Study On Atmospheric OH Radical Measurement Method By Laser Induced Fluorescence

OH radical is the most important oxidizing agent in the diurnal atmosphere.It can react with most trace gas components,such as CO,NO2,SO2,CH4,and VOCs,and plays an important role in the tropospheric oxidation reaction.The accurate measurement of the OH radical concentration has significant meaning to the research of OH radical atmospheric chemical reaction and its mechanism.Due to the low concentration(?1×106 molecules/cm3),short life(<1 s),high reactivity of OH radicals,the detection of OH radical concentration is difficult,only a few reliable monitoring technologies have been developed by some research groups.FAGE(Fluorescence Assy by Gas Expansion)technique has proved to be one of reliable OH radical detection methods,and it has some advantages like high sensitivity,good selectivity and low detection limit.In this paper,OH radical measurement method has been studied,and a new atmospheric OH radical monitoring instrument based on FAGE technology has been independently developed in our laboratory.The OH radical is calculated by detecting the resonance fluorescene excited by A-X(0,0)band.Some key topics in the OH radial measurement have been solved,including the high efficiency and low loss sampling,laser wavelength correction,time related photon counting measurement of resonance fluorescence and laboratory calibration of OH radical.And the FAGE system has been successfully used for OH radical measurement.1)The low pressure fluorescence cell has been designed and established,which can achieve the high efficiency and low loss sampling of OH radical.Combined with the free-expansion jet technique,the atmospheric OH radical is introduced into the low-pressure(350Pa)fluorescence cells with special designed nozzle(about 1mm diameter).The sampling flow field and the fluorescence excitation detection position are analyzed.The system can achieve high-speed sampling for high activity OH radical with a low collision loss.2)The laser output wavelength correction and feedback adjustment system is established to realize the stable excitation of the fluorescence signal.Because the OH radical fluorescence intensity is sensitive to the wavelength of the excitation line,a laser wavelength correction system is established,and the peak fluorescence intensity of the high concentration OH radical in the reference cell is monitored.Then the laser wavelength output is adjusted by the feedback of a Labview control program,and the wavelength is fixed on the Q1(2)excited line during the whole experiment,which has an accuracy of 0.1pm.3)An electronic gating system for photomultiplier(PMT)is designed,which can be used for the time related photon counting measurement of OH radical resonance fluorescence.The fluorescence is both excited and detected at 308nm and the liftetime of the fluorescence is quite short(200ns),and a time related detection based on gated PMT is needed.A high voltage gating circuit and a PMT divider circuit is desighed,and the gating on/off of the PMT is actualized by switching the voltage applied on the dynodes of the PMT.The circuits can obtain a volatage of 20 ns rise time and an extinction(on/off)ratio better than 105.The gating circuit can realize the gating on/off and response rescovery of the PMT in nonosenceond,which can feet the needs of the OH radical resonance fluorescence detection.4)The OH radical calibration method has been studied.A calibration system is established based on the simultaneous photolysis of the H2O and O2 by 185nm line of mecury lamp.OH radical with low concentration is stably generated in the flow tube of the calibration system,and the calibration procedure is accomplished by using a seris of OH radical concentration.5)The FAGE system has been tested and optimized,and OH measurements in laboratory and field environment have been carried out.The sensitivity and the detection limit of the FAGE is system is analysed,and the detection limit using the gated PMT and MCP as detector is calculated to be 9.4×105 molecules/cm3 and 1.6×105 molecules/cm3(S/N=2,integration time=60s),respectively.A molibe platform for field measurement is built,and the preliminary filed measurent is carried out.The research in this paper shows,the FAGE technique can be used for atmospheric OH radical measurement.FAGE system has many advantages,like high sensitivity and good selectivity,which can meet the needs of the real time OH radical measurement in our country,and also provide an important means for the monitoring OH radicals.