• 关键词：霾和沙尘复合污染  垂直加密观测  水平和垂直输送  大气稳定度

• 基金项目：北京市自然科学基金重点项目（No.8171022）；国家自然科学基金（No.41575010）

• 李青春
• 北京城市气象研究院, 北京 100089

• 王耀庭
• 北京城市气象研究院, 北京 100089

• 李伟
• 北京市气象信息中心, 北京 100089

• 郑祚芳
• 北京城市气象研究院, 北京 100089

• 程志刚
• 北京城市气象研究院, 北京 100089

• 李梓铭
• 京津冀环境气象预报预警中心, 北京 100089

• 摘要：为了弄清蒙古气旋外围出现的霾和沙尘复合污染特征及其形成的关键气象条件，本研究利用多种遥感设备（增强型云高仪、风廓线雷达和微波辐射计等）垂直加密观测数据，结合大气主要污染物（PM₁₀、PM_2.5、SO₂、NO₂）监测数据、加密自动气象站观测数据，以及常规地面和高空气象观测数据、NCEP再分析数据等，分析发生在北京春季的2次霾和沙尘重污染过程.结果表明，2017年5月4—5日为一次PM₁₀和PM_2.5混合污染过程，与上游地区强烈发展的蒙古气旋后部风沙区的输送有关.上游地区因受中-低空西来槽影响上升气流加强，使沙尘细颗粒物（粒径≤10 μm）悬浮于空中，由中-低空偏西风输送至下游地区，被北京及附近的弱下沉气流带至地面造成严重的PM₁₀、PM_2.5混合污染.其中，地面偏西风对上游地区的PM₁₀、PM_2.5的水平输送作用明显；2018年3月27—28日凌晨是受蒙古气旋底部低压区辐合作用和偏南气流输送作用形成的积累型霾（PM_2.5）污染.28日凌晨2：00开始蒙古气旋后部沙尘区随东-西向冷高压南压而向南扩散.随后冷高压不断东移形成回流偏东风，偏东风使北京及西北部地区的低层大气产生辐合上升运动，导致本地尘土扬起，造成PM_2.5重污染和PM₁₀极严重污染；浮尘天气引发的大气污染具有突发性特征，且持续时间较长.边界层高度低、低层大气存在逆温层（或等温层）并长时间维持是霾和沙尘复合污染形成和持续的重要条件.霾和沙尘复合重污染的形成是人为污染物、沙尘细颗粒物水平和垂直输送，以及大气层结稳定共同作用的结果.

• Abstract：In order to understand the pollution characteristics and crucial formation conditions of the combined pollution processes of haze and sand-dust in the periphery of Mongolian cyclone, comprehensive observational and reanalysis data are used to analyze the two pollution processes in Beijing during spring. The data include the intensive vertical observational data by multiple remote sensing devices (enhanced laser ceilometer, wind profiler radar, and microwave radiometer), the air pollutants data (PM₁₀, PM_2.5, SO₂, NO₂), high-resolution automatic weather stations observations data on fine-scale, conventional upper and surface meteorological observations data, and also NCEP reanalysis data. It is concluded that the heavy pollution process occurred on May 4-5, 2017 is featured by mixed PM₁₀ and PM_2.5, that was related to the dust transport from the wind-sand area in the rear of strongly developed Mongolian cyclone in upstream area. The strong updraft associated with westerly trough caused the fine particles (particles size ≤ 10 μm) of sand-dust suspended in the air, which were later transported to the downstream area by middle-lower level westerly airflow, and finally reached the surface by low level weak subsidence airflow over Beijing and surrounding areas. During this period, the westerly wind on the surface has significant effect on the horizontal transport of PM₁₀ and PM_2.5. From the evening of March 27 to the early morning of March 28, 2018, the accumulative PM_2.5 (haze) pollution was caused by the convergence at the bottom low-pressure area of Mongolian cyclone and the transport of the southerly airflow. Starting from the early morning of March 28, 2018, the high concentration plume of PM₁₀ diffused southward with the cold high-pressure system of the Mongolian cyclone rear. Then, the cold high pressure system kept moving eastward, and lead to the formation of air inverse flow (easterly wind). Finally, the easterly wind caused convergence and updraft motion in the low-level atmosphere in Beijing and northwest Hebei, and lifted local fugitive dust into the air. These are the complex causes of heavy pollution of PM_2.5 and extremely serious pollution of PM₁₀. In the serious combined pollution process featured by sudden-eruption and long-lasting, boundary layer height is relatively lower and inversion layer(or isothermal layer)existed and maintained in the low-level for a long time. Therefore, the serious combined pollution process are caused by the combination of anthropogenic emissions, horizontal and vertical transport of sand-dust fine particles, and atmospheric stratification stability.

• Key words：haze and sand-dust combined pollution  intensive vertical observation  horizontal and vertical transport  atmospheric stability

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