基于差分光学吸收光谱方法的OH自由基定标系统研究

介绍了一种基于差分光学吸收光谱(DOAS)方法的OH自由基定标系统, 该系统可产生一定浓度的OH自由基并同时进行精确测量. 系统采用紫外灯185 nm光线分解水汽产生OH自由基, 利用500 W氙灯准直光作为光源; 使用基长1.25 m、反射次数60次、总光程75.0 m的多次反射池来增加OH自由基的吸收光程; 以超高分辨率中阶梯光栅光谱仪(最高分辨率3.3 pm)作为光谱采集系统对光谱信号进行采集, 采用DOAS测量方法获得OH自由基的浓度. 通过改变腔内水汽的浓度, 系统准确测量了5×10⁸-1.8×10¹⁰ molecules/cm³浓度范围的OH自由基. 分析了OH自由基测量过程中受到的吸收截面偏差、气压等因素影响, 得到系统总测量误差小于7.3%. 在实验的浓度范围内, 系统可用于大气OH自由基气体扩张激光诱导荧光测量技术的定标.

Calibration system for OH radicals based on differential optical absorption spectroscopy

In the present paper, we describe a calibration system for OH radicals based on differential optical absorption spectroscopy (DOAS). In the system OH radicals can be produced by photolysis of H₂O which is irradiated by the 185 nm light in a cavity. The produced OH radicals with a certain concentration can be detected exactly. The system consists of a xenon lamp as light source in which the light has been collimated, a 1.25 m multiple-reflection cell in which the light can reflect 60 times to achieve 75.0 m whole path-length, and a double pass high resolution echelle spectrometer that is suitable for the measurement of OH radicals (best resolution: 3.3 pm). Utilizing the system the measurement spectra and lamp spectra can be obtained for OH concentration retrieval. OH concentration can be calculated by DOAS retrieval and during the DOAS retrieval the reference absorption cross section is obtained by applying the Voigt broadening method to the absorption lines. By changing water vapor concentration, the system accurately detects OH concentration ranging from 5×10⁸ molecules/cm³ to 1.8×10¹⁰ molecules/cm³. In the concentration range, OH concentration fluctuation is very small. For example, when the volume ratio between water vapor and pure N₂ reaches 0.3 L:24.7 L, the fluctuation is just ± 4%. Taking into account the effects of absorption cross section, gas pressure in the cavity and other factors, the total systematic error of the instrument is less than 7.3%. According to the results in the paper, the system can be used for the fluorescence assay by gas expansion technology calibration in field experiments.