Modeling the chemical evolution of nitrogen oxides near roadways

Abstract

The chemical evolution of nitrogen dioxide (NO2) 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 (O3), other oxidants and reactive species including hydroperoxyl radical (HO2), organic peroxyl radical (RO2), formaldehyde (HCHO) and acetaldehyde (CH3CHO) are considered in the transformation from NO to NO2. CFD-VIT-RIT is shown to be capable of predicting both NOx and NO2 profiles downwind. CALINE4 is able to capture the NOx profiles, but underpredicts NO2 concentrations under high wind velocity. Our study suggests that the initial NO2/NOx ratios have to be carefully selected based on traffic conditions in order to assess NO2 concentrations near roadways. The commonly assumed NO2/NOx 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 O3 concentrations and high traffic volumes would lead to the peak NO2 concentration occurring near roadways with elevated concentrations persistent over a long distance downwind.