Vertical observation and analysis on rapid formation and evolutionary mechanisms of a prolonged haze episode over central-eastern China

To clarify the rapid formation and evolutionary mechanisms of an extremely severe and persistent haze and fog (HF) episode that occurred in central-eastern China from Dec 20 to 25, 2015, a novel campaign was conducted and vertical profiles of wind, temperature, light extinction coefficient (LEC) and PM_2.5 concentration were used to analyze the rapid formation and evolutionary mechanisms of this HF episode. The substantial downward transportation of regional pollution from high layers and stagnant weather conditions favorable for the local pollution accumulation were the two main causes of the rapid increase in pollutant concentration. Southwest wind speeds of 4m/s between 300 and 600m and obvious downward flows were observed, whereas the southwest wind speeds were low below 300m, and strong temperature inversion with intensity of 4.5°C/100m expanded vertically to a height of 600m. Two peaks of PM_2.5 concentration were observed at 200 and 700m, corresponding to 235 and 215μg/m³, respectively. The frequent change in wind direction and wind speeds resulted in the fluctuation of PM_2.5 concentration. The turbulence within lower layers of the troposphere was enhanced by easterly and northerly winds which decreased the pollution level; however, the strength and stretching height of the winds were insufficient to fully clear the air of pollutants. The PM_2.5 concentration revealed 2-high concentration layers in the vertical direction. The maximum concentration layer was below 100m, while the second high-concentration layer was at 400m.