Detection of the atmospheric pollutant by using passive DOAS

This paper is aimed to present our method for finding the atmospheric pollutant by using passive DOAS. For this purpose, we have introduced an optical tomography system based on the passive difference optical absorption spectrum (DOAS), that is, how to do rea-l time monitoring activities with the spatial distribution of pollution gases in the atmosphere so as to prevent the atmosphere from being polluted effectively. The goal of the system is to employ the concentration profiles, rather than just a point value, of the pollutant gases concentrations. However, the reconstruction area (20×20 pixels) of the concentration of the pollutant gases are to be scanned while doing on-line monitoring activities by using two sets of passive difference optical absorption spectrum (DOAS) ratherthan one. In addition, 36 light paths are needed to be adopted in the form of the fan projection scheme to scan the area of the polluted gases. At the same time, it is to apply a novel image reconstruction algorithm, known as the simu-l taneous iterative reconstruction technique (SIRT), to extract the data and information from the two passive DOAS systems. The disadvantage of the SIRT is slow in speed though some improvements have a-l ready been made by adding different relax factors to reduce the relative errors. The new system of ours can work out the integrated concentration of the gases with the difference optical absorption spectrum (DOAS) algorithm. Besides, we have also introduced the detection principle of difference optical absorption spectrum (DOAS) in the paper in a detailed way by means of numerical simulation for analyzing and evaluating the reconstruction effect. The results of trial application of our system showthatthe theory is conformedwiththe practice, with the maximum error of the optical tomography being 0.02, which proves to meet the expected indicators nicely. And, if the different random errors were added to the projection data, the results of our testing also indicate that the projection errorwon’t basically affect the quality of the rebuilt image. Moreover, the experimental results prove to be in conformity with the actual situation, with the measurement relative error being about 7%. All the above facts showthat the system helps to realize the gas two-dimensional space reconstruction, and the optical tomography is feasible formonitoring the atmospheric env-i ronment. Thus, we can believe that the tomography system developed herein by us is applicable for other atmosphere measurements. And, it is natural thatthe image quality should be improved in the future by increasing the number of passive difference optical absorption spectra (DOAS).