京津冀典型城市一次重污染过程特征及边界层结构变化对其影响

为探究典型重污染过程的污染特征与大气边界层结构演变规律，基于PM_2.5采样数据、气象观测数据及WRF-Chem模式，以北京市和石家庄市2016年12月27日—2017年1月10日一次重污染过程为研究对象，对气象要素、PM_2.5化学组分、天气背景场、边界层结构演变特征，以及大气边界层结构变化对ρ(PM_2.5)及其主要化学组分的影响进行分析.结果表明:①研究期间，北京市和石家庄市ρ(PM_2.5)分别为(165.63±110.89)(247.67±95.22)μg/m3，石家庄市污染程度高于北京市；高空纬向环流和地面弱高压控制的天气背景场，低于1.75 m/s的风速以及超过75%的相对湿度是造成北京市与石家庄市重污染的不利气象条件.②重污染时段北京市与石家庄市SNA(SO₄2-、NO₃-、NH₄+三者的统称)与碳质组分(OC、EC)占比之和超过76%，是PM_2.5中的两大主要组分；重污染时段ρ(SNA)占比明显上升，北京市与石家庄市ρ(SNA)占比由非重污染时段的42.23%、45.93%分别升至重污染时段的58.87%、59.62%；北京市与石家庄市ρ(OC)/ρ(EC)分别为5.13、3.51，表明在重污染时段两城市存在明显的二次有机气溶胶污染.③WRF-Chem模式模拟结果表明，PM_2.5污染严重时北京市与石家庄市在300~500 m处均出现明显的逆温，垂直风场主要表现为低层偏南风顺时针向上切变为偏西风，切变高度在400~1 000 m，逆温层结与明显垂直风切变的边界层特征共同抑制了污染物的湍流与扩散.④北京市与石家庄市重污染时段的PBLH(Planetary Boundary Layer Height，大气边界层高度)日均值与非重污染时段相比分别下降了202、128 m，PBLH每下降100 m，北京市与石家庄市ρ(PM_2.5)分别上升18.81、29.85 μg/m3，PBLH下降是导致两城市ρ(PM_2.5)快速上升的重要因素.北京市与石家庄市的PBLH与PM_2.5组分质量浓度之间的相关性不同，北京市PBLH与ρ(SNA)的相关性高于与碳质组分质量浓度的相关性，石家庄市PBLH与ρ(EC)相关性最高，表明此次重污染过程中北京市PM_2.5污染特征以二次形成为主，而石家庄市以一次排放为主.研究显示，北京市与石家庄市此次重污染过程与大气边界层结构变化密切相关. 

Characteristics of Heavy Air Pollution Process and Influence of Structure Variation of Planetary Boundary Layer in Typical Cities of Beijing-Tianjin-Hebei Region

In order to study the characteristics of typical heavy pollution and structure variation of planetary boundary layer, taking a typical heavy pollution process in Beijing and Shijiazhuang from December 27th, 2016 to January 10th, 2017 as an example, the meteorological conditions, PM_2.5 chemical components, synoptic background, characteristics of planetary boundary layer vertical structure and its impact on ρ(PM_2.5) and components were evaluated based on PM_2.5 sampling data, meteorological observation data and WRF-Chem model. The results indicated:(1) In the study period, the average ρ (PM_2.5) of Shijiazhuang ((247.67±95.22)μg/m3) was higher than that of Beijing ((165.63±110.89)μg/m3). Synoptic situation controlled by zonal circulation at high altitude and weak high pressure of ground, wind speed below 1.75 m/s and relative humidity above 75% were unfavorable meteorological conditions for heavy pollution in Beijing and Shijiazhuang. (2) The total proportion of SNA (SNA including SO₄2-, NO₃- and NH₄+) and carbon components (OC, EC) exceeded 76%, which were the two main components of PM_2.5 during the heavy pollution period. The proportion of ρ(SNA) increased from 42.23% and 45.93% in other pollution level to 58.87% and 59.62% during the heavy pollution period in Beijing and Shijiazhuang, respectively. The concentration ratios of OC and EC were 5.13 and 3.51, respectively, indicating that the obvious secondary organic aerosols pollution existed in Beijing and Shijiazhuang during heavy pollution period. (3) The WRF-Chem Model simulation results showed that during the period with severe PM_2.5 pollution in Beijing and Shijiazhuang, obvious temperature inversion could be found at 300-500 m, and the characteristic of wind shear showed variation from south to west in clockwise direction and from low altitude to high altitude, with the shear height of 400-1000 m. The turbulence and diffusion of pollutants were restrained by the factors mentioned above. (4) The daily average Planetary Boundary Layer Height (PBLH) during the periods with heavy pollution decreased by 202 and 128 m compared with that during the period with other pollution level in Beijing and Shijiazhuang, respectively. For every 100 m decrease in PBLH, the ρ(PM_2.5) increased by 18.81 and 29.85 μg/m3 in Beijing and Shijiazhuang, respectively. This indicated that the decrease of PBLH was an important factor leading to the rapid increase of ρ(PM_2.5) in the two cities. The correlation between PBLH and different PM_2.5 components showed different characteristics in Beijing and Shijiazhuang. The correlation between PBLH and ρ(SNA) in Beijing was better than that between PBLH and the mass concentration of carbon components. The correlation between PBLH and ρ(EC) in Shijiazhuang was better than those between PBLH and the mass concentration of other components. It implies that the PM_2.5 pollution in Beijing was mainly secondary, while in Shijiazhuang was mainly primary. The results show that the heavy pollution process in Beijing and Shijiazhuang is closely related to the structural variation of the planetary boundary layer.