Sensitivity analysis and correction algorithms for atmospheric CO2 measurements with 1.57-µm airborne double-pulse IPDA LIDAR

Preliminary analysis of global column-averaged CO2 concentration data from the spaceborne aerosol and carbon dioxide detection lidar onboard AEMS

In contrast to the passive remote sensing of global CO2 column concentrations (XCO2), active remote sensing with a lidar enables continuous XCO2 measurements throughout the entire atmosphere in daytime and nighttime. The lidar could penetrate most cirrus and is almost unaffected by aerosols. Atmospheric environment monitoring satellite (AEMS, also named DQ-1) aerosol and carbon dioxide detection Lidar (ACDL) is a novel spaceborne lidar that implements a 1572 nm integrated path differential absorption (IPDA) method to measure the global XCO2 for the first time. In this study, special methods have been developed for ACDL data processing and XCO2 retrieval. The CO2 measurement data products of ACDL, including the differential absorption optical depth between the online and offline wavelengths, the integral weighting function, and XCO2, are presented. The results of XCO2 measurements over the period from 1st June 2022 to 30th June 2022 (first month data of ACDL) are analyzed to demonstrate the measurement capabilities of the spaceborne ACDL system.

Simulation evaluation of a single-photon laser methane remote sensor for leakage rate monitoring

We propose a novel methane leakage rate remote sensor that combines a single-photon avalanche diode detector with a near-infrared 1653.7 nm low-power laser. The proposed M sequence and triangle wave signal modulation method simultaneously realizes the detection of methane leakage and target point clouds. Innovatively, the sensor's methane concentration and leakage rate quantification ability were simulated by combining the Gaussian plume diffusion model and the Risley prism. The effects of the prism rotation ratio, wind speed, leakage rate, atmospheric stability (AS), target reflectivity, signal averaging period, and concentration spatial interpolation method on leakage rate are discussed. When plume methane concentrations reduce from 10,000 to 500 ppm·m, the relative concentration bias rise from 1% to 30%, the absolute concentration bias is approximately 100 ppm·m. Two spatial concentration interpolation methods introduced leakage rate bias ranging from 6%-25%. For a low AS, the leakage rate bias under the cubic interpolation method was small (approximately 1.6%). In addition, when the initial leakage rate increased from 100 to 1,000 mg/s, the leakage rate bias was approximately 20% smaller.

Robust algorithm for precise XCO2 retrieval using single observation of IPDA LIDAR

CO₂ column-weighted dry-air mixing ratio (X_CO2) products with high precision and spatial resolution are essential for inverting CO₂ fluxes and promoting our understanding of global climate change. Compared with passive remote sensing methods, IPDA LIDAR, as an active remote sensing technique, offers many advantages in measuring X_CO2. However, a significant random error in IPDA LIDAR measurements causes X_CO2 values calculated directly from LIDAR signals to be unqualified as the final X_CO2 products. Hence, we propose an efficient particle filter-based inversion of CO₂ for single observation (EPICSO) algorithm to precisely retrieve the X_CO2 of every LIDAR observation while preserving the high spatial resolution of LIDAR measurements. The EPICSO algorithm adopts the sliding average results as the first estimate of the local X_CO2; subsequently, it estimates the difference between two adjacent X_CO2 points and calculates the posterior probability of X_CO2 based on particle filter theory. To evaluate the performance of the EPICSO algorithm numerically, we perform an EPICSO to process pseudo-observation data. The simulation results show that the results retrieved by the EPICSO algorithm satisfy the required high precision and that the algorithm is robust to a significant amount of random errors. In addition, we utilize LIDAR observation data from actual experiments in Hebei, China, to validate the performance of the EPICSO algorithm. The results retrieved by the EPICSO algorithm are more consistent with the actual local X_CO2 than those of the conventional method, indicating that the EPICSO algorithm is efficient and practical for retrieving X_CO2 with high precision and spatial resolution.

Calibration experiments based on a CO2 absorption cell for the 1.57-µm spaceborne IPDA LIDAR

