Numerical study on the ozone formation inside street canyons using a chemistry box model.

Tropospheric ozone is a secondary air pollutant produced in the presence of nitrogen oxides (NOx), volatile organic compounds (VOCs), and solar radiation. In an urban environment, ground-level vehicular exhaust is the major anthropogenic source of ozone precursors. In the cases of street canyons, pollutant dilution is weakened by the surrounding buildings that creates localized high concentration of NOx and VOCs, and thus leads to high potential of ozone formation. By considering the major physical and chemical processes, a chemistry box model is employed to investigate the characteristics of ozone formation due to vehicular exhaust inside street canyons under the worst case scenario, i.e. the calm wind condition. It is found that a high level of ozone concentration, of the order of 100 ppbv and higher, would occur inside the street canyons, in particular, when the emission rate (concentration) ratio of VOCs to NOx is greater than 10. This elevated ozone concentration appears at the transition from VOCs to NOx sensitivity and may extend to a few hundreds.