How is ozone formed?

Ozone is formed through a photochemical reaction that occurs when sunlight acts on pollutants in the atmosphere, particularly volatile organic compounds (VOCs) and nitrogen oxides (NOx). In the presence of sunlight, these two types of compounds react to create ozone, which is a key component of smog in urban areas.

This process is influenced by a variety of factors, such as temperature, sunlight intensity, and the concentration of the reactants. The formation of ground-level ozone is significant because, while ozone high in the atmosphere protects against ultraviolet radiation, at ground level it can have harmful effects on human health and the environment.

The other options do not accurately reflect the chemical process leading to ozone formation. For instance, water and carbon do not directly contribute to ozone creation, and sulfur dioxide primarily leads to the formation of acid rain rather than ozone. Though chlorofluorocarbons (CFCs) do break down to release chlorine, which can deplete ozone in the stratosphere, they are not a primary source of ozone at ground level. Thus, the correct answer regarding the formation of ozone involves the reaction between oxides of nitrogen and VOCs due to sunlight.