Study On Miniaturized Environmental Pollution Gas Monitoring System Based On DOAS

With the development of economy, industry and transportation business developincreasingly fast, which leads to more and more serious environmental pollution.Because environmental pollution directly affects human health, environmentalprotection has become an important theme in the development of the new century.Monitoring pollutants is an important prerequisite for environmental protection. Inorder to continuously monitor air pollutants in a large range, according to theprinciple of differential optical absorption spectroscopy, the paper proposes a portableair quality monitoring system with combined transmitting-receiving fibers based onCassegrain telescope system.The main work of this paper is listed as follows.Firstly, On the basis of the Lambert-Beer law, the paper analyzes the principleand characteristics of the existing DOAS air-polluting gas monitoring system. On thisbasis, this paper proposes a modified monitoring system. The modified DOAS systemuses optical fibers sharing a common terminal to realize the transmission of the light.The main optical path of the modified system reserves a Cassegrain telescopestructure, which implement the collimation and reception of the optical path through aparabolic reflector and an ellipsoid sub-reflector. Compared with the traditionalDOAS system，it has many advantages such as simple structure, high efficiency ofutilizing light energy, small size and portability.Secondly, the data processing is divided into two main lines. Firstly, whendealing with the standard absorption section, an interpolation method was adopted toobtain the same resolution as the observational spectrum. Secondly, when dealingwith absorption spectrum, the Savitzky-Golay digital smoothing filter technology isapplied to fit the slow change, and by removing the slow change spectrum the fastchange spectrum, which presents the gas absorption spectrum was extracted. Becausethe differential absorption spectrum contains high frequency noises, in this paper, thewavelet transform method was used to remove high-frequency noises.Finally, According to measuring different concentrations of SO₂and NO₂gasusing a self-developed prototype，the paper evaluates the accuracy and linearity of thesystem, and monitors live air at the same time. The measurement shows the maximum monitoring error of SO₂gas is4.20%and2.27%for NO₂. At last, the essay analyzesthe causes of error in detail.