Modeling the chemical evolution of nitrogen oxides near roadways

The chemical evolution of nitrogen dioxide (NO ₂) and nitrogen monoxide (NO) in the vicinity of roadways is numerically investigated using a computational fluid dynamics model, CFD-VIT-RIT and a Gaussian-based model, CALINE4. CFD-VIT-RIT couples a standard k- ɛ turbulence model for turbulent mixing and the Finite-Rate model for chemical reactions. CALINE4 employs a discrete parcel method, assuming that chemical reactions are independent of the dilution process. The modeling results are compared to the field measurement data collected near two roadways in Austin, Texas, State Highway 71 (SH-71) and Farm to Market Road 973 (FM-973), under parallel and perpendicular wind conditions during the summer of 2007. In addition to ozone (O ₃), other oxidants and reactive species including hydroperoxyl radical (HO ₂), organic peroxyl radical (RO ₂), formaldehyde (HCHO) and acetaldehyde (CH ₃CHO) are considered in the transformation from NO to NO ₂. CFD-VIT-RIT is shown to be capable of predicting both NO _x and NO ₂ profiles downwind. CALINE4 is able to capture the NO _x profiles, but underpredicts NO ₂ concentrations under high wind velocity. Our study suggests that the initial NO ₂/NO _x ratios have to be carefully selected based on traffic conditions in order to assess NO ₂ concentrations near roadways. The commonly assumed NO ₂/NO _x ratio by volume of 5% may not be suitable for most roadways, especially those with a high fraction of heavy-duty truck traffic. In addition, high O ₃ concentrations and high traffic volumes would lead to the peak NO ₂ concentration occurring near roadways with elevated concentrations persistent over a long distance downwind.