What is ground-level ozone primarily formed by?

Ground-level ozone is primarily formed by chemical reactions in the presence of sunlight, specifically through a process that involves precursor pollutants such as volatile organic compounds (VOCs) and nitrogen oxides (NOx). During sunny days, these chemicals react with each other in the atmosphere, leading to the formation of ozone near the Earth's surface. This type of ozone is considered a significant air pollutant and can have harmful effects on human health and the environment.

The involvement of sunlight is crucial in this process because the energy from sunlight drives the chemical reactions that result in ozone production. Without sunlight, the reactions would not occur at the same rate, leading to significantly lower levels of ground-level ozone.

Other options misrepresent the formation process of ground-level ozone. For instance, while burning fossil fuels does contribute to emissions of VOCs and NOx, it is not the exclusive or direct formation factor for ozone itself, as it requires sunlight for the reaction to take place. Additionally, moisture plays a role in various atmospheric reactions, but it is not the primary method of ozone formation. The natural decay of organic matter does contribute to volatile organic compounds, but, like moisture, it does not directly produce ground-level ozone in the way that sunlight-driven reactions do.