After 80 years, elusive molecules reveal how ozone starts turning plant emissions into haze — Phys.org

RESOLVING THE LEDGER

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Add as preferred source Scientists have directly observed a fleeting chemical reaction that creates a key ingredient in urban smog. The research offers insight into the earliest stages of pollution formation and could help improve air quality forecasting models.

The researchers directly tracked short-lived molecules called Criegee intermediates as they formed when ozone reacted with isoprene, a gas released in large quantities by trees and other plants. These highly reactive molecules help drive atmospheric chemistry that can ultimately affect air pollution and particle formation, but scientists had previously understood them mainly through indirect evidence.

The study, published in Nature Communications , was conducted by UC Riverside chemist and first author Lei Yang, corresponding author Jingsong Zhang, a UCR chemistry professor, and several doctoral students.

Though unfamiliar to most people, isoprene is one of the most abundant chemicals released into the atmosphere. Trees and other plants emit hundreds of millions of tons of this gas every year, making natural vegetation one of the largest sources of reactive gases in the atmosphere.

Isoprene itself poses little direct harm. It belongs to a family of compounds called alkenes, which react with ozone in the atmosphere. When that happens, they launch a chain of chemical reactions that create particles called secondary organic aerosols , which make up much of the haze hanging over cities, scatter sunlight and can travel deep into the lungs.

Although trees produce most of the world's isoprene, the findings do not suggest forests are driving air pollution. Instead, they highlight the importance of controlling ground-level ozone, which fuels these reactions.

"We can't do anything about the alkenes or isoprene from trees," Zhang said. "If you want to solve the air quality problem, you have to reduce ozone in the air. Ozone is the main driver. In practice, this means reducing emissions of its chemical precursors, especially nitrogen oxides and volatile organic compounds."

Scientists have known for decades that ozone reacts with isoprene and other alkenes, but the first moments of that reaction have remained largely invisible. The chemistry unfolds within hundredths of a second, creating unstable molecules called Criegee intermediates that disappear almost as soon as they form. Although German organic chemist Rudolf Criegee postulated them 80 years ago, these intermediates are so short-lived that they disappear before conventional techniques can detect them and have eluded direct observation or characterization in ozonolysis reactions.

"It's like nature was presenting us with a dish, and we had to guess how it was made," Zhang said. "Now we have a recipe."

To capture these elusive molecules, the researchers used cavity ring-down spectroscopy, an ultrasensitive optical technique that passes light between mirrors roughly 10,000 times. Rather than collecting the molecules for analysis, the system observes them where they are created, allowing scientists to detect compounds that exist only briefly and in extremely low concentrations.

Using this approach, the team detected Criegee intermediates produced during isoprene's reaction with ozone and tracked how they formed and disappeared over time. Those measurements allowed the researchers to determine reaction rates and test atmospheric reaction networks with direct observations instead of relying solely on the stable products left behind after the chemistry is complete.

The work builds on an earlier study, also published in Nature Communications , from Zhang's laboratory that captured Criegee intermediates in simpler reactions involving smaller alkenes. Isoprene, with its more complex molecular structure and central role in atmospheric chemistry, presented a much greater challenge and carries broader environmental significance.

Better measurements of these early reactions could improve the atmospheric models scientists use to predict pollution events and understand how fine-particle pollution forms under different environmental conditions. While the study was conducted under controlled laboratory conditions, the researchers say it establishes a foundation for examining increasingly complex chemical reactions occurring in the real atmosphere.

Next, the team plans to apply the same techniques to pinenes, another major class of compounds emitted naturally by trees, to better understand how they react with ozone and contribute to air pollution.

"For eight decades, scientists could only infer what happened during the first instant of these reactions," Zhang said. "Now we can watch them happen. That gives us the best picture yet of how ozone transforms natural emissions into the compounds that shape our atmosphere."

Lei Yang et al, Direct measurement of Criegee intermediates in isoprene ozonolysis, Nature Communications (2026). DOI: 10.1038/s41467-026-73307-6

Journal information: Nature Communications

Provided by University of California - Riverside

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After 80 years, elusive molecules reveal how ozone starts turning plant emissions into haze — Phys.org | AIPROPX