2026 News & Events

New aircraft research finds that declining nitrogen oxide emissions are unlocking an understudied atmospheric chemistry pathway, quietly adding particle-forming compounds to urban skies.

The air over American cities is getting cleaner. Decades of emissions reductions and cleaner technologies have dramatically cut levels of nitrogen oxides (NOx), byproducts of combustion that are central to ozone formation and "smog".

While NOx reductions have helped to curb ozone, a new study published in Science Advances suggests that this progress is reshaping how pollutants form in urban air, unlocking a type of chemical reaction that may already be contributing to particulate matter (or PM_2.5) pollution.

Based on chemical measurements collected from aircraft over major urban areas including New York, Chicago, Toronto, and Los Angeles during the 2023 Atmospheric Emissions and Reactions Observed from Megacities to Marine Areas (AEROMMA) campaign, researchers see evidence for production of a particular form of oxidized volatile organic compound (VOC), collectively referred to as "highly oxidized molecules", or HOMs. These relatively large and sticky HOMs are especially good at forming PM_2.5.

"We know from lab experiments that these HOMs can form when NOx levels are very low, and so the common thinking has been that they can't form at any significant rate in our current urban atmosphere," explained Michael Robinson, lead author of the new study and a research scientist with the Cooperative Institute for Research in Environmental Sciences (CIRES) working at NOAA's Chemical Sciences Laboratory. "Our measurements are showing that, for some VOCs, conversion to HOMs is actually already occurring in urban areas and it is likely widespread across North American cities."

A shift in urban chemistry

When sunlight drives the oxidation of VOCs, the process generates highly reactive organic peroxy radicals. In a high-NOx environment like the urban air of decades past, these radicals are quickly consumed by reacting with nitric oxide (NO). Those reactions produce ground-level ozone, which itself is a pollutant, but also tends to shut down the radical chemistry quickly.

As NOx levels fall, peroxy radicals have more time before they encounter a reaction partner. Using the AEROMMA aircraft measurements combined with data from NASA's TEMPO geostationary satellite, Robinson calculated this time window (known as the "bimolecular lifetime") and found that it is substantially longer than expected in cities like New York, Chicago, and Toronto – averaging around 17 seconds – compared to Los Angeles, where the basin's still-elevated NOx levels keep lifetimes closer to 7 seconds.

Seventeen seconds may sound brief, but in atmospheric chemistry this is a lifetime – especially for highly reactive, and highly impatient, peroxy radicals.

For larger peroxy radicals, this time window is long enough for them to undergo what chemists call isomerization: rather than waiting for another molecule to react with, they instead internally rearrange, effectively reacting with themselves. Larger molecules more readily isomerize because they are essentially floppy enough to fold in on themselves.

Each self-rearrangement adds oxygen to the molecule, and the cascade can repeat multiple times through a process known as autoxidation – ultimately producing HOMs with extremely low volatility and a strong tendency to condense into fine particles known as secondary organic aerosol (SOA).

These HOMs are extremely difficult to measure in the atmosphere with current instrumentation, which is at least part of the reason why they have been understudied to date. To be able to constrain these reaction rates and pathways from aircraft measurements, Robinson explained, he first spent years in the laboratory painstakingly calibrating his instruments to detect and quantify various reaction products.

"It has been a 5 year saga to get to this paper," he added with a laugh.

A widespread, underrecognized process

The new research finds that for several common VOCs, isomerization is already happening at appreciable rates. For alpha-pinene, a compound emitted abundantly by pine trees (responsible for that well-known piney aroma) and common in urban and suburban environments, the researchers calculated that 12–17% of peroxy radicals in New York, Chicago, Toronto, and Los Angeles are currently reacting through the isomerization pathway, much higher than expected in urban air. For 2-ethoxyethanol, a solvent found in paints and cleaning products, the fraction is much higher in these cities at 44%, making this chemistry a dominant fate for peroxy radicals. For hexanal, a compound associated with cooking emissions, the figure approaches 50%.

"These percentages will only grow as NOx emissions continue their long-term decline," said Robinson.

Yet, many air quality models still do not fully include these isomerization pathways and only a small handful of VOCs have even had their isomerization rates studied in laboratories. Without that data, those VOCs are not included in models at all.

That gap could lead to underestimates of particulate pollution in air quality models, particularly in the summer months, and inaccurate predictions of how air quality will respond to future emission changes. As NOx and ozone levels drop, the potential for increased PM_2.5 production means that the net effect for air quality and public health remains an open question.