A hidden pollutant that contaminates Southern California’s nighttime air is much more prevalent than previously thought, suggesting that air quality alerts underestimate the health effects of breathing smoggy air, a study released Wednesday reported.

Researchers at Caltech and other academic institutions found that levels of ammonium nitrate made up half of the fine particulate pollution (PM2.5) on the region’s worst air days but are unaccounted for by air pollution control agencies. This means air quality indexes of “good,” “moderate,” “unhealthy,” or “hazardous” released to the public may be worse than indicated.

“When PM2.5 is measured and ammonium nitrate levels are high, the PM2.5 is underestimated because most of it evaporates before it is measured,” wrote the lead author of the study published in “Science Advances” on May 21, Ryan Ward. He did the work while a graduate student at Caltech in Pasadena and is now a post-doctoral fellow at Columbia University in New York.

PM2.5 are tiny pollution aerosols that originate from tailpipes of cars, trucks, ships in port and also fossil-fuel powered garden equipment. These particles can lodge deep into the lungs and the brain and are linked to poor health outcomes.

Scientists and medical doctors have known for at least 20 years that breathing smog peppered with high PM2.5 levels is linked to cardiovascular issues, respiratory problems, and premature death. Chronic exposure to PM2.5, the size of 2.5 micro meters or less, and the slightly larger variety, PM10 that’s 10 micro meters leads to hospitalizations, higher incidents of asthma and COPD and even death, according to a recent article by Dr. Olawale Amubieya, pulmonologist and junior faculty in the Division of Pulmonary and Critical Care Medicine at UCLA Health.

Ward wrote that it’s not known if this inorganic chemical is more toxic than the other compounds found within PM2.5, he said, adding that in total, microscopic particulate pollution is a public health hazard.

Measurements of the Air Quality Index (AQI) don’t say what the smog is made of. With the new data, Caltech scientists and clean-air advocates suggest air quality regulators re-focus their attention on reducing ammonium nitrate, an often ignored element of smog.

“This study should urge the U.S. EPA and California Air Resources Board (CARB) to address ammonium nitrate,” said Bill Magavern, policy director for the nonprofit Coalition For Clean Air in Los Angeles. “Caltech has provided an excellent basis. Now these agencies should bring in other experts and do their own science and if borne out, regulators should act to bring ammonium nitrate down.”

Ammonium nitrate was measured using the Atmospheric Science and Chemistry mEasurement NeTwork (ASCENT), a collection of scientific instruments that measure continuous air quality. ASCENT is funded by the National Science Foundation (NSF).

Scientists could differentiate the compounds within PM2.5 by using the portal of ASCENT in Pico Rivera, combined with a mass spectrometer, Ward said. The spectrometer was first tested on the roof of Caltech Hall, the tallest building on the Pasadena campus, from January to June 2023. They were able to identify ammonium nitrate and its percentage, something not done previously.

The measurements continued at later dates. Ward stressed the study was not done during the Eaton or Palisades fires of January 2025. “This study represents the day-to-day fluctuations in LA smog,” he emphasized, not the episodic smoke production from wildfires. But the ACENT system was used by other Caltech scientists to measure lead and other metals present in continuous air flow during the fires.

Ammonium nitrate is formed after oxides of nitrogen (NOx) molecules change into nitric acid, which combines with ammonia, creating ammonium nitrate. Ironically, smog-reducing regulations and less-polluting internal combustion engine automobiles burning gasoline or diesel fuels have reduced NOx and particulate levels in the South Coast Air Basin.

However, this has produced a counter-effect, resulting in ozone, the primary smog component formed when chemicals react in sunlight, leaving the daytime air but hanging around in the nighttime air. The lingering, nighttime ozone reacts with NOx molecules, forming ammonium nitrate, Ward explained.

“A lot of this bad pollution is made at night when it is cool,” he said. That begs the question whether joggers are breathing cleaner air at night, essentially waiting for the sun to go down.

Also, many Angelenos take advantage of the summer breezes and leave their windows open overnight when they sleep. This can potentially let in more PM2.5 and ammonium nitrate, he said. However, the Caltech study does not examine health effects.

Ammonium nitrate is produced primarily through the burning of fossil fuels, Caltech reported. By identifying this inorganic compound in PM2.5, the researchers can trace it backward.

“The sources of an inorganic molecule like ammonium nitrate — NOx-burning fossil fuels — are more straightforward to address,” the study reported. “The work thus suggests the importance of powering cars, trucks, and gas-driven appliances like lawnmowers with electricity and continuing to monitor air quality with sensors that can capture the full chemical makeup of the aerosols.”

California has a mandate that the state must sell no more gasoline-powered passenger vehicles. Starting in 2035, only battery electric vehicles (BEVs) and plug-in hybrid electric vehicles (PHEVs) can be purchased.

“The good news is California is trying to move cars, trucks, buses, lawnmowers to zero emission. The bad news is the Trump administration and Congress is trying to block all that progress and force us to burn more fossil fuels and breathe all that poison,” said Magavern.