Which gas is a criteria pollutant associated with combustion processes?

Nitrogen Dioxide (NO2) is a criteria pollutant that is closely associated with combustion processes, particularly those occurring in vehicular emissions and industrial activities. It is a byproduct of burning fossil fuels such as coal, oil, and natural gas. Its formation is primarily attributed to high-temperature combustion conditions, where nitrogen in the air reacts with oxygen to form various nitrogen oxides, including nitrogen dioxide.

As a criteria pollutant, nitrogen dioxide is regulated by the Environmental Protection Agency (EPA) due to its significant impact on human health and the environment. Exposure to elevated levels of nitrogen dioxide can lead to respiratory problems and increased susceptibility to infections, particularly for individuals with asthma or other respiratory conditions. Furthermore, nitrogen dioxide contributes to the formation of ground-level ozone and fine particulate matter, which are associated with further health and environmental risks.

In contrast, while carbon dioxide is a significant greenhouse gas resulting from combustion, it is not classified as a criteria pollutant under the Clean Air Act. Methane, although a potent greenhouse gas, also does not fall under the criteria pollutants category, and argon is an inert gas that does not participate in combustion processes or produce harmful effects, thus not becoming a concern for air quality regulations.