嘉兴夏季臭氧污染的近地层垂直变化特征

基于2018年浙江省嘉兴市14个环境国控站点的O₃历史资料与气象要素，研究O₃与气象要素的关系，并结合差分吸收臭氧雷达的垂直臭氧探测资料，分析近地层O₃廓线变化特征.结果表明，嘉兴地区发生高浓度O₃污染的关键气象要素为24~36℃的大气温度和36%~77%的相对湿度，24℃以上的大气温度与77%以下的相对湿度可作为启动预警O₃污染的气象指标.差分吸收臭氧雷达监测结果显示，无论O₃超标天与清洁天，在垂直方向上其浓度随高度先升后降，在600~800m范围出现峰值；O₃污染时段，在中午到午后低空形成持续向下的O₃输送带，这种低空O₃与地表O₃的叠加机制加重地表O₃污染程度，导致地表O₃超标与低空高浓度相伴出现；其廓线日变化规律表现出800m以下浓度在夜间和凌晨梯度不显著，日出后近地层O₃随时间快速增加，中午和午后持续高值，傍晚随时间逐渐下降的特征.后向轨迹分析表明，10，500，1000m高度层的气流后向轨迹聚类有相似性，500m处沿闽浙海岸线的轨迹簇对应O₃较10m处来自海上的轨迹簇高，这与500m处前体物输送堆积和紫外线辐射增强有关.污染过程近地层气流来向紧贴地面，但中高层有明显下沉气流使得O₃前体物在500m附近堆积，是造成2个典型污染过程中垂直方向上O₃高值出现在500m左右的原因之一.

Characterizing the near-surface vertical variations of summertime O3 in Jiaxing

Based on O₃ surface data in 2018 from 14 monitor stations and meteorological factors of Jiaxing city, Zhejiang Province, this study analyzed the relationship between O₃ and meteorological factors. The vertical profile of O₃ was characterized by a differential absorption O₃ lidar. Temperature and relative humidity were the dominating factors for high concentration O₃ pollution, with the range of 24~36℃ and 36%~77%, respectively. The temperature above 24℃ and the relative humidity below 77% can be used as the meteorological threshold for early warning of O₃ pollution. The O₃ lidar results showed that the vertical O₃profiles were similar between episodic and clean days, which exhibited an O₃ peak between 600~800m, while the O₃ concentrations decreased beyond this range. During the O₃ episodic period, O₃ was transported downward to the ground~level from noon to afternoon. This superposition of near-surface ozone and ground-level ozone aggravates ground-level O₃ pollution, leading to an exceedance of air quality standard for O₃ at both near-surface and ground-level. The O₃ below 800m was relatively constant at night and in the early morning. Ground-level O₃ increased rapidly after sunrise until reaching a peak in the afternoon. O₃ decreased gradually in the later afternoon. The backward trajectories clustering analysis showed that there was a similarity in the clusters of air masses at 10m, 500m, and 1000m. Air masses that traveled along the coastline of Fujian and Zhejiang provinces at 500m exhibited higher O₃ than the air masses from the ocean at 10m. That was related to the transport accumulation of the precursors and the stronger ultraviolet radiation at 500m. During the episodic events, the near-surface air masses exhibited an unchanged height. The downdraft in the middle and upper boundary layer led to the accumulation of O₃ precursor near 500m. That was one of the reasons for the high O₃ near 500m observed in two typical pollution episodes.