Study on the concentration retrieval of SO₂ and NO₂ in mixed gases based on the improved DOAS method

NO₂ and SO₂ are important components of air pollutants, and their absorption spectra are superimposed at 193-253 nm. The superposed spectra affect the gas concentration retrieval based on the ultraviolet differential optical absorption spectroscopy (DOAS) method. In this study, a suitable wavelength band was chosen for concentration retrieval, moreover, the characteristics of the quasi-periodic variation of absorption cross-section with wavelength was given sufficient attention and then the superposed spectra were separated by the Fast Fourier transform (FFT) method. The concentration of the gases to be measured was calculated according to the relationship between the amplitude of absorbance after FFT and the gas concentration. The experimental results prove that by using an absorption cell with a 700 mm optical path, the relative deviation absolute value of the retrieval concentration of SO₂ in a NO₂ and SO₂ gas mixture is less than 1.471%, and that of NO₂ in a NO₂ and SO₂ gas mixture is less than 7.207%. The method has good adaptability, high detection precision, whether single SO₂, NO₂ or a mixture of both, and important reference value for the development of DOAS and future research on the high-precision detection of more types of mixed gases in the ultraviolet band, such as gas mixtures of NO₂, SO₂ and NO in flue gas.