Scientists Finally Capture the Elusive Chemical Reaction That Sparks Urban Smog

Researchers have successfully captured the fleeting chemical origins of urban smog, providing a clearer look at how natural plant emissions interact with man-made pollutants to degrade air quality. By directly observing a critical, short-lived reaction, the team has solved an 80-year-old puzzle that could fundamentally improve how scientists forecast atmospheric pollution.

The study, published in Nature Communications, details the detection of Criegee intermediates—highly reactive, unstable molecules that exist for only a fraction of a second. These molecules emerge when ozone encounters isoprene, a gas emitted in vast quantities by forests and vegetation. While isoprene itself is harmless, its reaction with ozone triggers a cascade of chemical events that produce secondary organic aerosols, which are primary components of the haze obscuring city skylines and pose significant risks to respiratory health.

Watching Atmospheric Chemistry in Real Time

For decades, the existence of Criegee intermediates was accepted in theory, first proposed by organic chemist Rudolf Criegee in the 1940s. However, their incredibly brief lifespan made them nearly impossible to study directly. Scientists previously had to rely on indirect evidence, effectively working backward from the stable end products to guess how the reaction occurred.

To overcome this, a team led by University of California, Riverside chemist Jingsong Zhang and first author Lei Yang utilized an advanced method known as cavity ring-down spectroscopy. This technique employs high-precision mirrors to bounce light roughly 10,000 times, creating an environment sensitive enough to detect trace compounds in the exact moment they are born. This allowed the researchers to observe the formation and decay of the intermediates as they happened, rather than analyzing what was left over after the fact.

Addressing the Ozone Driver

The findings emphasize that while trees are responsible for the massive release of isoprene, they are not the root cause of the smog problem. Instead, the research points to ground-level ozone as the primary catalyst. Ozone is largely formed through industrial and vehicular emissions of nitrogen oxides and other volatile organic compounds.

“We can’t do anything about the alkenes or isoprene from trees,” Zhang explained. “If you want to solve the air quality problem, you have to reduce ozone in the air. Ozone is the main driver.”

Future Implications for Air Quality Modeling

This direct observation provides a vital “recipe” for atmospheric chemists, allowing them to replace estimated reaction rates with empirical data. Improving the accuracy of these chemical models is essential for developing better strategies to predict and mitigate pollution events. By understanding the precise pathways through which natural emissions are transformed by human-made ozone, policymakers may be better equipped to manage the factors that lead to particle formation.

Building on their success with isoprene, the research team is now preparing to apply these spectroscopic techniques to pinenes—another significant class of tree-emitted compounds—to further decode the chemistry shaping the Earth’s atmosphere. By moving from theoretical inference to direct observation, the researchers believe they have finally unlocked a clearer view of the initial moments of pollution formation.