Theories And Technique Studies On Ultra-Violet Fluorescence Detection Of Sulfur Dioxide In Air At The Parts Per Billion Level

With the development of economic, environment pollution problem arise people's concerns. One of major pollutants, sulfur dioxide, has done great harm to our environment, so it is more and more necessary to operate an continuous monitoring for pollution by sulfur dioxide.In our country, all monitoring systems for atmospheric pollutants, including sulfur dioxide have imported for a long time, such situation do no good to national environment protection technology and environmental protection industry, we should develop our environment protection technology including detection of sulfur dioxide in air. Among all methods for detecting sulfur dioxide, ultraviolet fluorescence method is popular in many countries, including our country, and also is promoted by Word Health Organization.By the great support from Guangzhou Environment Protection Administration, we start the theories and technique studies on ultraviolet fluorescence for detecting sulfur dioxide in air based on photon counting method. The achievements include.-(1) On the base of single wavelength and small light beam fluorescence detection theory, fluorescence detection theory using multiple excitation wavelength and limited excitation light beam is presented; the results show the later can improve the fluorescence response of sulfur dioxide.(2) Two physical model for studying the performance of cell, the key part, are developed; numerical calculation results indicate the effect of both radius of cross section and absorption coefficient of the surface of cell on cell's signal-noise ratio, and there is a reasonable radius value for cell to reach the best ratio of signal to noise without an bigger radius; those results are helpful for the design optimization of cell;(3) Two experimental system are designed and many factors having an effect on system's performance are studied; using the first experimental system with the detector H6240-02 PMT, the results show that the photon counts is linear to the concentration of sulfur dioxide from 0 to 97ppm in volume; using another experimental system with the detector H6180-01 PMT, the detector response is linear from 18ppb to l00000ppb; the response of SO2 fluorescence signal is about 7counts/ppb.s ,and the detection limit is about 8 ppb, with an integration time of one minute .(4) The major source of measurement error at low ppb concentrations is fluctuations of excitation light. The dependence of fluorescence response of zero air on temperature and flux is not obvious. Fluorescence response at 48000ppb and lOOOOOppb increased with the increase of the flux, however is lower with the higher temperature at 48000ppb.(5) The differences on fluorescence counts between ambient air and zero air are studied.(6) The origin of background counts, the spreading of excitation light and fluorescence are analyzed.(7) One data acquisition soft was developed by use of National Instrument's virtual development language.