The spatial and yearly trends of haze during 1980-2013 in Anhui Province and its possible reasons were analyzed based on daily observations from ground-level stations.The effects of urbanization, industrialization and climate change on haze were discussed through an analysis of trends in haze frequency at urban(town) sites and comparison with provincial coal consumption, SO2
emission, tropospheric NO2
column content, vehicle usage and East Asia monsoon index.The main conclusions were as follows:(1) The number of annual haze days averaged over all sites increased evidently with large fluctuations.The zones with frequent haze were different in different periods.During the 1980s, the provincial annual average haze days was 5.5 d, with scattered high values from the zone near the Yangtze River to the Huaihe River.During the 1990s, the provincial annual average haze days was 8.5 d, with high values at some county sites and Hefei, the capital city.During the 2000s, the provincial annual average haze days was 8.7 d, with three evident high-value zones in the central area between the Yangtze River and Huaihe River, the central area along the Huaihe River, and the central to the eastern area along the Yangtze River, respectively.(2) Based on geographical locations, the province was divided into six sub-regions and the annual haze days averaged over all sites in each sub-region showed different trends.For example, it varied gently in southern Anhui, increased rapidly since 2000 in the region along the Huaihe River, and first increased and then decreased in the region north to the Huaihe River and the region along the Yangtze River.(3) Based on site locations, all sites were divided into two groups:urban sites and town sites.The number of annual haze days increased evidently at the urban sites, while it increased slowly at the town sites until 2008, and then decreased.(4) Urbanization and the rapid increase of car usage, which led to a rapid increase of NOx
emissions, might be the major impact factors involved in the evident increase of haze days at urban sites;whereas, the driving factor of variation of haze days at town sites is likely to be climate change, e.g., the intensity of the monsoon in East Asia.