Los Angeles has long struggled with air pollution, and despite progress over the decades, smog is still a major issue in the region.
Now, scientists at Caltech have discovered that the amount of a specific smog-forming chemical in the air—called ammonium nitrate—is much higher than previously believed, especially during the worst pollution days.
Ammonium nitrate is a major component of smog and is formed when gases from cars, trucks, and industrial activities—specifically nitric oxides (NOx)—react with other chemicals in the atmosphere.
This process often includes a reaction with ammonia, a compound commonly released from agriculture or other sources.
While experts have known about ammonium nitrate for years, it’s been notoriously hard to measure because it can evaporate during sampling, making levels seem lower than they really are.
To get a clearer picture, Caltech researchers used advanced equipment from a nationwide air monitoring program called ASCENT.
These tools allowed them to track tiny particles in the air more accurately and continuously.
The instruments were set up in Pico Rivera, California, as part of a larger effort to study how smog forms and changes over time.
What they found was surprising: ammonium nitrate levels were much higher than previous estimates. Even though LA has made significant cuts to NOx emissions over the years—through cleaner fuel, better vehicle technology, and regulations like catalytic converters—this smog-forming chemical still lingers in large amounts.
The team, led by Caltech graduate Ryan Ward (now at Columbia University), believes part of the problem is a chemical process that happens at night. Increased nighttime ozone levels may be helping to convert NOx into nitric acid, which eventually becomes ammonium nitrate. This explains why smog can remain stubborn even as overall NOx emissions fall.
The discovery highlights how air pollution is more complex than it may appear. Not all smog is the same—some particles are organic and come from many hard-to-control sources, like plants, fires, and even cooking. But inorganic particles like ammonium nitrate mostly come from fossil fuel combustion, which is more straightforward to target with policy and technology.
The findings suggest that Los Angeles still needs to reduce NOx emissions even further. This could mean switching more vehicles and machines—like gas-powered lawnmowers and trucks—to electric versions and improving air-quality monitoring to better understand what pollutants are actually in the air.
As Professor Paul Wennberg of Caltech explains, knowing what smog is made of is key to cleaning it up. With this new information, officials and scientists can design smarter strategies to tackle one of LA’s longest-standing environmental challenges.
