Which of the following gases is known as a volatile organic compound (VOC) at ground level?

Ozone is classified as a volatile organic compound (VOC) at ground level due to its formation through chemical reactions between other pollutants, particularly in the presence of sunlight. VOCs are organic chemicals that can evaporate into the air and contribute to smog, which is particularly prevalent in urban areas. Ground-level ozone is not directly emitted, but rather formed by reactions involving VOCs and nitrogen oxides, primarily from vehicle emissions and industrial processes. This type of ozone contributes to air quality issues and has significant health and environmental effects, such as respiratory problems and harm to vegetation.

The other gases mentioned do not fit the definition of VOCs at ground level. While nitrogen dioxide is a significant air pollutant and can affect ozone formation, it is classified differently. Oxygen and carbon dioxide are essential gases for life but are not categorized as VOCs. Their roles in the atmosphere and ecology are critical but distinct from the behavior and impact of volatile organic compounds.