Seasonal variations in haze and its dominant meteorological factors in Anhui Province

Based on the ground-based observations from 63 national stations from 1980 to 2010 in Anhui province, the seasonal variations in haze and its closely related meteorological factors (i.e., relative humidity and wind speed) were analyzed . Results show that: ① before 1995, the largest frequency for haze appeared in winter, followed by spring, autumn and summer; after that, the seasonal variations in haze frequency became less obvious. ②During the recent 30-year period, the haze frequencies in summer and autumn increased significantly, especially after 1995, while slight increases were observed both in spring and winter. ③When haze occurred, the wind directions in four seasons all exhibited features of high frequencies of northeasterly wind and calm wind, and low frequency of Southerly wind. The wind speed in summer and autumn decreased noticeably under haze-weather conditions, and ~48.5% (54.5%) of the haze in summer (autumn) occurred when the wind speed (14h) declined to below 2m/s. ④Haze formations are more closely related to high relative humidity (>70%) conditions in most regions of Anhui province. However, in more polluted regions ( e.g., Hefei and Bengbu), ~ 71.2% and 68.9% of haze events are associated with low relative humidity conditions (<70%). ⑤The frequencies of haze under different relative humidity conditions increased significantly in summer and autumn, with a confidence level of over 95%. The increases in the days of small wind speed (≤2 m/s) are one possible factor for the increases in haze frequencies in summer and autumn.