Roadside noise barrier helps to reduce downwind pollutants concentration from vehicle emission. This positive characteristic of the construction feature can be explained by its interaction with flow distribution and species transportation. In this thesis, 3-D numerical model has been developed to simulate highway pollutant transportation - realizable k-e model was employed to model turbulent flow; non-reaction species dispersion was applied to simulate species transport. First, numerical models were validated with experimental data. Good agreement was observed. Then detailed simulations were conducted to study double barriers’ effects on highway pollutant dispersion under different settings: noise barriers with different heights, noise barriers with and without edge effects, different atmospheric thermal boundary conditions. Results show that: (1) Noise barrier feature helps reduce downwind pollutant concentration. For 4m tall double barriers without edge effect case, 80% less concentration can be found than non-barrier case at downwind 100m; (2) Reduction of concentration increases as barrier height increases. 1m higher can lead 0.002 more concentration reduction at ground level. (3) Unstable condition has the least concentration and stable condition has the highest concentration at the same location. (4) Barrier with edge effect has higher concentration than barrier without edge effect downwind; (5) Oblique wind condition enlarges barrier edge effect. The larger the oblique angle is, the higher turbulence intensity can be found near barrier edges. These findings will be provide valuable input to noise barriers design so as to improve roadside neighborhood air quality.