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Real-world emission factors Of PM2.5 (particle’s aerodynamic diameter of less than 2.5 mu m), NOx (the sum of NO2 and NO), and CO from mixed vehicles in the Shing Mun Tunnel in Hong Kong have been determined. A total of 16 runs were made during varying times that encompassed different traffic fractions of diesel-fueled vehicles. During the study, a total of 27,310 vehicles traversed the tunnel, comprising approximately 50% diesel-fueled vehicles, 41% gasoline-fueled vehicles, and 9% LPG-vehicles. The PM2.5 emission factors, derived from the DRI (Desert Research Institute, USA) particulate sampler, ranged from 0.066 to 0.190g veh(-1) km(-1). Result from a t-test showed no statistical significance for PM2.5 difference in summer and winter. The PM2.5 emission factors were found to correlate well with NOx (R-2 = 0.65) and both had a positive relationship with the proportion of diesel-fueled vehicles. Two DustTrak air monitors were also used to monitor PM2.5 masses in the tunnel. The mean emission factor derived from DustTrak, 0.150 +/- 0.045 g veh(-1) km(-1), had reasonable agreement with the filter method, 0.131 +/- 0.037 g veh(-1) km(-1), demonstrating that the DustTrak air monitor can be used in tunnel studies, but needs careful calibrations with the filter method. The emission factors for PM2.5 mass and NOx were obtained for on-road diesel-fueled vehicles based on the linear regression analysis and they were 0.257 +/- 0.031 and 1.931 +/- 0.309 g veh(-1) km(-1), respectively. (c) 2006 Elsevier Ltd. All rights reserved.