水汽吸收对基于宽带腔增强吸收光谱的NO_3自由基测量的影响

Influence of Water Vapor Absorption on NO_3 Radical Measurement Based on Broad-Band Cavity-Enhanced Absorption Spectroscopy

非相干宽带腔增强吸收光谱(IBBCEAS)技术凭借其高选择性、高灵敏度、高时空分辨率等优势而逐渐成为NO_3自由基的主要测量方法之一。然而其使用的光谱仪分辨率有限,不足以分辨水汽的精细吸收结构,导致水汽的吸收非线性,进而影响NO_3自由基浓度的准确反演。介绍了一种基于插值法获取水汽有效吸收截面的方法,并将其用于消除IBBCEAS装置中水汽吸收对NO_3自由基浓度反演的干扰。利用不同浓度的水汽吸收谱结合插值法获得了水汽的有效吸收截面,使用该有效吸收截面来反演不同浓度的水汽,反演结果与商用湿度计测量结果的线性相关系数为0.99789。在此基础上测量并拟合了不同水汽浓度下NO_3自由基和NO_2气体的吸收,在拟合残差上未发现水汽残余结构,水汽反演结果与商用湿度计测量值的线性相关系数为0.999。在30 s的积分时间内,NO_3自由基和NO_2的探测极限分别为5.8×10^(-12)和3.6×10^(-9)。将本装置应用于夜间大气中进行NO_3自由基和NO_2浓度的测量,测得NO_3自由基体积分数为18.4×10^(-12)～22.9×10^(-12),平均体积分数为20.2×10^(-12),NO_2体积分数为0.6×10^(-9)～16.0×10^(-9),平均体积分数为9.9×10^(-9)。实验结果表明:利用插值法获得的水汽的有效吸收截面能够有效消除水汽吸收对NO_3自由基浓度反演的干扰,提高NO_3自由基和NO_2气体浓度测量的准确度。

Incoherent broad-band cavity-enhanced absorption spectroscopy(IBBCEAS) is gradually becoming one of the primary methods for measuring NO3 radical with the advantages of high selectivity, high sensitivity and high spatial resolution. However, due to the limited spectral resolution of the adopted spectrometers, it is not enough to distinguish the fine absorption structures of water vapor, which results in the non-linear absorption of water vapor and thus affects the accurate retrieval of NO3 radical concentration. A method based on interpolation for obtaining the effective cross section of water vapor absorption is introduced, which is used for the elimination of the interference of water vapor absorption on the concentration retrieval of NO3 radical in the IBBCEAS device. The water vapor spectra under different water concentrations are measured to obtain the effective water vapor absorption cross section by the interpolation method. The effective water vapor absorption cross section is used to retrieve water vapors with different water concentrations. The linear correlation coefficient between the retrieval results and the data from the commercial hygrometers is 0.99789. On this basis, the absorption spectra of NO3 radical and NO2 gas with different water vapor concentrations are measured and fitted. There is no water vapor absorption structure in the residual spectra, and the linear correlation coefficient between the retrieved water concentrations and the measurement values from the commercial hygrometers is 0.999. The detection limits of NO3 radical and NO2 within an integration time of 30 s are 5.8×10-12 and 3.6×10-9, respectively. This system is applied to measure the concentrations of NO3 radical and NO2 in the atmosphere at night, the measured volume fraction of NO3 radical is 18.4×10-12-22.9×10-12 with an average volume fraction of 20.2×10-12, while the measured volume fraction of NO2 is 0.6×10-9-16.0×10-9 with an average volume fraction of 9.9×10-9. The experimental results show that the effective water vapor absorption cross section obtained by the interpolation method can be used to effectively eliminate the effect of water vapor absorption on the retrieval of NO3 absorption and to improve the accuracy of NO3 radical and NO2 gas concentration measurement.