Scientists discover hidden driver of urban air pollution

Air pollution particles are harmful to human health, reduce visibility – for example, in traffic – and affect the weather and climate. Understanding how these particles form in the atmosphere is essential for improving air quality predictions and developing effective pollution control strategies.

For many years, atmospheric scientists have considered nitric oxide to be a compound that suppresses the formation of low-volatility condensable vapours that can contribute to aerosol formation. However, the new study demonstrates that the opposite can occur for an important group of urban pollutants, such as aromatic carbonyl compounds.

“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.

The researchers investigated the behaviour of aromatic carbonyl compounds in the atmosphere following their release from sources such as vehicle emissions, industrial activities and a range of consumer products. Using advanced laboratory experiments and computational modelling, the team identified a previously overlooked chemical pathway that rapidly converts these compounds into aerosol precursors.

“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.

The discovery is particularly relevant for urban environments, where aromatic pollutants and nitrogen oxide emissions often coexist. The newly identified pathway may therefore play an important role in the formation of particulate matter in cities around the world.

Despite decades of research, the formation of aerosol particles, the most detrimental aspect of air pollution, remains poorly understood in urban environments.

“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 towards explaining why predicting urban aerosol loads has been so difficult,” says Professor Matti Rissanen. 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.

The research article, titled “Nitric oxide can enhance secondary aerosol precursor formation from aromatic carbonyls”, was published in Nature Communications on 7 May 2026.

Contact

Shawon Barua, shawon.barua [at] tuni.fi, +358 451 584 222

Avinash Kumar, avinashkumar [at] tuni.fi, +358 458 044 343

Matti Rissanen, matti.rissanen [at] tuni.fi