Evaluation of Photostationary and Non-Photostationary Operational Models for NOX Pollution in a Street Canyon

To predict pollutant concentration in urban areas, it is crucial to take into account the chemical transfor-mations of reactive pollutants in operational dispersion models. In this work, we derive and discuss two photostationary (with constant or varying transformation rates) and one non-photostationary chemical models for NO - NO2 - O3 pollution in a street canyon. In the analytical derivation, we focus on the chemical and transport time scales to evaluate the applicability of the models in different urban contexts. We then assess their performance in predicting NO2, NO and O3 concentration at three locations within an urban district by comparing the model predictions with measurements acquired in a field campaign. The results are in line with analytical speculations and highlight in which street types non-photostationary models can bring substantial advantages. In courtyards with limited ventilation and without direct emissions, the performance of the photostationary model with meteorology-based transformation rates is satisfactory. On the other hand, the application of a non-photostationary model significantly improves the predictions in urban canyons with direct vehicular emissions. The applicability of the proposed models in operational tools at the city scale is finally discussed.