| 北京2019年冬季一次典型霾污染特征与成因分析 |
| 摘要点击 3413 全文点击 870 投稿时间：2020-08-26 修订日期：2020-10-23 |
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| 中文关键词 霾污染特征 污染成因 气流输送 边界层结构 湍流特征 |
| 英文关键词 haze pollution characteristics pollution causes air transport boundary layer structure turbulence characteristics |
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| 中文摘要 |
| 为研究北京冬季霾污染过程的污染特征和成因，以北京2019年12月一次典型PM2.5污染过程为分析对象，利用气溶胶垂直探测资料、边界层气象场和近地湍流资料，对霾不同污染阶段的特征与边界层理化特性的演变进行综合分析.结果表明：①观测期间北京共经历两次污染生消，历时5 d，PM2.5小时浓度最高220 μg·m-3，超过重度污染标准的时次占比53%.②高空稳定形势和地面均压场配置下，来自北京西南城市群地表的气溶胶和水汽传输（占比48%），在近地层偏南弱风（风速1~2 m·s-1），贴地强逆温［0.8 K·（100 m）-1］和高湿（相对湿度80%以上）等不利扩散的气象条件下不断吸湿增长，加之本地污染排放，成为霾日维持的主要原因.且随污染加重，气溶胶球形特征逐步显著（退偏比从0.05降至0.02）.③各湍流统计量（湍流强度、摩擦速度和湍流动能）在重污染发生与结束前提前出现规律性异常突降（小时波动率77%）与激增（超过一个量级的峰值）现象，表明湍流统计量可作为重污染过程发生和结束的预报指标，其中湍流强度响应提前的时长与其峰值后的持续湍流强弱有关.污染累积阶段摩擦速度（0.04~0.21 m·s-1）、湍流强度（均值0.678）和湍流动能（均值0.643 m2·s-2）等均维持在较低水平，底层大气混合扩散能力较差，对污染持续累积起重要作用.另外晴天和霾天感热通量均由地面向大气输送，且呈明显的日单峰变化特征，霾天感热通量（20W·m-2）较晴天小（60W·m-2）；潜热通量则全程在0值附近. |
| 英文摘要 |
| In order to study the pollution characteristics and causes of winter haze pollution in Beijing, a typical PM2.5 pollution process in Beijing in December 2019 was used as the analysis object using aerosol vertical detection data, boundary layer meteorological field and near-ground turbulence data, and the difference in haze. The characteristics of the pollution stage and the evolution of the physical and chemical characteristics of the boundary layer were comprehensively analyzed. The results showed that ① the pollution process in Beijing during the observation period lasted 5 d and experienced two generations and eliminations. The maximum hourly PM2.5 concentration was 220 μg·m-3 and the time exceeding the severe pollution standard was 64 h, thereby accounting for 53% of the total time. ② The aerosol optical properties and meteorological field observation data showed that the pollution originated from the regional transmission of aerosols and water vapor on the surface of the southwest urban agglomeration in Beijing, which accounted for 48% of the total pollution transmission, followed by a stable high-altitude situation and ground pressure field configuration. The near-surface layer maintained weak southerly winds (wind speed: 1-2 m·s-1), a strong inversion temperature close to the ground ［0.8 K·(100 m)-1］, high humidity (relative humidity above 80%), and other unfavorable diffusion weather conditions, thereby promoting the accumulation of pollutants and the conversion of moisture absorption. Superimposing local pollution emissions were the main reasons for the maintenance of haze days. In addition, the near-ground extinction coefficient increased from 0.070 km-1 to 5.954 km-1, and the depolarization ratio decreased from 0.05 to 0.02 during the two pollution generation and disappearance processes, thereby indicating that the spherical characteristics of aerosols gradually became significant as the pollution increased. ③ The analysis of the turbulence observation data showed that the characteristic quantities of different pollution stages were significantly different and negatively correlated with the pollutant concentration. Before the occurrence of heavy pollution, the turbulence statistics (turbulence intensity, friction velocity, and turbulent kinetic energy) suddenly decreased from high values (the hourly variation rate was 77%, thereby far exceeding the daily fluctuation of 33%), and the turbulence intensity responded first. During the pollution accumulation stage, the friction velocity (0.04-0.21 m·s-1), turbulence intensity (average: 0.678 m2·s-2), and turbulence energy (average: 0.643 m2·s-2) were maintained at a low level, and the bottom atmosphere had a poor mixing and diffusion ability, which is important for continuous pollution accumulation. Four hours before the end of the pollution event, the turbulence intensity again showed a sharp increase (increment of more than one order of magnitude); thus, the turbulence intensity can be used as a predictive indicator of the occurrence and end of a heavy pollution event, and the response time is the same as the continuous turbulence intensity after the turbulence peak. In addition, the sensible heat fluxes on sunny days and haze days were both transported from the ground to the atmosphere, and showed clear daily single-peak changes. The sensible heat flux on haze days (20 W·m-2) was smaller than that on sunny days (60 W·m-2). The latent heat flux was approximately 0 W·m-2 in the whole process. ④ There was a feedback effect between the meteorological conditions of the pollution layer and the boundary layer. On the one hand, unfavorable diffusion of the meteorological conditions was conducive to the accumulation of pollution. On the other hand, the aerosol layer and water vapor cooling effect that accumulated near the ground were worse than the night cooling radiation on the inversion layer The contribution was greater, thereby further inhibiting the development of turbulent motion and ultimately resulting in increased pollution. |
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