大气HOx自由基湍流标定系统研究

HO_x(OH，HO₂)自由基是大气中重要的氧化剂, 准确测量大气HO_x自由基的浓度对研究大气光化学反应机理有着重要作用。气体扩张激光诱导荧光技术(FAGE)已广泛应用于HO_x自由基的外场观测, 准确标定是FAGE系统准确测量大气HO_x自由基的重要前提。介绍了一种可以产生准确OH和HO₂自由基浓度的便携式湍流标定系统。该系统是基于低压汞灯产生的185 nm线辐射处于湍流状态的H₂O和O₂产生一定浓度的HO_x自由基。该系统中产生的自由基浓度分布均匀, 适用于多种平台的系统标定。为了准确计算出湍流标定装置中产生HO_x自由基的浓度, 分别开展了氧气和水汽吸收截面的测量。利用高精度的腔衰荡光谱(CRDS)系统测量臭氧浓度, 并用冷镜式露点仪对温湿度计测量水汽的浓度进行修正, 提高标定系统HO_x自由基浓度计算的准确度。为了便携化湍流标定系统的外场应用, 快速获取标定系统中产生的HO_x自由基的浓度, 测量了用于探测汞灯光强的光电倍增管的灵敏度因子, 实现用汞灯光强代替标定系统中产生的臭氧浓度。考虑到HO_x自由基的活性比较高, 在湍流标定系统传输的过程中会有一定的壁碰撞损失, 通过改变汞灯和标定装置出气口之间的距离对HO_x自由基在标定系统中的壁碰撞损失进行定量测量。将搭建好的湍流标定系统应用于基于气体扩张激光诱导荧光技术HO_x自由基探测系统(FAGE-HO_x)的准确标定测试, 根据OH自由基在标定系统中的壁碰撞损失对FAGE系统中探测的OH自由基荧光数进行修正, 实验结果表明修正后的OH自由基荧光数和OH自由基浓度之间有着良好的相关性, 这说明HO_x自由基湍流标定系统具有很好的准确性, 并且体积小方便携带, 适用于外场复杂环境条件下的系统标定。

The Study of Turbulent Calibration System of HOx Radical Detection

HO_x radicals are important oxidants in the atmosphere. Accurate measurement of atmospheric HO_x radicals plays an important role to study the mechanisms of atmospheric photochemical reactions. Gas expansion laser-induced fluorescence technology (FAGE) has been widely used in the field measurement of HO_x radicals. Accurate calibration has always been an important prerequisite for the FAGE system to accurately detect atmospheric Hox radicals. A turbulent calibration system producing an accurate concentration of OH and HO₂ radicals is developed in this work. It is based on the photolysis of H₂O and O₂ radiated by 185 nm UV light produced by a low-pressure mercury lamp. HO_x radicals generated in the calibration system are uniformly distributed and suitable for system calibration on multiple platforms. The measurement of oxygen and water vapor absorption cross-sections were carried out to accurately calculate the concentration of HO_x radicals in the turbulent calibration device. The high-precision cavity ring-down spectroscopy (CRDS) system was used to measure the ozone concentration, and a chilled mirror dew point meter was used to correct the concentration of water vapor measured by humidity & temperature probe to improve the calculation of HO_x radicals concentration. In order to simplify the field application of the turbulent calibration system and quickly obtain the concentration of HO_x radicals, the sensitivity factor of the phototube used to detect the intensity of mercury lamp was detected, and the relationship between mercury light intensity and ozone concentration was measured. There will be a certain loss during the transmission process of the turbulent calibration system. By changing the distance between the mercury lamp and the gas outlet of the calibration device, the loss of HO_x radicals during the transmission process is a quantitative measurement in the turbulent calibration system. And then, the built-up turbulent calibration system is applied to the system, which is based on gas expansion laser-induced fluorescence technology (FAGE). The fluorescence signal of OH radical detected in the FAGE system is corrected according to the transmission loss of OH radical in the calibration system. The experimental result shows a good correlation between the corrected OH fluorescence signal and the concentration of OH radicals, which intimates that the turbulent calibration system has good accuracy. And the high accuracy and small size of the turbulent calibration system is more suitable for the field measurement of HO_x radicals.