Modeling NO2 concentrations in real urban areas using computational fluid dynamics: A comparative analysis of methods to assess NO2 concentrations from NOx dispersion results

Major cities worldwide constantly deal with health hazards caused by air pollution. Modeling this pollution on an urban scale is essential for assessing the impact of local policies and promoting sustainable urban development. However, there are practical difficulties when using microscale modeling in applied context, and particularly for nitrogen dioxide modeling (NO2). In this study, a Computational Fluid Dynamics (CFD) model was employed to assess monthly NO2 concentrations in Antwerp, Belgium, and the results were compared to a one -month measurement campaign using 73 passive samplers. The result showed that using CFD with conventional assumption - such as neutral atmospheric stability consideration and using a turbulent Schmidt number (Sct) set to 0.7 - yield satisfying results according to air quality model acceptance criteria. Optimal outcomes were achieved by considering NO2 background concentration instead of NOx and employing Bachlin et al.'s empirical function to convert modeled NOx concentrations to NO2, dismissing the need for straightforward chemical mechanisms - such as photostationary steady-state equilibrium (PSS) -, or more expensive models in terms of computing resources. This approach yielded an overall error of less than 15 % and a correlation coefficient R of 0.78, affirming its effectiveness in modeling NO2 air quality in applied context.