Final answer:
Oxidation begins with the reaction of highly reactive HO₂ with gases in the presence of sunlight, leading to the formation of ground-level ozone (O₃) from NOx and VOCs.
Step-by-step explanation:
Oxidation starts when the highly reactive species HO₂ attacks a gas in the troposphere. Ground-level ozone (O₃) is not emitted directly into the air but is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC). This process occurs in the presence of sunlight, where pollutants from cars, power plants, and other sources react to form ozone. In the upper atmosphere, ozone forms naturally by the action of ultraviolet light on oxygen, creating a protective layer against UV radiation. However, at ground level, ozone can be harmful, causing respiratory health issues and damaging crops and ecosystems.