Abstract：Benzene-toluene-xylene (BTX) is an important component of atmospheric pollutants VOCs(Volatile Organic Compounds). Long term exposure to benzene series will greatly increase the risk of human carcinogenesis. BTX has obvious characteristic absorption characteristics in the ultraviolet band. The 250～275 nm band is selected as the research band. This band can include the characteristic absorption of BTX, and a set of equipment for preparing standard gaseous BTX from standard BTX liquid is designed. The continuous light source and differential absorption spectroscopy are used to monitor the single-component benzene and BTX mixed gas. Wavelet transform filtering and polynomial smoothing filtering method are used to evaluate the denoising effect. The results show that the polynomial smoothing filtering is often used in the traditional method of processing the absorption spectral noise, and the detailed information or high-frequency components on the absorption section will be lost in this method. The wavelet transform has good time-frequency localization characteristics. It can perform multi-resolution analysis on the signal through scaling and translation and can focus on any details of the signal. The wavelet transform denoising method can maintain the characteristic structure of spectral lines better, and the signal-to-noise ratio is better than the polynomial smoothing filtering. The absorption cross-section obtained through experiments is compared with the absorption cross-section in the HITRAN database, and it is found that if the absorption cross-section value in the HITRAN database is directly used, it will cause concentration inversion errors due to temperature and pressure changes. In order to be consistent with the actual monitoring environment, the absorption cross-section obtained in the laboratory is used as the standard absorption cross-section. The integrated area method and the least square method are used to retrieve the concentration of benzene. The results show that the measurement accuracy of the two methods can meet the environmental protection requirements, but the least square method is more stable and accurate. For the measurement of BTX mixed gas, the method of inverting the differential absorbance by the concentration value is used to invert the concentration values of benzene-toluene-xylene one by one. The study found that the measurement error of xylene inversion for the concentration of mixed BTX mixtures is less than 2%, but the measurement error of toluene and benzene gradually increased. The maximum error of benzene inversion reaches 9.07%, and the measurement accuracy of benzene is affected by the measurement accuracy of xylene, toluene and the characteristic absorption band characteristics of benzene.
Key words：Benzene-toluene-xylene (BTX); Differential optical absorption spectroscopy (DOAS); Wavelet transform filtering; Integral area method; Least square method; Monitoring
