腔衰荡光谱方法测量大气中痕量NH_3的浓度

Measurement of Trace NH_3 Concentration in Atmosphere by Cavity Ring-Down Spectroscopy

NH_3是大气二次细颗粒物的主要前驱物之一,NH_3浓度的准确测量对于大气环境监测和保护具有重要意义。近红外波段激光器的成本较低,但采用其测量NH_3时,普遍存在受环境中H_2O、CO_2气体干扰以及吸收光程较短等问题。为克服环境中H_2O、CO_2干扰气体的影响,筛选出中心波数为6521.97 cm^(-1)的吸收谱线,利用该谱线对大气环境中痕量NH_3的浓度进行测量。该谱线不受环境中CO_2吸收的影响,且在低压条件下与H_2O吸收谱线的重叠范围较小,通过多峰拟合可以准确提取出NH_3的光谱吸收率。基于分布反馈式激光器搭建了一套腔衰荡吸收光谱测量装置,在该装置中,衰荡光腔由一对反射率高达99.996%的高反镜构成,空腔衰荡时间约96μs,有效吸收光程可达1.6×10~4 m。利用该装置对大气环境中痕量NH_3的浓度进行测量,结果表明:该测量系统的探测灵敏度可以达到3.9×10^(-10)。

Ammonia （NH3） is one of the main precursors of secondary fine particulate matters in the atmosphere. The accurate measurement of NH3 concentration plays an important role in the monitoring and protection of atmospheric environment. Although the cost of a near-infrared laser is relatively low, there always exist some problems such as the H2 O, CO2 gas interference in the environment and the limited absorption path when it is used for the NH3 concentration measurement. In order to reduce the H2O and CO2 interference in the environment, we screen out the absorption line with a central wavenumber of 6521.97 cm-1 , which is used for the measurement of the trace NH3 concentration in the atmospheric environment. Moreover, this spectral line is not affected by the CO2 absorption in the environment and its overlap range with the H2 O absorption line is small under the condition of low pressures. The spectral absorptivity of NH3 can be accurately extracted through multi-peak fitting. A measurement device based on cavity ring-down spectroscopy is built with a distributed feedback laser, in which the ring-down cavity is composed of a pair of high reflectivity mirrors with a reflectivity of up to 99. 996%, the empty ring-down time is about 96 μs, and the effective absorption path is about 1.6 × 10^4 m. The trace NH3 concentration in the atmospheric environment is measured with this device, and the results show that the detection sensitivity of this measurement system can approach 3.9 × 10^-10 .