Abstract: To investigate the impacts of air parcel transport on the concentrations of trace gases (SO₂, NO_x, CO and O₃) at regional background stations, backward air parcel trajectories from July 2005 to June 2007 are calculated for Lin' an, Shangdianzi, and Longfengshan stations using the H YSPLIT4 model and the NC EP reanalysis meteorological data. Four trajectories are calculated for each day, with starting time at 02: 00, 08: 00, 14: 00 and 20: 00 Beijing Time and a travelling time of 72 hours. Individual trajectories are clustered to obtain the mean trajectories and monthly statistics are made for different clusters of trajectories. Statistics of trace gas concentrations observed at the stations during the same period are conducted, corresponding to different clusters of trajectories. However, these statistics do not facilitate interpreting relative influences of different trajectories on the pollutants concentrations at the stations, because both trajectories and pollutants concentrations vary seasonally.Therefore, seasonal trends in the time series of the O₃, SO₂, NO_x and CO concentrations are filtered by 30-day smooth-averaging to obtain the short-term variations of these gases, which is nearly independent of seasonal variations. The impacts of air parcel transport over different areas on the pollutants concentrations are studied by combining the trajectories and the corresponding short-term variation data of the gases. The results show that air parcels originating from higher altitudes and transported at higher speeds can effectively remove gaseous pollutants from surface air over the stations. However, the surface O₃ level may occasionally be enhanced by downward transport of O₃-rich air from near the top of boundary layer. About 50 %-60 % of the backward trajectories for the stations are related to the transport of polluted air parcels, leading to increases in the concentrations of the gaseous pollutants. The polluted air parcels are mainly from the S-SW and N-SE sectors for Lin' an, from the SE-SW sector for Shangdianzi, and from the S-WSW sector for Longfengshan. On the average, air parcels from the S-SW sector can enhance the concentrations of SO₂, CO and O₃ at Lin' an by 1.2 ×10^-9, 57.5 ×10^-9 and 4.6 ×10^-9, respectively, while those from the E-SE sector can abate concentrations of SO₂, CO and O₃ by 0.7 ×10^-9, 5.0 ×10^-9 and 9.5 ×10^-9, respectively. For Shangdianzi, air parcels from the SE-SW sector can increase the concentrations of SO₂, NO_x, CO and O₃ by 3.6 ×10^-9, 2.8×10^-9, 406×10^-9 and 1.8×10^-9, respectively, while those from the high altitudes in the NW sector can reduce concentrations of SO₂, NO_xand CO by 8.4×10^-9, 10×10^-9 and 599×10^-9, respectively. For Longfengshan, air parcels from the SW sector can increase the concentrations of SO₂, NO_x, CO and O₃ by 1.3 ×10^-9, 0.4×10^-9, 53×10^-9 and 1. 4×10^-9 respectively, while those from the other sectors decrease the concentrations of SO₂, NO_x and O₃ by about 0.3 ×10^-9-5.9×10^-9, and that of CO by about 24×10^-9-100 ×10^-9.