Urban air quality modeling with full O3–NOx–VOC chemistry: Implications for O3 and PM air quality in a street canyon

We examine transport and chemical transformation of reactive pollutants on an urban street using a computation fluid dynamics (CFD) model coupled with full photochemistry of reactive pollutants. An extensive comparison between simulated results and observations is conducted to evaluate the model, focusing on a field campaign occurred in Dongfeng Middle Street in Guangzhou, China. Observed CO and NO concentrations vary diurnally following traffic volumes. The model captures this observed diurnal variation and magnitudes of CO concentrations successfully. However, simulated NO concentration is three times higher than observation. This high bias is significantly reduced in the sensitivity simulation with lower NOx emissions. We find that oxidation products of O3 photochemistry such as NO2 and O3 vary differently from primary pollutants, indicating important effects of photochemical reactions on their fates. The model appears to reproduce observed O3 and NO2 variability with time and altitude. Our analysis shows that high NOx concentrations in the urban street canyon may efficiently produce aerosol nitrate in the presence of NH3. Simulated inorganic NO3− aerosol concentration reaches up to 0.3 μg m−3 in July but increases an order of magnitude higher at lower temperature that favors partitioning of gas-phase HNO3 to aerosol-phase, implying a serious concern for urban air quality in winter.