Experimental study of nonlinear phenomenon of NO ultraviolet broadband absorption spectroscopy

Abstract

Ultraviolet broadband absorption spectroscopy (UV-BAS) has been widely used to measure the concentration of gas pollutant, such as NO. However, the nonlinear dependence of the absorbance on the optical thickness (<i>XL</i>) caused by the broadening effect of instrument function is observed. In this paper, the nonlinear behavior of NO absorbance is investigated both theoretically and experimentally, and a database using a polynomial to describe the nonlinearity is established to present a simple method of measuring NO concentration. First, the nonlinear relationship between absorbance and <i>XL</i> is deduced. Second, the nonlinearity of an isolated spectral line is simulated, and the dependence of nonlinear behavior on instrument width is investigated. Third, the nonlinerities of peak absorbance in <i>γ</i> (0, 0) band with different instrumental widths are calculated, the nonlinear expression is given in a polynomial form, and the corresponding coefficient database is established. In addition, the nonlinearities in different vibration bands with the same instrumental width are compared with each other. Finally, two spectrometers are used to measure NO absorption spectra in different instrumental widths in order to validate the above-mentioned results of theoretical analysis. The relative error between the measured peak absorbance and theoretical calculation is less than 4%, and that between experimental results and the interpolation polynomial results is less than 8%. The experimental results demonstrate the accuracy of theoretical calculation and the reliability of database.