The role of flow structures in the effective removal of NOx pollutants by a TiO2-based coating in a street canyon

Photocatalytic titanium dioxide (TiO2) coatings are known to effectively remove harmful nitrogen oxides (NOx) in so-called laminar flow reactors (LFRs). However, their effectiveness in turbulent flow environments, such as street canyons, is not well understood. In this study, we applied physical modelling principles to simulate NOx photocatalysis on the walls of a street canyon polluted by idealised traffic, and compared the results of this modelling with those of experiments in a standard LFR. The results show that the LFR is partially able to simulate photocatalysis in such a turbulent environment, but with about 18 times higher flow rate than recommended in the standards (3 l min-1). However, the results from the street canyon experiment show that the performance of the photocatalyst is spatially dependent due to the mean flow structures that develop in the canyon. The mean vertical recirculation transports the NOx pollutants from the line source first to the leeward wall and then to the windward wall, making the windward wall more favourable for pollutant removal. The secondary corner vortices that form at the bottom of the canyon increase the reaction time, so that NOx pollutants are removed more effectively (up to 33%) than at other locations in the canyon. In contrast, the lowest removal efficiencies were found at the leeward wall (6%), where pollutants are mainly advected from the traffic and have less contact with the wall. These results provide valuable insights into the effectiveness of photocatalytic coatings in turbulent flow environments and may be useful for the development and implementation of these coatings in realistic urban pollution scenarios.