Poorly quantified trends in ammonium nitrate remain critical to understand future urban aerosol control strategies

Creators: Ward, Ryan X.¹; Baliaka, Haroula D.¹; Schulze, Benjamin C.¹; Kerr, Gaige H.²; Crounse, John D.¹; Hasheminassab, Sina^{1, 3}; Bahreini, Roya⁴; Dillner, Ann M.⁵; Russell, Armistead⁶; Ng, Nga L.⁶; Wennberg, Paul O.^{1, 7}; Flagan, Richard C.¹; Seinfeld, John H.¹

1. California Institute of Technology
2. George Washington University
3. Jet Propulsion Lab
4. University of California, Riverside
5. University of California, Davis
6. Georgia Institute of Technology
7. Environmental Defense Fund

Abstract

Despite decades of progress in reducing nitrogen oxide (NO_x) emissions, ammonium nitrate (AN) remains the primary inorganic component of particulate matter (PM) in Los Angeles (LA). Using aerosol mass spectrometry over multiple years in LA illustrates the controlling dynamics of AN and their evolution over the past decades. These data suggest that much of the nitric acid (HNO₃) production required to produce AN in LA occurs during the nighttime via heterogeneous hydrolysis of N₂O₅. Further, we show that US Environmental Protection Agency–codified techniques for measuring total PM_2.5 fail to quantify the AN component, while low-cost optical sensors demonstrate good agreement. While previous studies suggest that declining NO_x has reduced AN, we show that HNO₃ formation is still substantial and leads to the formation of many tens of micrograms per cubic meter of AN aerosol. Continued focus on reductions in NO_x will help meet the PM_2.5 standards in the LA basin and many other regions.

Copyright and License

© 2025 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution License 4.0 (CC BY).

Acknowledgement

We thank H. Pye for useful discussions using the CMAQ Equates data, NOAA CSL for the HNO₃ measurements during the RECAP campaign, CARB for the NH₃ measurements during the RECAP campaign, and the Megacities project for meteorological data.

Funding

R.X.W. acknowledges support from the Resnick Sustainability Institute Graduate Fellowship. H.D.B. acknowledges support from the Resnick Sustainability Institute and the Onassis Foundation - Scholarship ID: F ΖT 006-1/2023-2024. Funding for the ASCENT project is provided by the National Science Foundation (AGS-2131914). We thank NOAA (NA21OAR4310224) and the California Air Resources Board (21RD017 and 22RD015) for supporting observations and analysis of data obtained on the Caltech campus. Criteria air quality instrumentation at Caltech used in this analysis was supported by the Resnick Sustainability Institute and the Linde Center for Global Environmental Science.

Conflict of Interest

G.H.K. reports serving as a consultant for the Environmental Defense Fund, New York State Attorney General, Department of Justice, and California Air Resources Board. The authors declare that they have no other competing interests.

Supplemental Material

Supplementary Text; Figs. S1 to S23; References (PDF)

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