Sun Pengshuai

In this study, a method for double-beam quantum cascade laser absorption spectroscopy (DB-QCLAS) was developed. Two mid-infrared distributed feedback quantum cascade laser beams were coupled in an optical cavity for the monitoring of NO and NO₂ (NO at 5.26 μm; NO₂ at 6.13 μm). Appropriate lines in the absorption spectra were selected, and the influence of common gases in the atmosphere, such as H₂O and CO₂, was avoided. By analyzing the spectral lines under different pressure conditions, the appropriate measurement pressure of 111 mbar was selected. Under this pressure, the interference between adjacent spectral lines could be effectively distinguished. The experimental results show that the standard deviations for NO and NO₂ were 1.57 ppm and 2.67 ppm, respectively. Moreover, in order to improve the feasibility of this technology for detecting chemical reactions between NO and O₂, the standard gases of NO and O₂ were used to fill the cavity. A chemical reaction instantaneously began, and the concentrations of the two gases were immediately changed. Through this experiment, we hope to develop new ideas for the accurate and rapid analysis of the process of NOx conversion and to lay a foundation for a deeper understanding of the chemical changes in atmospheric environments.

At present, gas monitoring technology based on optical principle has been widely used in atmospheric environment, industrial emissions, smart city and other monitoring fields. For the requirement of high precision and high stability control of temperature and pressure of laser absorption spectroscopy measuring instrument, the temperature and pressure control scheme of the system is studied. The temperature and pressure control system is identified and the simulation model is established. The consistency of temperature and pressure models is 99.35% and 95.55% by using Process Models module to debug continuously. The simulation compares the output response curves of different control methods with the same step signal input, and the Fuzzy Adaptive PID parameter control algorithm achieves the design requirements in response time, overshoot and other aspects.

In order to achieve the accurate measurement of gas stable isotope, according to the structure of gas cavity and the characteristics of optical elements, a constant temperature and pressure gas isotope measurement experimental system based on tunable diode laser absorption spectroscopy (TDLAS) technology is designed, which can achieve high-precision and stable control of temperature and pressure in the box. The hardware circuit system including high-precision constant-current source, two-channel temperature sampling module, conditioning circuit and pressure control circuit is designed in the system. The corresponding system closed-loop control software is developed to ensure the stability of the temperature and pressure of the optical cavity. Through the experimental verification, the temperature is controlled at 35ºC and the pressure is controlled at 150 mbar in the present work. The measurement results verified the stability of the temperature and pressure system, as follows: the optimal temperature fluctuation control is ±0.005ºC and the pressure stability is ±0.1 mbar. Then, in our experiment, the CH4 standard gas is introduced to measure carbon isotope (12CH4 and 13CH4). The results clearly show that the control system is sufficiently stable to be used and the δ13C value can reach 0.52‰ according to the Allan variance.