基于MAX-DOAS的上海市区NO2对流层柱浓度研究

二氧化氮（NO₂）是大气中的主要污染物之一，在对流层和平流层大气化学中发挥关键作用，不仅参与对流层臭氧的催化形成，而且还有助于气溶胶的生成并导致酸雨等气候灾害，危害人体健康。人为源排放（工业，电厂、交通等排放）的NO₂占氮氧化物排放总量的大部分。传统的监测手段例如卫星遥感技术对对流层底部没有足够的敏感度，原位采样仪器则只能获得近地面的污染物浓度信息。近年来广泛使用的多轴差分吸收光谱技术（MAX-DOAS）不仅对近地面观测敏感，还拥有时间分辨率高，探测下限低，可以同时监测多种污染物等优点。为了实时监测上海市NO₂对流层柱浓度的变化特征，在上海市徐汇区搭设了地基MAX-DOAS仪器，进行了长期的持续观测。分析2019年6月至9月的MAX-DOAS观测数据，发现NO₂VCDs(垂直柱浓度)受交通排放影响显著，一般上午9:00左右达到峰值（1.56×1016 molec·cm-2），随光照增强浓度降低明显，午后达到最低值（1.21×1016 molec·cm-2），傍晚交通排放增强16:00以后浓度再次抬升。工作日早高峰期间的NO₂VCDs明显高于周末（高出约11.8%），而周末傍晚NO₂VCDs较工作日傍晚大幅上升。将MAX-DOAS观测结果与TORPOMI卫星观测数据对比发现，两个数据具有良好的一致性，相关性系数r为0.87。采用HYSPLIT后向轨迹模型对观测期间500 m高空气团输运后向轨迹进行聚类分析，发现上海市NO₂污染受沿海区域污染气团输送影响较大。研究表明，地基MAX-DOAS系统作为一种实时、快速、连续的大气监测手段，可以广泛应用于城市区域污染监测应用中。上海市对流层NO₂的观测研究为上海市大气污染防治提供了一定的数据支持。

Study on the Tropospheric Column Density of NO2 in Shanghai Based on MAX-DOAS

Nitrogen dioxide (NO₂) is one of the main pollutants in the atmosphere, which plays a key role in tropospheric and stratospheric atmospheric chemistry processes. It not only participates in the catalytic formation of tropospheric ozone, but also contributes to the formation of aerosols, which leads to climate disasters such as acid rain and endangers human health. NO₂ from anthropogenic sources (industrial, power plant, transportation, etc.) accounts for the majority of the total NO₂ emissions. Traditional monitoring methods, such as satellite remote sensing technology, are not sensitive enough to the bottom of the troposphere, while in-situ sampling instrument can only obtain the concentration information of pollutants near the surface. The multi-axis differential optical absorption spectroscopy (MAX-DOAS) technique, which has been widely used in recent years, is not only sensitive to near surface observation, but also has the advantages of high temporal resolution, low detection limit and simultaneous monitoring of multiple pollutants. In order to monitor the variation characteristics of the tropospheric column density of NO₂ in Shanghai in real time, a ground-based MAX-DOAS instrument was set up in Xuhui District of Shanghai for long-term continuous observation. By analyzing the MAX-DOAS measurement data from June to September 2019, it is found that NO2 VCDs generally reach the peak value (1.56×1016 molec·cm-2) at about 9:00 in the morning, and the concentration decreases significantly with the increase of illumination, and reaches the lowest value (1.21×1016 molec·cm-2) in the afternoon, and increases again after 16:00 in the evening. In addition, the NO₂ VCDs during the morning peak of weekday was significantly higher (about 11.8%) than that at the weekend, while the NO₂ VCDs in the weekend evening increased significantly compared with that in the weekday evening. Comparing the MAX-DOAS observation results with the data of TORPOMI satellite, it is found that two data sets have a good agreement, and the correlation coefficient R is 0.87. Using the HYSPLIT backward trajectory model to cluster analysis the backward trajectory of air mass transport at 500 m height during the campaign, it is found that NO₂ pollution in Shanghai is significantly influenced by the transportation of polluted air masses in coastal areas. Research shows that the ground-based MAX-DOAS system, as a real-time, rapid and continuous atmospheric monitoring method, can be widely used in urban area pollution monitoring applications. The observational research of tropospheric NO₂ in Shanghai provides some data support for the prevention and control of air pollution in Shanghai.