基于差分吸收光谱技术监测苯-甲苯-二甲苯的实验研究

Experimental Research on Monitoring of BTX Concentration Based on Differential Optical Absorption Spectroscopy

苯-甲苯-二甲苯(BTX)是大气挥发性有机物(VOCs)的重要组成成份,人类长期暴露在苯系物的环境中致癌风险将极大提高。利用BTX在紫外波段有明显的光谱吸收特征,选取250~275 nm作为研究波段,该波段可将BTX的主要特征吸收包含在内。设计了一套由标准BTX液体制备标准气态BTX的装置,采用连续紫外光源和差分吸收光谱技术分别对单组分苯与BTX混合气体进行连续监测研究。为最大程度去除外界噪声干扰,分别采用小波变换滤波和多项式平滑滤波法,并对两种方法的去噪效果进行评价。研究表明尽管传统处理吸收光谱噪声的方法常采用多项式平滑滤波,但该方法会使吸收截面上的细节信息或高频分量丢失。而小波变换滤波具有良好的时频局域化特性,能通过伸缩和平移对信号进行多分辨率分析并可聚焦到信号的任意细节,更能保持光谱谱线的特征结构且信噪比优于多项式平滑滤波。通过实验获得了BTX的吸收截面并与HITRAN数据库吸收截面对比,发现若直接采用HITRAN数据库中吸收截面值将造成由于温度与压强变化导致的浓度反演误差。为了能够与实际监测环境相符合,采用实验室获取的吸收截面作为标准吸收截面。对单组分苯浓度的反演分别采用积分面积法和最小二乘法,研究表明两种方法的测量精度均能满足环保监测法规要求,且最小二乘法更加稳定、精度更高。针对BTX混合气体的测量,采用通过浓度值反演差分吸光度的方法进而逐一反演苯-甲苯-二甲苯的浓度值。研究发现对BTX混合物的浓度反演时二甲苯测量误差均在2%以下,但对甲苯和苯的测量误差逐渐增大,苯浓度的反演最大误差达到了9.07%,苯的测量精度受到了二甲苯、甲苯测量精度以及苯光谱特征吸收波段的影响。

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.