中文摘要：

利用全国756基准站1994—2013年MICAPS地面常规观测资料和47个城市2002—2012年国家环境保护总局公布的空气污染指数（the Air Pollution Index,API）数据,对北京、南京、重庆沙坪坝、广州4个大城市霾日和API的气象条件进行分析。结果表明：1）我国霾日和API的高值区主要分布在中东部地区,年霾日自1998年开始上升,并持续增长,到2013年达到最多。1月霾日最多,8月霾日最少。2002—2012年期间,与年霾日不同,年API呈下降趋势,12月API最高,8月API最低。四大城市的霾日和API的年际变化和月际变化均存在较大差异。2）一般情况下,霾发生时,北京、南京、重庆沙坪坝、广州对应的相对湿度较高;霾发生时,北京西南风占主导作用,南京和广州均东南风占主导作用,重庆沙坪坝西北风占主导作用,风速多集中在1~4 m/s,负变压及低层大气稳定时更易产生霾日。3）低层稳定度（γ850）对四大城市的API的影响各不相同,北京和重庆沙坪坝与γ850呈负相关,即越稳定,API值越高,南京和广州与γ850呈正相关,即越不稳定,API值越高。4）多元线性回归方程的结果表明：热力因子（相对湿度、γ850）和动力因子（地面风速、24 h变压）对四大城市冬季霾日能见度和冬季API各有贡献。

英文摘要：

The distribution of haze days and air pollution index（ API）,and their relationship w ith meteorological elements w ere analyzed for Beijing,Nanjing,Shapingba district of Chongqing,and Guangzhou,using the M ICAPS ground conventional observation data during 1994—2013 from 756 national base stations and the API data for 47 citiesduring 2002—2012 published by M inistry of Environmental Protection of the PRC. The results indicate thatthe haze and high API mainly occurs in central and eastern China; the annual frequency of haze days has begun to rise since 1998 and reached its maximum in 2013,and most haze occurs in January and least in August. While the annual frequency of high API is on the decline,and API is generally highest in December and low est in August during the period of 2002—2012. The interannual and monthly variations of haze days and API are different forBeijing,Nanjing,Shapingba of Chongqing,and Guangzhou. As for the relationship betw een haze and meteorological elements,the results indicate that the relative humidity is generally high in the four large cities / district in haze days; w hen haze occurs,Beijing,Nanjing and Guangzhou,and Shapingba district of Chongqingare dominated by southw estly,southeastly,and northw estly w ind,respectively,w ith w ind speed mainly in the range of 1—4 m / s.Haze w ill easily occur inkatallobaric and stable low er atmosphere. The low er atmosphere stability（ γ850） is correlated negatively w ith API in Beijing and Shapingba district of Chongqing,w hile positively in Nanjing and Guangzhou. M ultiple linear regression equations show that both thermal factor and dynamic factor have contributed to the visibility at haze days and API in w inter in the four cities / district.