The impacts of the meteorology features on PM2.5 levels during a severe haze episode in central-east China

The most polluted urban agglomeration including 13 cities in Central-East China (112-122 degrees E, 34-42 degrees N) were selected to study the impacts of meteorology features on PM2.5 levels during the severe haze episode by using observational PM2.5 concentration, surface and balloon sounding meteorology data. The study results showed that the temporal changing of PM2.5 in the 13 cities showed well correlation at the haze beginning, maintenance, and ending period due to the similar 500 hPa circulation and surface sea level pressure pattern. The increasing of surface relative humidity (RH) and temperature preceded PM2.5 accumulation when haze began. RH usually reached up to 90-95% during the period of PM2.5 peak, suggesting the possible contribution of high humidity to extreme PM2.5 values. In contrast to the similar circulation of upper air and surface pressure pattern, the divergences of local PBL meteorology, especially their vertical structure, were very obvious, which was the major meteorology cause for the different PM2.5 levels in these cities. The temperature rise at 850 hPa layer was higher than that at 925 hPa, which was higher than that at 1000 hPa, leading to the formation of temperature inversion. This was the most important trigger factor for haze. PM2.5 maximum generally occurred within 12 h after the formation of the strongest inversion in each city. PM2.5 levels in the 13 cities strongly depend on their reverse intensity: for the cities where the inversion was strong and long lasting, its PM2.5 was often the highest. Stable inversions are more likely to form in the transitional area from the northwestern mountains to the southeastern plains because of the mountain's blocking of cold air and the warming of boundary layer by sinking airflow from the mountaintop. This is the major meteorology cause for the frequent occurrence of the extreme PM2.5 levels in middle-south plain of Hebei province. This study reminds us that local boundary layer and inversion conditions, closely related with geographical location and local topography, contributes greatly to local PM2.5 levels and should be fully considered in the emission reduction and industrial layout policy by government.