Study On Continuous Air Quality Monitoring System Based On Differential Optical Absorption Spectroscopy

Air pollution destroys ecological condition and does great harm to human beings. To prevent air pollution, monitoring of polluted gases' concentrations is needed to be done firstly. At present, most of air quality monitoring systems operated in our country are imported. These instruments only provide average data during a period of time in one point. So we try to develop a set of real-time air quality monitoring system that can measure concentrations of trace gases continuously during a long distance. Our research emphasizes on how to utilize the technique of differential optical absorption spectroscopy(DOAS) in the system. The system is divided into three parts including differential optical absorption spectrometer, PM10 monitoring device and metrological instrument. Other than the lectotype of some parts of the system and electronic interface, the main job we do is as follows: Firstly, we did research on spectral analysis and data process. Based on the theory of DOAS, an algorithm is developed by two cardinal lines including the process of reference spectra and of atmospheric spectra. The former is accomplished by getting absorption cross section applicable for the system through convoluting standard cross section with instrumental function, and then separating the differential part from slow varying part by polynomial approximation method. The latter includes the process of eliminating effect of spectra deviation, tension and contraction, and extracting the differential part in the same way. At last, least squares fit is done between differential absorption cross section and differential absorption spectra to retrieve concentrations of polluted gases. We applied two advanced method to data process. One is extracting the differential part by digital filtering instead of polynomial approximation and the other is fitting the area of spectra instead of multiple linear regression. Experiments show that by using a the digital filter, retrieval accuracy is improved and the operation time is reduced by almost 8 percent, which is prefered to real-time monitoring. For area fitting method, process such as spectra deviation is not required, therefore, it needs less computation time, and will improve the accuracy of retrieval, too. Secondly, we evaluated the system by experiments. We established an experimental setting for the evaluation. The reproducibility, accuracy and linearity of the system were detected on the setting by testing standard gases with known concentrations. After its installation, the system measured real atmosphere and compared with another instrument of the same kind. It showed that the test results of our system were close to those of international advanced instrument of the same kind. At last, we discussed how the error was produced and how to eliminate it.