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. Beijing occurred severe haze events in winter of recent years. To understand the extreme events, the air pollution case from 9 January to 15 January 2019 was selected. The PM 2.5 concentration of 34 monitoring sites which were hourly measured, the nationwide synoptic conditions at surface and 850hpa, the meteorological factors including temperature, wind speed and humidity and the vertical variation in Planetary boundary layer (PBL) during the case were analyzed. It is found that synoptic condition, meteorological factors and urban PM 2.5 (fine particles with diameter less than 2.5μm) concentration distribution changed regularly with time. Clean stage, transport stage, cumulative stage and dispersion stage during haze are defined in this study. Three overall research results are: (1) The synoptic variation is distinct. Beijing is alternately dominated by Siberian High-pressure and Western Pacific Subtropical High-pressure system in winter. The transport stage is with predominant Subtropical High-pressure which brings temperature and humidity rise. High humidity facilitates formation of secondary aerosols. The cumulative stage ends along with boosted Siberian High-pressure. (2) The severe haze is characterized by temperature inversion, low wind speed and high humidity, especially nearby surface, and experienced PM 2.5 explosive growth. The stability of PBL structure will be strengthened with decreased PBL height, which inhibits the dispersion of pollutants. (3) The PM 2.5 concentrations during different stages had different regional distribution. The southern PM 2.5 concentration is much high than northern in transport stage. During dispersion stage, urban central PM 2.5 is the high for the reason that buildings, vehicle exhaust and urban heat island effects all impedehaze disappearing.

Analysis of meteorological factors on different stages of heavy haze pollution in Beijing, 2019