Why is surface-level ozone considered both an air pollutant and a greenhouse gas?
Surface-level ozone () is categorized as both an air pollutant and a greenhouse gas due to its distinct chemical properties and its impact on human health and the Earth's energy balance.
An Air Pollutant
Surface-level ozone is considered a major air pollutant because it is toxic and irritating to living organisms, even at low concentrations. It is formed in the lower atmosphere through photochemical reactions between nitrogen oxides and hydrocarbons, often resulting in photochemical smog in urban areas. Because of its harmful effects on the eyes and mucous membranes, it is designated as one of the six criteria air pollutants monitored by the U.S. Environmental Protection Agency.123
A Greenhouse Gas
As a greenhouse gas, surface-level ozone contributes to the greenhouse effect by absorbing infrared radiation emitted from Earth’s surface and reradiating it back toward the ground. While it exists in lower concentrations than major gases like carbon dioxide, it exerts a measurable influence on the planet's energy budget, with a net radiative forcing of approximately 0.35 watt per square metre due to human-driven emissions.3425