[Cause and Effect Evaluation of PM_2.5 During Three Red Alerts in Beijing from 2015 to 2016]

Beijing authorities issued three red alerts for heavy air pollution at 6 pm on December 7 and 7 am on December 18, 2015, and at 1 pm on December 15, 2016, respectively. To better understand the different causes of the three red alerts and assess the effectiveness of emergency measures dealing with heavy air pollution, the PM_2.5 concentrations, meteorological conditions, weather conditions, and air mass transmission were analyzed during the red alerts using data from environmental and meteorological monitoring stations in Beijing. All three red alerts were affected by secondary transformation, but the high PM_2.5 concentrations were mainly affected by meteorological conditions. During the first and second red alerts, the ground in Beijing was controlled by a uniform pressure field and the regional transport was mainly affected by southwestern and southern air masses. During the third red alert, the ground was under the control of a wide range of low pressure and affected by the superposition of southwestern and local air masses. During the third red alert, the PM_2.5 pollution was the most serious. Its average concentration was the highest (273.6 μg·m^-3), that is, 1.2 times and 1.3 times higher than that of the previous two alerts, respectively. The WRF-CMAQ model was used in combination with the emission reduction of each source to evaluate the effects of the emergency measures related to the third red alert. The results show that the average daily emission reduction of pollutants was 678.4 t, the average concentration of PM_2.5 decreased by 79.1 μg·m^-3, and the average reduction ratio was 26.9% after the emergency measures were implemented. The increase of coal-fires, traffic, and other sources control the emergency measures, early alert start-up time, and inter-region joint prevention and effectively mitigate the increasing PM_2.5 concentration.