Laser-induced fluorescence instrument for measuring atmospheric SO2

We report on the development of a high-sensitive detection system for measuring atmospheric SO2 using a laser-induced fluorescence (LIF) technique at around 220 nm. Second harmonics of a tunable broad-band optical parametric oscillator (OPO) pumped by the third harmonic of a Nd:YAG laser is used as a fluorescence excitation source. The laser wavelength is alternatively tuned to the peak and the bottom wavelengths in the photoabsorption spectrum of SO2 at 220.6 and 220.2 nm, respectively, and the difference signal at the two wavelengths is used to extract the SO2 concentration. This procedure can give a good selectivity for SO2 and avoid interferences of fluorescent or particulate species other than SO2 in the sample air. The SO2 instrument developed has a sensitivity of 5 pptv in 60 s and S/N=2S/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 two instruments is measured up to 70 ppbv. A good linear relationship between the LIF measurements and commercial instrument measurements is obtained.