The influence of catalytic coating walls on O3 in urban street canyon based on CFD simulation

With the acceleration of urbanization, the building density and pollution emission sources have increased, and the problem of urban tropospheric ozone has become increasingly severe. Traditional pollution control strategies have focused on source reduction. However, emission reductions have reached their limits, making substantial further reductions difficult to achieve while maintaining socio-economic stability. Moreover, ozone is a secondary pollutant whose formation exhibits a non-linear relationship with its precursors (VOC and NO_x). Addressing the issue solely through source reduction of these precursors proves insufficient. Consequently, there is an urgent need for atmospheric ozone self-purification technologies to tackle air pollution. By applying catalytic materials to building facades, atmospheric ozone pollution can be self-purified at low cost and with zero energy consumption. Under ambient temperature and pressure, alongside typical wind speeds and sunlight conditions, these catalytic materials decompose ozone into oxygen.

Application experiments have been conducted under real meteorological conditions in a park. Results indicate that coating park perimeter walls with catalytic materials can reduce nearby ozone concentrations by 5%-20%, with effects extending up to 18 m. Moreover, the higher the temperature, the greater the wind speed and the higher the relative humidity, the overall level of ozone will also increase. These results further confirm that wall catalysis significantly reduces ozone in a small near-wall range, but on a larger spatial scale, the distribution of ozone is still controlled by the atmospheric background and flow field. Therefore, numerical simulations at the urban block scale are required to evaluate the effectiveness of self-purification materials in ozone removal.

The study selected a real building complex in Nanchang as the computing domain, with a horizontal range of approximately 1000 m × 600 m, and constructed a three-dimensional physical model through the urban building outline. In this model, we first examined the impact of varying inflow wind speeds (1 m/s, 3 m/s, and 6 m/s) on ozone distribution. The results show that higher wind speeds correlate with overall elevated ozone concentrations, indicating that atmospheric background transport plays a dominant role. We have paid particular attention to several typical street canyon configurations. These include combinations with aspect ratios of 0.75 and 1.0, as well as scenarios where the canyon is parallel to the wind direction or forms a 40° angle with it. Ozone concentration profiles reveal that different combinations of aspect ratio and wind direction significantly alter vortex structures, thereby influencing ventilation within the canyon and pollutant residence times. Preliminary findings indicate that deep street canyons with larger aspect ratios and those aligned parallel to the prevailing wind are more prone to forming high ozone exposure zones, where ozone catalytic effects are enhanced. Conversely, canyons with wider openings or those angled relative to the wind direction exhibit superior ventilation, resulting in ozone concentrations closer to background levels. In summary, this study confirms the effectiveness of applying ozone-catalysing materials to building facades for urban ozone control.