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Los Angeles smog: Higher-than-expected ammonium nitrate levels found

The Los Angeles region has some of the most polluted air in the United States, failing to meet standards set by the Environmental Protection Agency for the last decade. Now, Caltech researchers have quantified the levels of a component of smog called ammonium nitrate, a molecule that has been notoriously difficult to measure, and have found that there is much more of it than previously calculated, especially on the most polluted days.

The findings emphasize the need for the continued reduction of fossil fuel emissions from cars, trucks, and other industrial processes, which produce the precursor to ammonium nitrate, called nitric oxides or NO_x.

The research was conducted in the Caltech laboratories of Paul Wennberg, the R. Stanton Avery Professor of Atmospheric Chemistry and Environmental Science and Engineering, and Richard Flagan, the Irma and Ross McCollum-William H. Corcoran Professor of Chemical Engineering and Environmental Science and Engineering.

A paper describing the study, led by former graduate student Ryan X. Ward, Ph.D., now a postdoc at Columbia University, in the journal Science Advances.

The late Caltech professor Arie Haagen-Smit first linked air pollution to automobile exhaust and other industrial fuel combustion in the 1950s. Since then, the Los Angeles region has made significant progress in cleaning up aerosols that cause smog through measures such as prohibiting the burning of sulfur in fuels and mandating the use of catalytic converters that scrub out NO_x from vehicle exhaust. Still, on some days, smog limits visibility to only a few miles.

"In many ways, we're re-contextualizing the now-decades of work Caltech has put into atmospheric chemistry since Haagen-Smit," Ward says. "By applying new techniques to answer a decades-old problem, we see just how persistent the inorganic aerosol smog is in LA. Despite significant reductions in NO_x emissions, we still have work to do to clean it up."

Ammonium nitrate has been difficult to measure with traditional sensors; due to its temperature and pressure sensitivity, it can evaporate when being sampled. In 2023, Caltech researchers began operating the Pico Rivera location of the Atmospheric Science and Chemistry mEasurement NeTwork (ASCENT), a collection of scientific instruments across the nation that measure air quality.

ASCENT, which is managed by Nga Lee (Sally) Ng, Ph.D., of the Georgia Institute of Technology, is designed to monitor particulate matter in the air continuously to get a more complete picture of the changing chemical composition and physical properties of these aerosols. Notably, ASCENT instruments can document the presence of ammonium nitrate.

Using ASCENT, Ward and his collaborators measured high levels of ammonium nitrate in the air. The chemical is formed after NO_x molecules are converted into nitric acid, which—in the presence of ammonia—creates ammonium nitrate. Though NO_x levels have declined, the researchers theorize that a complex chemical process involving increased nighttime levels of ozone leads to the persistently high levels of ammonium nitrate, indicating the need to continue to reduce NO_x precursors to ammonium nitrate formation.

The aerosols that make up smog can be categorized as either organic or inorganic. Organic aerosols are difficult to mitigate because they come from myriad sources, but the sources of an inorganic molecule like ammonium nitrate—NO_x-burning fossil fuels—are more straightforward to address. The work thus suggests the importance of powering cars, trucks, and gas-driven appliances like lawn mowers with electricity and continuing to monitor air quality with sensors that can capture the full chemical makeup of the aerosols.

"Measurements like the air-quality index, or AQI, don't tell you the composition of the smog, what it's made of," Wennberg says. "It's important for us to know the breakdown of chemicals in the air. Now that we understand what a major component of ammonium nitrate is, we can develop specific strategies to reduce it."