Ultrasensitive NO Monitoring System Based on Fourier Transform Ultraviolet Differential Optical Absorption Spectrum

Nitric oxide (NO) is cross-sensitive with NO2 and SO2, which is a key obstruction for accurately monitoring its concentration. In this work, a NO monitoring system was developed. An innovative approach based on differential optical absorption spectroscopy (DOAS) was proposed. Fast Fourier transform (FFT) was performed on differential optical density (DOD) signals of NO, NO2, and SO2. The differences in characteristic frequency distribution were found. Characteristic frequencies were utilized to identify and separate NO characteristic frequencies in mixed gas. Inverse FFT (IFFT) was performed to reconstruct the NO DOD signal, and thus, the interference of NO2 and SO2 was greatly alleviated. Furthermore, a novel concentration retrieval algorithm that enhanced the selectivity of the system was proposed. The experimental results show that the detection limit (DL) was 0.047 mg/m3, which is among the best ones. The quantitative limit (QL) was 0.156 mg/m3. A nonlinear error was 0.62%F.S (full scale). Meanwhile, the gas temperature influence on the retrieval concentration was also studied. A simple but effective temperature compensation method is used to enhance the adaptability of the system. The comparison experiment between the commercialized NO monitoring system and the self-developed system was performed in a power plant, and the correlation of the results reached 0.91, which means that the measurement results are highly consistent.