Analysis on the cause of a persistent pollution episode of Yichang

A persistent heavy haze episode occurred in Yichang during 15 to16 January 2015. Based on the pollutant on centration data and meteorological observation data, using the backward trajectory model, meteorological conditions and source of the pollution during the period were analyzed. The results showed that during the study period, the diurnal variation of PM2.5 (particle matter 2.5 μm or less in aerodynamic diameter) concentration was very significant, being the highest at midnight and noon, and the lowest before sunset around 16-17 PM. The analysis of historical climate data from 1981 to 2010 indicated a large positive anomaly (10%) of relative humidity and a negative anomaly (-1 m·s^-1) of wind speed during the study period. The topographic convergence favored the accumulation of aerosols. The sounding data also showed the presence of temperature inversion layers over Yichang and high relative humidity maintained at 80%-90% below 1 km. Hourly averaged wind profile and C_n² (refractive index structure constant) showed that the formation of the haze event was accompanied with a low atmospheric boundary layer height, and closely related with light wind or calm and downdraft. The above meteorological conditions were favorable for the hygroscopic growth and accumulation of aerosols. The trajectory analysis showed that compared to weather in YiChang from January 4th to 5th, the accumulation and the round trip of pollution at local residence was caused by the South trajectory during haze period from January 15th to 16th. And six hours before reaching the region, the pollution was entering from the east to the city, which is a trumpit-shaped region opening to the east, leading to more severe and persistent pollution.