Which gas is commonly used to illustrate photochemical ozone formation in urban air?

Which gas is commonly used to illustrate photochemical ozone formation in urban air?

• A. Ozone
• B. Nitrogen
• C. Methane
• D. Helium

Answer: (A)

Photochemical ozone formation in urban air is driven by sunlight acting on nitrogen oxides and volatile organic compounds, and ozone ends up as the observable result of this chemistry. The key steps show NO2 absorbing sunlight and splitting to NO and atomic oxygen; that O quickly combines with O2 to form ozone. VOCs help regenerate NO2, allowing ozone to build up during sunny conditions. Because ozone is the secondary pollutant produced by these reactions and is measured to gauge photochemical smog, it is the gas used to illustrate this process. Nitrogen is part of the NOx family but isn’t the illustrative gas itself; methane is a VOC that can influence ozone formation but is not the standard indicator; helium is inert and does not participate in the chemistry.

Photochemical ozone formation in urban air is driven by sunlight acting on nitrogen oxides and volatile organic compounds, and ozone ends up as the observable result of this chemistry. The key steps show NO2 absorbing sunlight and splitting to NO and atomic oxygen; that O quickly combines with O2 to form ozone. VOCs help regenerate NO2, allowing ozone to build up during sunny conditions. Because ozone is the secondary pollutant produced by these reactions and is measured to gauge photochemical smog, it is the gas used to illustrate this process.

Nitrogen is part of the NOx family but isn’t the illustrative gas itself; methane is a VOC that can influence ozone formation but is not the standard indicator; helium is inert and does not participate in the chemistry.