The rapid development in Beijing,the capital of China,has resulted in serious air pollution problems.Meanwhile great efforts have been made to improve the air quality,especially since 1998.The variation in air quality under the interaction of pollution and control in this mega city has attracted much attention.We analyzed the changes in ambient air quality in Beijing since the 1980s using the Daniel trend test based on data from long-term monitoring stations.The results showed that different pollutants displayed three trends:a decreasing trend,an increasing trend and a fiat trend.SO2,dustfall,B[a]P,NO2 and PM10 fit decreasing trend pattern,while NOx showed an increasing trend,and CO,ozone pollution,total suspended particulate (TSP),as well as Pb fit the flat trend.The cause of the general air pollution in Beijing has changed from being predominantly related to coal burning to mixed traffic exhaust and coal burning related pollution.Seasonally,the pollution level is typically higher during the heating season from November to the following March.The interaction between pollution sources change and implementation of air pollution control measures was the main driving factor that caused the variation in air quality.Changes of industrial structure and improved energy efficiency,the use of clean energy and preferred use of clean coal,reduction in pollution sources,and implementation of advanced environmental standards have all contributed to the reduction in air pollution,particularly since 1998.