City air has a chemistry problem that researchers thought they understood.
For years, atmospheric chemists treated it as a suppressor of the reactions that turn ordinary vapors into airborne particles.
Shawon Barua, a doctoral researcher at Tampere University in Finland, set out to test that idea against a group of pollutants that crowd city air.
The reactions ran fast enough that some molecules picked up as many as 12 oxygen atoms in under a second.
Fixing that gap could sharpen forecasts of particulate matter – the fine pollution tied to heart and lung disease – and help officials judge how cleaner-burning engines will change city air.
City air has a chemistry problem that researchers thought they understood. Exhaust gases from vehicles react with sunlight and other compounds to produce tiny particles – and certain gases were believed to slow that process down.
One of them, a colorless gas that streams from tailpipes and power plants, had been labeled a damper on particle formation for decades. Recent work out of Finland has turned that assumption around.
A long-held belief
The gas is nitric oxide, or NO, and it streams from tailpipes, power plants, and just about anything that burns fuel.
For years, atmospheric chemists treated it as a suppressor of the reactions that turn ordinary vapors into airborne particles.
Shawon Barua, a doctoral researcher at Tampere University in Finland, set out to test that idea against a group of pollutants that crowd city air. The results ran the other way.
Those pollutants belong to a family known as aromatic carbonyls – reactive compounds that pour from vehicle exhaust, industrial work, and a long list of scented household products.
On their own, they readily form the vapors that seed particles. That picture was never airtight.
An earlier study had found that small amounts of NO could actually boost particle precursors from the vapors trees give off, a sign the rule might bend under the right conditions.
Inside the experiments
To watch the chemistry happen, the team pushed the pollutants through a tube reactor wired to a mass spectrometer fast enough to catch products as they formed.
The readout is a dense pattern of peaks, each one a molecule of a specific weight.
The team focused on three reactive compounds that show up in vehicle exhaust, solvents, and scented products.
Benzaldehyde carries the smell of almonds; phenylacetaldehyde and acetophenone bring honey and orange-blossom notes.
By dialing the amount of NO up and down, from a trace to far higher levels, the researchers tracked how it reshaped the products.
At the low end, the peaks marking oxygen-rich molecules climbed rather than shrank – the reverse of what the textbook account predicted.
Nitric oxide and urban pollutants
For two of the three compounds, adding a modest dose of NO multiplied the yield of those oxygen-rich molecules by as much as ten times for acetophenone, and several-fold for phenylacetaldehyde.
Until this study, no one had shown that effect for this class of urban pollutants.
“Our results show that it is more likely to enhance their formation from certain volatile compounds,” said Barua, the study’s lead author.
The enhancement held across a wide span of NO levels.
Once nitric oxide climbed past roughly 300 parts per billion – closer to the grime of a heavily trafficked corridor – the trend flipped. Rising NO began to suppress the same chemistry it had just boosted.
Speed was part of the surprise. The reactions ran fast enough that some molecules picked up as many as 12 oxygen atoms in under a second. Phenylacetaldehyde was the quickest off the mark.
Restarting the reaction
To explain the reversal, the team turned to computer models of the molecules and ran simulations of the reactions step by step.
The chemistry comes down to how partly-oxidized fragments behave once they form.
As a pollutant breaks down, it produces unstable fragments that keep grabbing oxygen, each step leaving the molecule heavier and stickier. NO would typically shut that process down early.
Here it may do the opposite – converting the fragment into a more reactive form that is thought to add still more oxygen before eventually breaking apart.
Why nearly identical compounds behave so differently has been a separate puzzle, and recent research has examined why these yields swing so widely.
In this case, the slower a compound’s own chain ran, the more room NO had to step in.
The benzaldehyde holdout
Not every pollutant responded the same way. Benzaldehyde, the almond-scented one, barely reacted to the added NO, and its output of oxygen-rich molecules held nearly flat.
Instead of building heavier particles, benzaldehyde mostly turned into a nitrogen-bearing compound called nitrophenol, a known urban pollutant in its own right.
The difference traces back to how each molecule starts breaking apart.
That split surprised the team, since the three compounds look so alike on paper.
Small structural differences can send near-identical pollutants down very different chemical paths. A change invisible to the naked eye reshapes what the air ends up carrying.
What this changes
Urban air-quality models have long struggled to predict how many fine particles a city will produce, and the chemistry uncovered here may be one reason why.
A separate paper has likewise found that high-NO reactions in cities have been underestimated.
What is new is the direction of the effect. For these common pollutants, low to moderate nitric oxide does not hold particle formation back – it pushes it forward. Standard models leave that pathway out entirely.
Fixing that gap could sharpen forecasts of particulate matter – the fine pollution tied to heart and lung disease – and help officials judge how cleaner-burning engines will change city air.
As cities cut NO emissions, the payoff for particle levels may not be a straight line.
The study is published in the journal Nature Communications.
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