Scientists Discover a Hidden Chemical Pathway That Makes City Air Hazy

Nitric oxide may play a larger role in city air pollution than scientists previously thought.

City air can look clear one moment and turn hazy the next, but the chemistry behind that change is often hidden molecule by molecule. In traffic corridors, near power plants, and around other combustion sources, nitric oxide (NO) has usually been treated as a chemical that limits some particle-forming reactions. Researchers from Tampere University and the University of Helsinki now report that, under certain urban conditions, it may do the opposite.

The finding matters because aerosol particles, tiny bits of material suspended in air, are among the most harmful parts of air pollution. They can enter the lungs, reduce visibility in places such as roads and streets, and influence both weather and climate. To forecast air quality more accurately, scientists need to know exactly how these particles form from gases in the atmosphere.

Nitric oxide may boost particles

For years, atmospheric scientists viewed nitric oxide mainly as a brake on the formation of low-volatility condensable vapors. These are gases that can cool, condense, and help build aerosol particles. That view made sense for many chemical systems, but it left an important question open: what happens when nitric oxide meets the kinds of aromatic carbonyl compounds often found in city air?

“Traditionally, NO has been viewed as a compound that limits the formation of atmospheric aerosol precursors. Our results show that it is more likely to enhance their formation from certain volatile compounds,” says doctoral researcher Shawon Barua from Tampere University.

Hidden chemistry reshapes urban air

To follow that chemical trail, the researchers focused on aromatic carbonyl compounds after they enter the atmosphere from vehicle exhaust, industrial activity, and consumer products. These compounds are volatile, meaning they can easily move into the air as gases, but they can also be transformed by atmospheric reactions.

Using advanced laboratory experiments and computational modeling, the researchers identified a pathway that had been largely overlooked. In this pathway, reactions involving nitric oxide can quickly turn aromatic carbonyl compounds into aerosol precursors, the chemical building blocks that help particles form.

“Our findings reveal that the chemistry of urban air is more complex than previously assumed. To accurately predict future air quality, we need to understand all the chemical pathways that contribute to particle formation,” says Dr. Avinash Kumar from Tampere University.

Models may be missing reactions

The result is especially important for cities, where aromatic pollutants and nitrogen oxide emissions are often present together. If this newly identified pathway is active in urban atmospheres around the world, it could help explain why models sometimes struggle to predict particulate matter, the pollution made of tiny airborne particles.

Despite decades of work, scientists still do not fully understand how aerosol particles form in city air. Professor Matti Rissanen from Tampere University argues that one reason may be that some important reaction chains have been left out of atmospheric models.

“Sequential oxidation reactions between common air pollutants, such as those highlighted in this study, have been missing from the existing model chemistries and may go a long way toward explaining why predicting urban aerosol loads has been so difficult,” says Professor Matti Rissanen from Tampere University. He believes the findings will help scientists improve atmospheric models used to predict air quality and assess the impacts of air pollution on human health and the climate.

Reference: “Nitric oxide can enhance secondary aerosol precursor formation from aromatic carbonyls” by Shawon Barua, Avinash Kumar, Prasenjit Seal, Mojtaba Bezaatpour, Sakshi Jha, Nanna Myllys, Siddharth Iyer and Matti Rissanen, 7 May 2026, Nature Communications.
DOI: 10.1038/s41467-026-72628-w

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