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

Area of Science:

• Atmospheric Chemistry
• Urban Air Pollution
• Environmental Science

Background:

• Tropospheric ozone is a secondary pollutant formed from nitrogen oxides (NOx) and volatile organic compounds (VOCs) under solar radiation.
• Urban vehicular exhaust is a primary source of ozone precursors.
• Street canyons trap pollutants, increasing local concentrations of NOx and VOCs, thus enhancing ozone formation potential.

Purpose of the Study:

• To investigate ozone formation characteristics from vehicular exhaust within urban street canyons.
• To analyze these characteristics under worst-case conditions, specifically calm wind scenarios.

Main Methods:

• A chemistry box model was employed to simulate key physical and chemical processes.
• The model considered the influence of building structures in street canyons on pollutant dilution.
• Simulations focused on worst-case scenarios with minimal wind.

Main Results:

• High ozone concentrations (≥100 ppbv) were predicted within street canyons.
• Elevated ozone levels were particularly pronounced when the emission ratio of VOCs to NOx exceeded 10.
• This high ozone concentration occurred during the transition from VOC-sensitive to NOx-sensitive regimes and could reach hundreds of ppbv.

Conclusions:

• Urban street canyons can experience significant ozone formation due to vehicular emissions.
• The ratio of VOCs to NOx is a critical factor influencing ozone levels in these environments.
• Calm wind conditions exacerbate ozone formation in street canyons, posing a substantial air quality risk.