Diagnose of Air Quality by Laser Sensing Technique

Abstract

To know the air quality of the urban atmosphere, precise measurement of key species is essentially important such as NOx and VOCs because they are very reactive and some of them are toxic. NOx indicates combination of NO and NO2 molecules. Exhaust of combustion mainly contains NO but it is easily oxidized by either ozone or peroxyl radicals in the atmosphere into NO2. Resultant NO2 is again converted to NO in the photolysis of solar UV. Thus, NO and NO2 achieve chemical equilibrium within a couple of minutes in daytime atmosphere. Selective measurement of NO2 is suffered from chemical interference when conversion technique to NO is applied. Therefore, direct measurement of NO2 is needed such as optical absorption and laser induced fluorescence (LIF) techniques. Although optical absorption technique does not need calibration process but sensitivity is not enough for lower concentration. Thus, we applied LIF technique for detecting NOx family molecules such as NO, NO2, NO3, and N2O5. We present herein instrumentation for both NO2 and NO3 sensors using pulsed laser excitation. We used an SHG of DPSS Nd:YVO4 laser and tunable dye laser pumped by it to measure N2 and NO3, respectively. The detection limits of radical sensors are 2 ppt for NO2 and 7 ppt for NO3 under some practical conditions. Using this instrument we measured N2O5 in sub-urban air of Tokyo. Using chemical model analysis we assessed heterogeneous loss processes of N2O5 on the aerosol surface and discussed chemical fate of NOx emitted in urban atmosphere. Photochemical oxidant formation is becoming great concern as revival issue from 1970's. To investigate photochemical ozone formation process in the atmosphere, HOx radicals, OH, HO2, and RO2 radicals, investigation is important as well as NOx measurements and great efforts are paid for detection of such radicals using either chemical ionization mass spectrometry or spectroscopic methods such as LIF. In addition to the information of HOx radicals, reactive VOCs measurements are also necessary for evaluate air quality. However, kind of VOCs existing in the urban atmosphere is believed to be more than 500. Individual measurements of chemical species are not practical to cover such a large number of molecules. Thus, we proposed to measure OH reactivity in the atmosphere and assess the air quality using OH reactivity as an index of pollution or photochemical activity. We present here the developed technique how to measure OH reactivity in the real atmosphere. We conducted a couple of field expedition using this technique and show some of the results.