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

在大气中，气态和气溶胶中的硫都会影响区域和全球的化学、气候变化以及各种生物的健康。SO2的人为排放转化为H_2SO_4，H_2SO_4是酸降水的主要前驱物质。我们研制了一种利用激光诱导荧光(LIF)技术在220 nm附近测量大气SO2的高灵敏度检测系统。用三次谐波泵浦的可调谐宽带OPO(光参量振荡器)激光器的二次谐波作为荧光激发源。由于OPO激光器是固态的，不需要液体染料溶液的循环系统，所以操作起来很容易。用侧面光电倍增管探测SO2的荧光，该光电倍增管在紫外区和可见光波长区都有灵敏度。将激光波长交替调谐到峰值波长220.6 nm和底部波长220.2 nm，并利用这两个波长的差值信号提取SO2浓度。该方法对SO2有很好的选择性，避免了样品空气中SO2以外的荧光或颗粒物的干扰。所研制的SO2测定仪的灵敏度为5pptv/60 S，S/N=2。检测系统的实际性能在郊区进行了测试。将该仪器与商用的氙灯激发荧光检测仪器进行了比对。这两台仪器之间的相关系数高达70ppbv。激光诱导荧光测量值与商用仪器之间具有良好的线性关系。该仪器证明了激光诱导荧光技术测量二氧化硫的可行性和潜力。

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.