SO₂ Detection By Using Laser Induced Fluorescence Method

Atmospheric pollution is one of the problems concerned by everycountry in the world. It always brings too many damages to both thedevelopment of economy and natural environment. Especially, it isharmful to the human being's health. SO2 is one of the most importantcomponents in atmospheric pollution. It generates from burning of coaland oil. Consequently, if we want to prevent atmospheric pollution, wemust study how to measure it accurately. Nowadays, the deterioration of the atmospheric pollution boosts thedevelopment of new detection technology. Today, semiconductor laserand quantum cascade laser have become new infrared sources, so peoplebegin to pay attention to the technique of infrared absorption. Thetraditional detection method of the atmospheric pollution is to collectsample outside and then analyze it. It is fussy. The technique of laserremote-analysis has more incomparable excellences than others, forexample, high sensitivity, high resolution, multi-component at thesametime, real-time, and very fast detection, and so on. First, this thesis reviewed several spectroscopic techniques otherthan ours, such as , Fourier transform infrared spectroscopy (FTIR),differential optical absorption spectroscopy (DOAS) , long-pathabsorption spectroscopy with laser, and differential absorption Lidar(DIAL). They have different detectable range , advantages and - 67 -吉林大学硕士学位论文disadvantages. Then,this thesis described the technique of laser-induced fluorescencethat we used for SO2 ultra-sensitiveanalysis. SO2 has three strong UV absorption bands, the first one is from190nm to 220nm, the second one is from 280nm to 310nm and the lastone is from 350nm to 390nm. In our experiment, we choose thewavelength of 300 nm which is within the absorption band between280nm and 310nm. We recorded fluorescence emission spectrum between280nm and 617nm, excited with 300nm light. All experiments wereperformed at room temperature. The experimental setup is shown inFigure.1. In these experiments a nanosecond laser system was employedfor producing the excitation laser pulses. An injection seeded Q-switchedNd:YAG laser (Continuum Precision II 8000),which worked at arepetition frequency of 10Hz, provides a frequency-tripled 355nm laserbeam, which was used to pump a tunable OPO laser with pulse duration6ns. Following the excitation, the fluorescence was collected in thedirection perpendicular to the incident beam using a quartz lens with4cm-diameter, 10cm-focal–length,and focused onto the 1mm entranceslit of monochromator. The exit slit of the monochromator was equippedwith a photomultiplier tube (Hamamatsu R3896). The signals fromphotomultiplier tube were monitored by a Tektronix TDS 620B 500MHzdigital oscilloscope, which was triggered by a photodiode. Data wereacquired through a Boxcar Integrator that was triggered by a photodiodeand transferred to a personal computer. The time-resolved fluorescencedecay curve was monitored with an oscilloscope. - 68 -吉林大学硕士学位论文 Fig.1 experimental setup of detecting SO2 concentration 0.7 300nm 0.6 0.5 364.0nm )v( 0.4 aget 0.3 600nm vol 0.2 0.1 0.0 -0.1 250 300 350 400 450 500 550 600 650 wavelength(nm) Fig.2 emission spectrum of SO2 excited at 300.0nm The gaseous mixture of SO2 and N2 were filled into five different gascells. Their concentrations were 2ppm, 20ppm, 50ppm, 100ppm,150ppm respectively. Through several experiments, we got fluorescencesignal intensity of five dif...