Impact of anomalous high pressure over the North Pacific on PM_2.5pollution in Beijing-Tianjin-Hebei Region

Taking the PM_2.5 heavy air pollution event that lasted for 4 days in Beijing-Tianjin-Hebei Region from March 2 to 5 in 2021 as an example, we analyze mainly the impact of anomalous high pressure over the North Pacific on the air pollution in this region by using the PM_2.5 mass concentration grid data, daily city air quality index (AQI), NCEP reanalysis data, sea surface temperature and the HYSPLIT trajectory tracking model. The main results are as follow. (1) The stability and maintenance of the anomalous warm high pressure system over the North Pacific at 500 hPa is favorable for the formation of air pollution in Beijing-Tianjin-Hebei Region. When this region is located on the west of the anomalous high pressure system and controlled by southerly winds in ground level, the air pollutants will be difficult to diffuse. (2) The anomalous high-pressure system at 850 hPa guides the warm and wet airflow from the Northwest Pacific to Beijing-Tianjin-Hebei Region and therefore increases the low-level temperature and humidity, which hinders the vertical diffusion and accelerates the hygroscopic growth of air pollutants. (3) The anomalous high pressure is positively correlated with the sea surface temperature in the central and western parts of the North Pacific and negatively correlated with the sea surface temperature in the central and eastern equatorial Pacific. This anomalous high pressure system, along with surface southerly wind, is an important indicator for air pollution in Beijing-Tianjin-Hebei Region.