Product (Lite_FP). For the first time, we evaluated XCO2 measurements from three versions of OCO-2 in terms of utilization efficiency, spatiotemporal coverage, and measurement accuracy compared with data (GGG2014) from the total carbon column observing network (TCCON). In data application, Lite_FP usually displayed the most efficient data volume and relatively stable spatial coverage, i.e., 42% in global scale. In addition, the spatial coverage of XCO2 measurements on land and ocean displayed opposite periodic seasonal fluctuations. However, no data were obtained in some areas where research on carbon ecology is highly significant. In terms of measurement accuracy, we considered the latitude distribution of TCCON sites and performed a site-by-site comparison at different latitude zones between XCO2 from three versions of OCO-2 and TCCON. Results demonstrated that the periodic variation trend of XCO2 from OCO-2 was consistent with that from TCCON. Moreover, the amplitude was similar to that of TCCON except that several sites had significant seasonal variation amplitude. The mean bias of OCO-2 was generally < 0.8 ppm, with 0.55% deviation. Among the three versions of OCO-2, Lite_FP showed good result in filtering and bias correction in the mid-low latitudes but still needs improvement in the high latitudes of the Northern and the Southern Hemispheres.

A CO 2 Profile Retrieving Method Based on Chebyshev Fitting for Ground-Based DIAL

The vertical profile of atmospheric CO2 is of great scientific significance in identifying carbon sinks and sources, and estimating CO2 emissions or uptakes. Differential absorption Light Detection And Ranging (DIAL), has been widely accepted as the most promising technique to sense atmospheric CO2. The classical method to retrieve measurements, generated from range-resolved detection, is derived from differentiating the measured column content, but its performance in dealing with aerosol backscatter signals is poor. To address this issue, this paper proposes a derivative method, which is based on Chebyshev fitting to the measured differential absorption optical depth. We created a performance evaluation model to assess the performance of the proposed method. Simulations revealed that the error of a single CO2 profile in data retrieval can be reduced to less than 4 ppm in 6000 m. The precision of long-term mean CO2 profile is expected to be less than 1 ppm. We believe that this novel method can be used in other applications also, e.g., trace gas measurements collected using DIAL, especially when the signal-to-noise-ratio of received signal is small.

Wavelet modulus maxima method for on-line wavelength location of pulsed lidar in CO 2 differential absorption lidar detection

Differential absorption lidar (DIAL) is an excellent technology for atmospheric CO2 detection. However, the accuracy and stability of a transmitted on-line wavelength are strictly required in a DIAL system. The fluctuation of a tunable pulsed laser system is relatively more serious than that of other laser sources, and this condition leads to a large measurement error for the lidar signal. These concerns pose a significant challenge in on-line wavelength calibration. This study proposes an alternative method based on wavelet modulus maxima for the accurate on-line wavelength calibration of a pulsed laser. Because of the different propagation characteristics of the wavelet transform modulus maxima between signal and noise, the singularities of a signal can be obtained by detection of the local modulus maxima in the wavelet transform maximum at fine scales. Simulated analysis shows that the method is more accurate than the general method such as quintic polynomial fitting and can steadily maintain high calibration precision at different signal-to-noise ratios (SNRs). Last, 16 groups of real experiments were conducted to verify the simulated analysis, which shows that the proposed method is an alternative for accurately calibrating an on-line wavelength. In addition, the proposed method is able to suppress noises in the process of wavelength calibration, which gives it an advantage in accurate on-line wavelength calibration with a low SNR.

An improved retrieving method of vertical CO 2 concentrations profile for dial

The vertical profile of atmospheric carbon dioxide is of great significance for carbon cycle and budget study. However, that parameter can be only obtained by flask samples from profiling aircraft till now. Ground-based differential absorption lidar is widely accepted as a promising remote sensing means to obtain the vertical CO2 concentration profile. Unfortunately, classic DIAL retrieving method cannot provide results of adequate accuracy and precision. Here, we propose an improved retrieving method to solve this problem. Experiments showed that the accuracy and precision of results are superior to 0.2 ppm (parts per million) and 4E-5 ppm respectively by means of the proposed method. That could be an ideal result for further carbon cycle and climate change research.

Development of differential absorption LiDAR system at 1.57 μm for sensing carbon dioxide in China

To facilitate understanding of the relationship between the most significant greenhouse gas carbon dioxide and human activities, we have developed a differential absorption lidar (DIAL) detection system at 1.57 μm. The goal of this lidar system is to detect the temporal and spatial distribution of atmospheric carbon dioxide gas from 0.3 km to 3 km in the atmosphere. Beginning in 2009, the system was initially completed in 2013. Since then, we have been constantly experimenting and repeated instrumentation improvements. From July 2015 to the present, we carried out vertical and horizontal measurement experiments in the urban area of Wuhan, Hubei Province and the suburb of Huainan, Anhui Province, China. This article presents a fast and optimized inversion algorithm to improve the speed and accuracy. Experimental results show that the DIAL system and inversion algorithm are stable and reliable.

Evaluation of XCO2 from OCO-2 Lite File Product compared with TCCON data

To evaluate the performance of the Orbiting Carbon Observatory 2 (OCO-2) Lite File Product (Lite_FP) which has the highest amount of data and the highest utilization efficiency among the three products of OCO-2, we compared global atmospheric CO2 observations for 20 months (September 2014 to April 2016) with GGG2014 data from the Total Carbon Column Observing Network (TCCON). We considered the latitude distribution of the TCCON sites and performed a site-by-site comparison at different latitude zones. The result demonstrated that the seasonal fluctuation of XCO2 from Lite_FP is consistent with TCCON, and the biases of XCO2 measurements ranged from −3 ppm to 4 ppm, with a 1% precision. Bias distribution differed in terms of latitude zones and observing modes. In addition, we analyzed the distribution characteristic of the bias of XCO2 observations under land target mode in detail combined with surface and atmospheric properties.

OCO-2 XCO2 validation using TCCON data

This work evaluates the performance of OCO (Orbiting Carbon Observatory) -2 on global observations since its launch in Sep 2014. It is 10%~30% coverage that could be detected to obtain the concentration of atmospheric carbon dioxide in space dimension. However, about 65% data of OCO-2 has not be utilized to retrieve because of special atmospheric environment, such as thick aerosol depth and low pressure. In the accuracy aspect, compared with the TCCON, OCO-2 has good consistence of XCO2 at most sites. The average bias of monthly value is about 0.87 ppm and the standard deviation is 1.8 over TCCON sites. The mean monthly value of CO2 can represent the actual value at a certain range.