Based on the observations at Lin’an regional background station in Zhejiang Province from December 2003 to November 2004,the seasonal and diurnal variations of NO_x and SO₂ concentrations are analyzed.Result shows that the winter background concentrations of acidic gases NO_x and SO₂in the Yangtze River Delta region are the highest with 23.81×10^-9and 37.3×10^-9(volume fraction),respectively,mainly affected by the relatively high concentration contamination transportation from the urban agglomeration of Shanghai-Nanjing area to the northeast of Lin’an.The concentrations of acidic gases are the lowest in summer,mainly contributed by the local emission sources.Rainfall can obviously scavenge SO₂,but it has low effect on NO_x.The diurnal variation of NO_x concentration presents a typical bimodal pattern with the higher value at 09:00 BST and the maximum at 18:00 BST in winter,while the variation is an unimodal pattern with the maximum at 07:00 BST in summer.The diurnal variation of SO₂ concentration is remarkable in winter and spring with the maximum around 06:00 BST and the minimum around 15:00 BST.The air quality of annual regional NO₂ conforms to the primary standard of GB 30952—1996(Ambient Air Quality Standard),but SO₂ concentration exceeds the allowable limit notably in winter and spring for the obvious influence of human activity.Air pollution indexes of NO₂ and SO₂ reach the maximums 50 and 93 in December,respectively.The weekend effect of NO₂ and SO₂ does not happen in this region.