Results and Discussions A comparison of the mass concentrations of CO2, CH4, and CO measured by the Picarro greenhouse gas analyzer and atmospheric OP-FTIR system yields correlation coefficients of 0.896, 0.840, and 0.906 for CO2, CH4, and CO mass concentrations, respectively, indicating the high reliability and accuracy of the atmospheric OP-FTIR system for monitoring greenhouse gas mass concentrations (Fig. 3). The results of the measured spectral fits show that the inversion spectral regions of 2102-2250 cm-1, 2920-3140 cm-1, and 2172-2210 cm-1 selected for CO2, CH4, and CO have root mean square errors of the spectral fit residuals of approximately 0.102%, 1.359%, and 0.551%, respectively (Fig. 4). In terms of meteorological conditions, the mass concentration indices of CO2, CH4, and CO show significant negative correlations with wind speed and temperature in most cases and positive correlations with humidity, with high mass concentrations of pollution mainly in the westerly and northwesterly wind directions (Figs. 6 and 7). In general, the daily average mass concentrations of CO2, CH4, and CO were 823.470 mg?m-3, 1.330 mg?m-3, and 0.510 mg?m-3, respectively. The mass concentrations of CO2, CH4, and CO in the ambient atmosphere vary periodically, in terms of mass concentration time series (Fig. 8). The correlations between the CO and CH4 mass concentrations and CO2 mass concentrations were analyzed, yielding correlation coefficients of 0.495 and 0.659, respectively (Fig. 9). The boundary-layer inverse temperature effect, intensity of atmospheric convection, and photochemical reactions are the main causes of the variations in CO and CH4 mass concentrations. The main factors causing the variation in CO mass concentrations are photosynthesis in plants, photochemical reactions, convective air transport, and motor vehicle emissions.
Curbing global warming and reducing greenhouse gas emissions have become important issues that urgently require worldwide attention. Therefore, the establishment and improvement of real-time continuous greenhouse gas monitoring systems that can identify the sources and mass concentrations of specific pollutants are essential for controlling atmospheric pollution. There is a high degree of consensus on the choice of technology used for the high-precision, continuous, and automatic monitoring of greenhouse gases. Techniques, such as Cavity ring-down spectroscopy (CRDS), off-axis integrated cavity output spectroscopy (OA-ICOS), tunable diode laser absorption spectroscopy (TDLAS), and Fourier transform spectroscopy (FTIR) have been extensively investigated. Among these, FTIR is a promising measurement technique because it offers the technical advantages of high scanning speed, high luminous flux, and high sensitivity. In the present study, a set of 90 m open-optical-path Fourier transform infrared spectroscopy (OP-FTIR) greenhouse gas analysis and measurement equipment was designed, and the equipment was used to perform high-precision observations of CO2, CH4, and CO. Through a detailed analysis of the measurement data from the system, we hope to provide an indication of the accuracy and precision of the entire measurement apparatus and technique for measuring greenhouse gases in the atmosphere. Simultaneously, we obtain a better understanding of the influence of meteorological conditions on greenhouse gas mass concentration and the processes of greenhouse gases in the ambient atmosphere over time.
In this study, FTIR technology was used for real-time monitoring of greenhouse gases based on an open-optical-path design. First, an external field experiment measuring CO2, CH4, and CO in the ambient atmosphere was performed for three consecutive months using the constructed greenhouse gas analysis system. The strong absorption interference of water vapor was then reduced by selecting appropriate inversion spectral regions for each of the three target gases of CO2, CH4, and CO. In addition, the monitoring data from a Picarro greenhouse gas analyzer and atmospheric greenhouse gas OP-FTIR system were compared to accurately assess the accuracy and precision of the overall measurement setup and technique for monitoring atmospheric greenhouse gases. Finally, data from 10 days of the measurement period were selected to investigate the correlations between temperature and humidity, wind direction and speed, and the ambient atmospheric mass concentrations of CO2, CH4, and CO, and to analyze the daily variation characteristics of the pollutants in detail.
This paper describes an OP-FTIR measurement system for greenhouse gas analysis. A comparison of the system measurement data with those of the Picarro analyzer shows that the OP-FTIR system monitors greenhouse gas mass concentrations with high degrees of reliability and accuracy. The inversion spectral regions selected for CO2, CH4, and CO in the OP-FTIR system can maximize the information content of this component, reduce the interference of water vapor and other components, shorten the identification time, and improve the identification rate. This study provides the characteristics of horizontal variation in CO2, CH4, and CO mass concentrations. In terms of meteorological conditions, the levels of temperature, humidity, wind speed, and wind direction have significant effects on local pollutant mass concentrations; in general, the average greenhouse gas mass concentrations remained at a high level in March. In terms of mass concentration time series, the mass concentrations of CO2, CH4, and CO in the ambient atmosphere show cyclical variations. The variations in CO and CH4 mass concentrations are mainly due to the boundary-layer inverse temperature effect, the intensity of atmospheric convection, and photochemical reactions. However, CO2 mass concentrations vary mainly because of plant photosynthesis, photochemical reactions, convective air transport, and motor vehicle emissions.
