What is Ozone primarily known for in relation to smog?

Ozone is primarily known as a significant component of photochemical smog, which forms when sunlight drives chemical reactions between volatile organic compounds (VOCs) and nitrogen oxides (NOx). This interaction leads to the production of ozone at ground level, particularly in urban areas with high traffic and industrial emissions. Ozone at ground level can be harmful to respiratory health and can negatively impact vegetation.

The other choices do not accurately reflect the main characteristics or impacts of ozone in the context of smog. For example, ozone does not cool the atmosphere; rather, it can contribute to warming effects when present as a greenhouse gas higher in the atmosphere. It is also not harmless to plants; in fact, ground-level ozone can cause damage to crops and forests, impairing photosynthesis and leading to reduced agricultural yields. While ozone is classified as a greenhouse gas, this characteristic is not its primary role regarding smog formation and its environmental impacts at or near the Earth's surface. Hence, the correct understanding of ozone's significance in relation to smog is closely tied to its role as a product of photochemical reactions.