Development of high-sensitive instrument for the detection of acid-rain precursors in the atmosphere

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In the atmosphere, sulfur in both gaseous and aerosol forms impacts regional and global chemistry, climate change, as well as the health of various living organism. The anthropogenic emissions of SO_2 are converted to H_2SO_4 which is the dominant precursor responsible for acid precipitation. We have developed a high-sensitive detection system for measuring atmospheric SO_2 using a laser-induced fluorescence (LIF) technique around 220 nm. Second harmonics of a tunable broad-band OPO (optical parametric oscillator) laser pumped by the third harmonic of a Nd : YAG laser is used as a fluorescence excitation source. Since the OPO laser is solid-state and does not need a circulation system of liquid dye solution, it is easy to operate. For the detection of the fluorescence from SO_2, a side-on photomultipliers is used, which has sensitivity in the ultraviolet and visible wavelength region. The laser wavelength is alternatively tuned to the peak and the bottom wavelengths at 220.6 and 220.2 nm, respectively, and the difference signal at the two wavelengths is used to extract the SO_2 concentration. This procedure can give a good selectivity for SO_2 and avoid interferences of fluorescent or particulate species other than SO_2 in the sample air. The SO_2 instrument developed has a sensitivity of 5 pptv in 60 s and S/N=2. The practical performance of the detection system is tested in the suburban area. The inter-comparisons between the LIF instrument and a commercial instrument using Xe flash lamp excitation for the fluorescence detection have been performed. The correlation between the two instruments is measured up to 70 ppbv. The good linear relationship between the LIF measurements and the commercial instrument is obtained. The instrument proved the feasibility and potential of the laser induced fluorescence technique for measuring SO_2.