Abstract. Gaseous pollutants, NO y /NO x , SO 2 , CO, and O 3 , were measured at an urban site in Beijing from 17 November 2007 to 15 March 2008. The average concentrations (with ± 1σ ) of NO, NO 2 , NO x , NO y , CO, SO 2 , and O 3 were 29.0 ± 2.7 ppb, 33.7 ± 1.4 ppb, 62.7 ± 4.0 ppb, 72.8 ± 4.5 ppb, 1.99 ± 0.13 ppm, 31.9 ± 2.0 ppb, and 11.9 ± 0.8 ppb, respectively, with hourly maxima of 200.7 ppb, 113.5 ppb, 303.9 ppb, 323.2 ppb, 15.06 ppm, 147.3 ppb, and 69.7 ppb, respectively. The concentrations of the pollutants show "saw-toothed" patterns, which are attributable mainly to changes in wind direction and speed. The frequency distributions of the hourly mean concentrations of NO y , SO 2 , CO, and O 3 can all be decomposed in the two Lorentz curves, with their peak concentrations representing background levels under different conditions. During the observation period, the average ratio NO x /NO y was 0.86 ± 0.10, suggesting that the gaseous pollutants in Beijing in winter are mainly from local emissions. Data of O 3 , NO z , and NO x /NO y indicate that photochemistry can take place in Beijing even in the cold winter period. Based on the measurements of O 3 , NO x , and NO y , ozone production efficiency (OPE) is estimated to be in the range of 0-8.9 (ppb ppb −1 ) with the mean(±1σ ) and median values being 1.1(±1.6) and 0.5 (ppb ppb −1 ), respectively, for the winter 2007-2008 in Beijing. This low OPE would cause a photochemical O 3 source of 5 ppb day −1 , which is small but significant for surface O 3 in winter in Beijing.Correspondence to: X. Xu (xuxb@cams.cma.gov.cn) Downward transport of O 3 -rich air from the free troposphere is the more important factor for the enhancement of the O 3 level in the surface layer, while high NO level for the destruction of O 3 . The concentrations of SO 2 , CO, and NO x are strongly correlated among each other, indicating that they are emitted by some common sources. Multiple linear regression analysis is applied to the concentrations of NO y , SO 2 , and CO and empirical equations are obtained for the NO y concentration. Based the equations, the relative contributions from mobile and point sources to NO y is estimated to be 66 ± 30 % and 40 ± 16 %, respectively, suggesting that even in the heating period, mobile sources in Beijing contribute more to NO y than point sources.