“`html
The Los Angeles area has some of the most contaminated air in the United States, consistently failing to meet benchmarks established by the Environmental Protection Agency over the past decade. Recently, Caltech scientists have measured the concentrations of a smog component known as ammonium nitrate, a molecule that has been notoriously challenging to quantify, discovering significantly higher levels than previously estimated, particularly on the most polluted days.
These results underscore the necessity for ongoing reductions in fossil fuel emissions from vehicles, trucks, and various industrial operations, which generate the precursor to ammonium nitrate, known as nitric oxides or NOx.
The study was carried out 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 detailing the research, led by former graduate student Ryan X. Ward (PhD ’25), now a postdoctoral researcher at Columbia University, is set to appear on May 21 in the journal Science Advances.
The late Caltech professor Arie Haagen-Smit first connected air pollution with vehicle emissions and other forms of industrial fuel combustion during the 1950s. Since that time, the Los Angeles area has made considerable strides in reducing aerosols responsible for smog through initiatives such as banning sulfur combustion in fuels and requiring catalytic converters that eliminate NOx from vehicle emissions. Nevertheless, on certain days, smog limits visibility to just a few miles.
“In many respects, we’re redefining the extensive work Caltech has invested in atmospheric chemistry since Haagen-Smit,” Ward expresses. “By utilizing advanced techniques to address a long-standing issue, we reveal just how enduring inorganic aerosol smog remains in LA. Despite significant decreases in NOx emissions, our task is not yet complete.”
Ammonium nitrate has proven challenging to detect with conventional sensors; owing to its sensitivity to temperature and pressure, it can evaporate during sampling. In 2023, Caltech researchers commenced operating the Pico Rivera site of the Atmospheric Science and Chemistry mEasurement NeTwork (ASCENT), a network of scientific instruments nationwide measuring air quality. ASCENT, financed by the National Science Foundation (NSF) and overseen by Nga Lee (Sally) Ng (PhD ’07) at Georgia Institute of Technology, is aimed at continuously monitoring particulate matter in the air to provide a comprehensive understanding of the changing chemical composition and physical characteristics of these aerosols. Notably, ASCENT instruments can identify the presence of ammonium nitrate.
Using ASCENT, Ward and his team detected elevated levels of ammonium nitrate in the atmosphere. This chemical forms when NOx compounds convert into nitric acid and subsequently interact with ammonia to produce ammonium nitrate. Although NOx levels have diminished, the researchers hypothesize that a complex chemical reaction involving elevated ozone levels at night contributes to the persistently high concentrations of ammonium nitrate, highlighting the continued need to lower NOx precursors for ammonium nitrate synthesis.
The aerosols responsible for smog can be classified as either organic or inorganic. Organic aerosols are challenging to reduce due to their diverse sources, while the origins of inorganic molecules like ammonium nitrate—NOx from fossil fuel combustion—are more manageable to address. This research thus emphasizes the importance of transitioning vehicles, trucks, and gas-powered equipment like lawnmowers to electric options, as well as the ongoing monitoring of air quality with sensors capable of capturing the complete chemical profile of the aerosols.
“Metrics such as the air quality index, or AQI, fail to reveal the composition of smog, what it actually consists of,” Wennberg remarks. “It is crucial for us to determine the chemical breakdown in the atmosphere. Now that we recognize the significance of ammonium nitrate, we can formulate targeted strategies to diminish it.”
The paper is titled “Poorly quantified trends in ammonium nitrate remain critical to understand future urban aerosol control strategies.” Alongside Ward, Wennberg, and Flagan, Caltech co-authors include graduate student Haroula D. Baliaka (MS ’23); former graduate student Benjamin C. Schulze (PhD ’23); lead staff scientist John D. Crounse (PhD ’11); visitor in environmental science and engineering Sina Hasheminassab; and John H. Seinfeld, the Louis E. Nohl Professor of Chemical Engineering, Emeritus. Besides Ng, additional co-authors are Gaige H. Kerr from George Washington University; Roya Bahreini (PhD ’05) of UC Riverside, Ann M. Dillner of UC Davis, and Armistead Russell (PhD ’85) at Georgia Institute of Technology. Funding was supplied by the Resnick Sustainability Institute at Caltech, the Onassis Foundation, NSF, the National Oceanic and Atmospheric Administration, the California Air Resources Board, and the Ronald and Maxine Linde Center for Global Environmental Science at Caltech.
“`