The spaceborne IPDA LIDAR has the potential to measure the global atmosphere CO₂ column concentrations with high accuracy. For this kind of LIDAR, system calibration experiments in the laboratory are of high importance. In this study, a specially-customized CO₂ absorption cell is employed to simulate the CO₂ column absorption of the spaceborne platform. Then calibration experiments are constructed for the receiving system and the entire LIDAR system. The absorption of several different XCO₂ concentrations from 400 to 415 ppm in the atmosphere is equivalent to that of the absorption cell charged with different pressures of pure CO₂. Under the zero pressure of the absorption cell, the calculated equivalent column average concentration (XCO₂) is 12.53 ppm, which acts as system bias. In the calibration experiments, the absolute errors are all less than 1 ppm. And the standard deviations (STDs) are less than 1.1 ppm (148-shot averaging) and 0.8 ppm (296-shot averaging) for receiving system and less than 1.2 ppm and 0.9 ppm for the IPDA LIDAR system. All the results of different average times are close to each other and less than 1 ppm, which proves the high accuracy of the IPDA LIDAR system. In addition, the XCO₂ concentrations Allan deviation of 0.25 ppm and 0.35 ppm at 100 s shows that the receiving system and IPDA LIDAR system function with long-term stability. Using a CO₂ absorption cell as a standard calibration device in the laboratory validates the measurement accuracy and stability of the spaceborne IPDA LIDAR prototype. Furthermore, the proposed absorption cell may serve as a standard calibration device for related atmosphere trace gases sounding research.

Measurement of Atmospheric CO2 Column Concentrations Based on Open-Path TDLAS

Monitoring of CO₂ column concentrations is valuable for atmospheric research. A mobile open-path system was developed based on tunable diode laser absorption spectroscopy (TDLAS) to measure atmospheric CO₂ column concentrations. A laser beam was emitted downward from a distributed feedback diode laser at 2 μm and then reflected by the retroreflector array on the ground. We measured the CO₂ column concentrations over the 20 and 110 m long vertical path. Several single-point sensors were distributed at different heights to provide comparative measurements for the open-path TDLAS system. The results showed that the minimum detection limit of system was 0.52 ppm. Some similarities were observed in trends from the open-path TDLAS system and these sensors, but the average of these sensors was more consistent with the open-path TDLAS system values than the single-point measurement. These field measurements demonstrate the feasibility of open-path TDLAS for measuring the CO₂ column concentration and monitoring carbon emission over large areas.

High-Precision CO2 Column Length Analysis on the Basis of a 1.57-μm Dual-Wavelength IPDA Lidar

For high-precision measurements of the CO₂ column concentration in the atmosphere with airborne integrated path differential absorption (IPDA) Lidar, the exact distance of the Lidar beam to the scattering surface, that is, the length of the column, must be measured accurately. For the high-precision inversion of the column length, we propose a set of methods on the basis of the actual conditions, including autocorrelation detection, adaptive filtering, Gaussian decomposition, and optimized Levenberg–Marquardt fitting based on the generalized Gaussian distribution. Then, based on the information of a pair of laser pulses, we use the direct adjustment method of unequal precision to eliminate the error in the distance measurement. Further, the effect of atmospheric delay on distance measurements is considered, leading to further correction of the inversion results. At last, an airborne experiment was carried out in a sea area near Qinhuangdao, China on 14 March 2019. The results showed that the ranging accuracy can reach 0.9066 m, which achieved an excellent ranging accuracy on 1.57-μm IPDA Lidar and met the requirement for high-precision CO₂ column length inversion.

Airborne Validation Experiment of 1.57-μm Double-Pulse IPDA LIDAR for Atmospheric Carbon Dioxide Measurement

The demand for greenhouse gas measurement has increased dramatically due to global warming. A 1.57-μm airborne double-pulse integrated-path differential absorption (IPDA) light detection and ranging (LIDAR) system for CO2 concentration measurement was developed. The airborne field experiments of this IPDA LIDAR system were conducted at a flight altitude of approximately 7 km, and the weak echo signal of the ocean area was successfully received. The matched filter algorithm was applied to the retrieval of the weak signals, and the pulse integration method was used to improve the signal-to-noise ratio. The inversion results of the CO2 column-averaged dry-air mixing ratio (XCO2) by the scheme of averaging after log (AVD) and the scheme of averaging signals before log were compared. The AVD method was found more effective for the experiment. The long-term correlation between the changing trends of XCO2 retrieved by the IPDA LIDAR system and CO2 dry-air volume mixing ratio measured by the in-situ instrument reached 92%. In the steady stage of the open area (30 km away from the coast), which is almost unaffected by the residential areas, the mean value of XCO2 retrieved by the IPDA LIDAR system was 414.69 ppm, with the standard deviation being 1.02 ppm. Compared with the CO2 concentration measured by the in-situ instrument in the same period, bias was 1.30 ppm. The flight path passed across the ocean, residential, and mountainous areas, with the mean value of XCO2 of the three areas being 419.35, 429.29, and 422.52 ppm, respectively. The gradient of the residential and ocean areas was 9.94 ppm, with that of the residential and mountainous areas being 6.77 ppm. Obvious gradients were found in different regions.