A portable instrument for measurement of atmospheric Ox and NO2 based on cavity ring-down spectroscopy

Abstract

Atmospheric O_x (nitrogen dioxide (NO₂) + ozone (O₃)) can better reflect the local and regional change characteristics of oxidants compared to O₃ alone, so obtaining O_x accurately and rapidly is the basis for evaluating the O₃ production rate. Furthermore, O_x has proved to be a more representative indicator and can serve as a reflection of pollution prevention efficacy. A portable instrument for measuring atmospheric O_x and NO₂ based on cavity ring-down spectroscopy (O_x/NO₂-CRDS) was developed in this work. The NO₂ concentration is accurately measured according to its absorption characteristic at 407.86 nm. Ambient O₃ is converted into NO₂ by chemical titration of high concentrations of nitrogen oxide (NO), and the O₃ conversion efficiencies obtained are nearly 99%. The detection limit of the O_x/NO₂-CRDS system for O_x is 0.024 ppbv (0.1 s), and the overall uncertainty of the instrument is ± 6%. Moreover, the Kalman filtering technique was applied to improve the measurement accuracy of O_x/NO₂-CRDS. The system was applied in a comprehensive field observation campaign at Hefei Science Island from 26 to 30 September 2022, and the time concentration series and change characteristics of O_x and NO₂ were obtained for five days. The measured O_x concentrations were compared with those of two commercial instruments, and the consistency was good (R² = 0.98), indicating that this system can be deployed to accurately and rapidly obtain the concentrations of atmospheric O_x and NO₂. It will be a useful tool for assessing the atmospheric oxidation capacity and controlling O₃ pollution.

摘要

大气O_x (二氧化氮(NO₂)+臭氧(O₃))相比于O₃能够更好的反应区域氧化剂的变化特征, O_x也是反应大气污染防治效果的一个关键指标. 本研究基于腔衰荡光谱技术研发了一套大气O_x和NO₂同步测量系统 (O_x/NO₂-CRDS). NO₂浓度是利用其在407.86 nm处的特征吸收获取, 环境大气的O₃通过高浓度的NO被转化为NO₂进行间接测量, O₃转化效率高于99%, O_x/NO₂-CRDS的系统探测限为0.024 ppbv (0.1 s), 系统总不确定度为± 6%. 该O_x/NO₂-CRDS系统成功应用于2022年9月26日–30日的合肥市科学岛综合外场观测中, 获取了连续5天的NO₂和O_x的时间浓度序列和变化特征, 并将O_x的测量结果与商业化的设备进行了对比验证, 二者具有较好的一致性 (R² = 0.98), 表明O_x/NO₂-CRDS能够被应用于大气O_x和NO₂的高灵敏探测. 未来该系统也将会变成评估大气氧化性以及控制臭氧污染的重要工具.