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A B S T R A C T Ambient measurements of SO 2 , O 3 , NO x , NO y and CO were made at Shangdianzi (SDZ), a rural site in the northeast (NE) of Beijing, and urban Beijing (China Meteorological Administration) from 1 June 2008 to 31 August 2008. The pollutants at SDZ showed very different levels under different wind conditions, with the levels under the southwest (SW) wind being much higher than those under the NE wind. The SW wind facilitates the transport of urban plume to SDZ, whereas the NE wind provides a background condition. At SDZ, the Ozone (O 3 ) concentration in air masses from urban Beijing was found to be 33.4 9 0.4 ppbv higher than that from clean regions in summer. The ozone production efficiency (OPE x ) for the urban plume and background condition was 4.0 and 5.3, respectively. Based on these OPE x values and the NO z values for the respective conditions, the contribution of in-situ production in the urban plume to the level of O 3 at SDZ is estimated to be 8.6 ppbv, corresponding only to 25.7% of the total impact of urban plume transport. This suggests that direct transport of O 3 rather than in-situ photochemistry contributes mainly to the summer elevation of the level of O 3 at SDZ.