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How do photochemical reactions involving carbon monoxide create surface-level ozone?

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World map shaded to show annual carbon dioxide emissions by country in 2022, with China, the United States, and India highlighted as top emitters in dark red. A bar chart below lists the top ten emitting countries by million metric tons. — Greenhouse gas Map of annual carbon dioxide emissions by country in 2014.

Surface-level ozone (O₃) is a significant greenhouse gas and a major air pollutant. While naturally occurring ozone in the stratosphere is essential for the planetary radiation balance, surface-level ozone is primarily an anthropogenic byproduct of photochemical reactions in the lower atmosphere.1

The creation of surface-level ozone involves complex chemical processes triggered by sunlight. Specifically, carbon monoxide (CO) acts as a key precursor in these reactions. When CO is present in the atmosphere alongside other pollutants—most notably nitrogen oxides (NOₓ)—sunlight promotes a series of reactions that lead to the formation of ozone. In urban environments, these reactions are often associated with the development of photochemical smog, which can reach unhealthy concentrations, particularly in areas with stagnant air and high traffic emissions.12

It is important to note that while carbon monoxide is a primary anthropogenic source for this process, the overall formation of surface ozone is a complex atmospheric phenomenon involving various hydrocarbons and nitrogen compounds.12