Application of bi-directional long short-term memory in separating NO and SO2 ultraviolet differential absorption spectrum signals

To safeguard the environment, it is crucial to monitor the emissions of nitrogen oxide (NO) and sulfur dioxide (SO2), harmful pollutants generated during fossil fuel combustion in industries. However, accurately measuring ultra-low concentrations of SO2 and NO remains a challenge. In this study, we developed an optical measurement system based on ultraviolet differential optical absorption spectroscopy (UV-DOAS) to address this issue. The 200-230 nm cross-sensitivity band was chosen for SO2 and NO. Experimental data with a mixed gas concentration range of 1-25 ppm for SO2 and NO was utilized. We proposed a fast algorithm based on Bi-directional Long Short-Term Memory (Bi-LSTM) to extract the differential optical density, overcoming the mutual interference between SO2 and NO. A nonlinear calibration model was employed to invert the separated differential absorption spectra and determine the gas concentrations. The results demonstrated a detection limit (DL) of 0.27 ppm and a full-scale error of 3.15% for SO2, while for NO, the DL was 0.32 ppm and the full-scale error was 2.81%. The uncertainties in SO2 and NO detection were calculated as 1.73% and 1.96%, respectively.