Abstract This study explores the appropriateness of the locality of air monitoring stations which are meant to indicate air quality in the area. Daily variations in NO 2 and PM 10 concentrations at 14 monitoring stations in Hong Kong are examined. The daily variations in NO 2 at a number of background monitoring stations exhibit patterns similar to variations in traffic volume while variations in PM 10 concentration exhibit less discernible pattern. Principal component analysis (PCA) and cluster analysis (CA) are applied to analyse NO 2 and PM 10 measurements between January 2001 and December 2005. The results show that NO 2 concentrations at background stations within the urban area are highly influenced by vehicle emissions. The effect vehicle emission has on NO 2 at stations within new towns is smaller. CA results also show that variations in PM 10 concentrations are distinguished by the area the station is located in. PCA results show that there are two principal components (PC's) associated with variations in roadside concentration of PM 10. The strong influence of roadside emissions towards concentrations of NO 2 and PM 10 at a number of urban background stations may be due to their close proximity to busy roadways and the high density of surrounding tall buildings, which creates an enclosure that hinders dispersion of roadside emissions and results in air pollution behaviour that reflects variation in traffic.