Simulation model for NOx distributions in a street canyon with air purifying pavement

In Hengelo (Overijssel, NL) a street (Castorweg) will be paved with air purifying pavement. The top layer of this pavement contains TiO2, which serves as a catalyst for the degradation process of the car emission gases NO and NO2, under the influence of UV light. A 2‐dimensional simulation model is made that describes the distributions of pollutant concentrations and the effect of the air purifying pavement on these concentrations throughout a cross‐section of this street canyon. The simulation model assumes a wind direction perpendicular to the street, and takes the effects of wind speed, relative humidity, UV irradiance, traffic density, background concentrations and atmospheric reactions into account. The atmospheric reactions under the influence of UV irradiance generally account for a large reduction of NO and NO2 during the day. For a relatively quiet street like the Castorweg, simulations show that the air purifying pavement can reduce the NO and O3 levels in the street at a height of 1.50m with 2% and the NO2 levels with 10%, for an average late winter day. In more favourable winter conditions the NO and NO2 reduction can reach 3 and 17% respectively. For lower heights the NOx reduction increases significantly. A higher temperature could cause an increase in the NOx levels, consequently further increasing the reduction by the air purifying pavement. For streets with higher buildings, or with a wind direction parallel to the street the residence time of NOx near the air purifying pavement would be longer, yielding a higher NOx reduction.