The effect of photocatalytic coatings on NOx concentrations in real-world street canyons

The abatement performance of photocatalytic coatings on NOx concentrations in real-world street canyons remains an open question considering the very different conclusions reached by the few previous field studies. To fill this gap, an intensive experimental campaign was carried out in summer 2018 in the outskirts of Bologna in Italy. The experimental design involved two parallel street canyons in an open-air controlled environment fully instrumented for measuring air pollutant concentrations, meteorological and turbulence variables in presence of a photocatalytic coating. Specifically, the coating utilized TiO2 photocatalysts. Several controlled pollutant release experiments with a known pollutant source were conducted within the two canyons, of which one coated with TiO2 and the other free (reference canyon). The comparison of observations in the two canyons indicated a distinct behavior of airflows, leading to higher concentrations in the coated canyon. Three independent methods were developed to demonstrate the abatement performance of the coatings in an open-air controlled environment. The first method involved the comparison of the two canyons before and after the application of the coating; the second one regarded the derivation of normalized concentrations to remove the effect of the different geometry and dilution volume of the canyons; the third method involved dispersion modeling simulations conducted during the controlled pollutant release experiments. All the three methods demonstrated a photocatalytic reduction of NOx in the range of 14–21%, suggesting the potential effect of photocatalysis under real weather conditions.