||In recent years, the air quality of Hong Kong has deteriorated rapidly. Questions arise among the public, asking where these air pollutants originate from. Previous studies, including those approaches the problem by comparing total emissions or using receptor source apportionment method in terms of mass concentration, suggest regional emission sources in the Pearl River Delta (PRD) are the primary influence. This study is to test the proposition using a time-based approach. Sulphur dioxide (SO2) is used as the main tracer. The spatial and temporal SO2 and wind pattern are studied on a daily basis. Our results show that more than half (53%) of the days in 2006 the wind direction is not favourable for the transportation of pollutants from the PRD to Hong Kong, and therefore the air in Hong Kong is dominated by local emission sources during these days. An algorithm is then developed in order to approach the problem in a more objective way. This algorithm sets out the conditions for classifying a day whether local pollution or regional pollution are more important in affecting the air quality of Hong Kong. It also shows about half of the days (51%) have local emissions being the primary influence on the air quality of Hong Kong in 2006. On the other hand, analysis for the days from 1999 to 2006 shows that the number of days with the air quality in Hong Kong mainly affected by regional sources in the PRD has an increasing trend.