Detection of Benzene Concentration by Mid-Infrared Differential
Absorption Lidar
DUAN Ming-xuan1, LI Shi-chun1, 2*, LIU Jia-hui1, WANG Yi1, XIN Wen-hui1, 2, HUA Deng-xin1, 2*, GAO Fei1, 2
1. School of Mechanical and Precision Instrument Engineering, Xi'an University of Technology, Xi'an 710048, China
2. Shaanxi Collaborative Innovation Center for Modern Equipment Green Manufacturing, Xi'an 710048, China
Abstract：Benzene is an important component of volatile organic compounds (VOCs), and its pollution of the atmosphere has attracted increasing attention. The mid-infrared band is usually the fundamental frequency fingerprint absorption region of molecules, so it has become an important band for detecting trace gas molecules. Moreover, the differential absorption lidar is an important means of detecting atmospheric trace gases. Therefore, aiming at the problem of real-time remote sensing of regional benzene concentration, an integral path differential absorption (IPDA) lidar for detecting atmospheric benzene concentration based on inter-band cascade lasers (ICLs) is proposed. Firstly, we construct the retrieval algorithm of IPDA lidar and its error analysis model based on analyzing the detection principle of IPDA lidar. Secondly, the absorption spectra of benzene and major interfering gases (such as HCl, CH4 and H2O) near the mid-infrared vicinity region of 3 100 cm-1 from the HITRAN database are analyzed in detail. By considering comprehensively the influence of HCl, CH4 and H2O on the detection results, the measurement wavelength and reference wavelength of the IPDA lidar are selected to be 3 090.89 and 3 137.74 cm-1 respectively. Thirdly, we designed an IPDA lidar for detecting atmospheric benzene concentration based on two continuous-wave ICLs. The output wavelengths of these ICLs can be tuned by controlling the temperature and driving curren, so that their wavelengths can be stabilized in the strong absorption spectrum region and the weak absorption spectrum region respectively. And then, a spectroscopic system with a mid-infrared diffraction grating as the core is designed to realize synchronous detection of dual-wavelength receiving signals. Finally, combined with the mid-latitude standard atmospheric model, the performance of lidar under the conditions of different visibilities, path lengths, and water vapor concentrations is analyzed and discussed. And then, we carry out test experiments by building a mid-infrared band detection gas cell to verify the feasibility of the IPDA lidar. These results from simulations and experiments show that the relative error of benzene concentration is less than 10% within the concentration-path length product (CL) range of 0.1～24 mg·m-3·km, and the relative error of detection is better than 1%, while the CL of benzene is 5 mg·m-3·km, under the condition of atmospheric visibility of 5 km, and the water vapor concentration of less than 0.4%; and that the linear correlation coefficient R2 of differential absorption lidar detection in the mid-infrared band is about 98.7% by preliminary experiments.
