Research Of Nitrogen Dioxide Detecting System Based On Laser Modulation

As technology and industrial level is higher and higher in recent years, the concentration of pollution gases in the atmosphere is increasing, including the spread of acid rain, ozone depletion, greenhouse effect and water pollution, and so on. As an important part of gas pollutants, NO2 can cause acidic rain and photochemical smog, pollute environment severely and endanger people's health. Accordingly, it is very necessary to monitor the nitrogen dioxide of atmosphere.This paper mainly studies the concentration detective system of nitrogen dioxide,designing three NO2 detecting systems consisting of spectral absorption NO2 detecting system,differential detecting system based on outer modulation technology of laser and differential detecting system based on inner modulation technology of laser. The spectral absorption detection has the merits of detecting by measure the laser transmission spectral after transmitting the gas, in order to analysis concentration of NO2. Differential detecting system based on outer modulation technology of laser is a method by using light intensity modulation technology to modulate laser, a single light source and double light path. The difference detection technology and related detection technology used for weak signal detection.Differential detecting system based on inner modulation technology of laser is a method by driving current to obtain modulation for waveforms and frequencies of output intensity.According to the spectrum characteristics of nitrogen dioxide, this paper chooses the diode laser operating at 450 nm, the experimental measurement and analysis are carried on three detection system. Spectral absorption detection system collects and analyses data by using WGD-8 grating spectrometer and computer. The differential detecting system based on outer modulation technology using the chopper modulated light source, detection signal after A/D conversion, difference amplifier, related testing and collect data through the phase-locked amplifier, finally by computer processing data. The differential detecting system based on inner modulation technology of laser is the use of FPGA platform for detecting light source modulation and related functions, then by C8051F021 single-chip microcomputer control the display module, and concentration information will be displayed. It is confirmed that therationality and feasibility of system by simulating. The experimental data and system performance are analyzed and evaluated using the least squares fitting, indication error,sensitivity, precision and repeatability.The results show that the spectral absorption detection system is easy to be jammed as a result of light source fluctuation, spectrometer work noise and spectral response. Outer modulation technology of laser could meet the detection requirements, but will be affected by the stability of the chopper, introducing the noise and reduce the system accuracy. Compared with these two methods, inner modulation technology of laser can overcome the effects of the modulation signal is not stable, not only has the concentration of low detection limit, but also has higher accuracy, sensitivity and stability.