Why the air in your city is a complex chemical factory

Countries & Cities · Published 3 October 2026 Swipe more like this →

In short: NOx is the shorthand for a pair of gases—nitric oxide and nitrogen dioxide—that define urban air quality. While they are natural byproducts of lightning, human activity has turned them into a persistent atmospheric challenge, driving the formation of smog, acid rain, and ground-level ozone that impacts human health.

At its core, NOx is a product of high-temperature combustion. When fuel burns in car engines or power plants, nitrogen and oxygen from the air react to form these oxides. This process requires extreme heat, typically above 1300 °C, to break the stable bonds of atmospheric nitrogen. While lightning naturally produces NOx at high altitudes, human-made emissions are concentrated near the surface, where they interact with sunlight and volatile organic compounds to create photochemical smog.

The chemistry of NOx is a delicate, sunlight-driven dance. During the day, nitrogen oxides and ozone exist in a photostationary state, where the intensity of sunshine dictates the ratio between NO and NO2. However, when NOx reacts with volatile organic compounds, it shifts this balance, leading to the net production of ground-level ozone. This ozone is a potent respiratory irritant, particularly for children and those with asthma, and can be transported by wind far from its original source.

Beyond combustion, our environment is shifting in unexpected ways. Agricultural fertilizers and nitrogen-fixing plants contribute to NOx levels through soil microbial processes, with some studies suggesting fertilizer accounts for 25 percent of California's NOx pollution. Even forests are changing; in the eastern United States, certain tree species like maples and tulip poplars are associated with bacteria that emit reactive nitrogen, potentially altering air quality as these species displace others like oak and hickory.

The consequences are far-reaching. NOx contributes to acid rain through the formation of nitric acid and acts as a precursor to toxic compounds like nitroarenes and nitrosamines. Because tropospheric ozone absorbs infrared radiation, the indirect effect of these emissions is an intensification of global warming. Understanding these pathways is essential, as nearly 50 percent of United States inhabitants currently live in counties that fail to meet ozone compliance standards.

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