宽带腔增强吸收光谱法测量大气NO2定标方法研究

非相干宽带腔增强吸收光谱法定量探测大气痕量气体浓度需要准确定标。以定量探测大气NO₂为目的，建立了基于蓝色发光二极管光源的非相干宽带腔增强吸收光谱测量系统，研究了(1)仅使用浓度已知的NO₂吸收光谱、(2)同时使用浓度已知的NO₂和纯氧气中氧气二聚体O₂-O₂吸收光谱、(3)利用纯氮气和纯氦气的瑞利散射消光差异等三种方法，分别获取非相干宽带腔增强吸收光谱在430～490 nm波段的镜片反射率定标曲线。三种方法得到的镜片反射率最大值对应波长均约为460 nm，但这些最大值存在一定差异，分别为0.999 25，0.999 33和0.999 37。利用NO₂样气吸收测量对比了三种定标方法，发现方法(1)与另外两种方法的测量结果不一致性分别约为14%和19%，而后两种方法所测结果的不一致性仅为4%。测量结果表明，NO₂标准气体浓度的不准确性以及壁损耗等因素恶化了方法(1)的定标精度，应尽量避免使用该定标方法。通过对实际大气中NO₂和O₂-O₂在440～485 nm波段内的同时测量，进一步验证了非相干宽带腔增强吸收光谱法的高灵敏度以及所用标定方法的有效性。

Calibration Method of Broadband Cavity Enhanced Absorption Spectroscopy for Measuring Atmospheric NO2

Accurate calibration of the spectrometer based on incoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS) is very essential for quantitative measurements of trace gases in the atmosphere. An incoherent broadband cavity enhanced absorption spectrometer using a blue light emitting diode as light source is developed for quantitative detection of atmospheric NO₂. This paper describes three methods for calibrating the mirror reflectivity of the spectrometer in the spectral region of 430～490 nm, which are: (1) only using known absorption of standard NO₂; (2) using known absorptions of standard NO₂ and oxygen collisional pair in pure oxygen; (3) using Rayleigh extinction coefficient of pure nitrogen and helium. The wavelengths corresponding to maximum reflectivity calibrated with these methods are all about 460 nm, whereas different maximum reflectivity and respectively 0.999 25, 0.999 33 and 0.999 37. NO₂ samples with various mixing ratios are measured with the IBBCEAS spectrometer in order to compare these three methods. Disagrees of about 14% and 19% between method (1) and other two methods are respectively found, whereas only about 4% disagree between method (2) and (3). The measuring results show that the concentration uncertainty of NO₂ sample and inner wall loss maybe deteriorate the calibration accuracy of method (1) and hence this approach is not recommended. High sensitivity and calibration methods of the IBBCEAS spectrometer are further demonstrated by simultaneously measuring the absorption of atmospheric NO₂ and oxygen collisional pair in the atmosphere in the spectral region of 440～485 nm.