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Published May 23, 2025 | Version Published
Journal Article Open

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

Creators

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

Abstract

Despite decades of progress in reducing nitrogen oxide (NOx) 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 PM2.5 fail to quantify the AN component, while low-cost optical sensors demonstrate good agreement. While previous studies suggest that declining NOx 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 NOx will help meet the PM2.5 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)

Files

sciadv.adt8957.pdf

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Additional details

Identifiers

PMCID
PMC12094242

Related works

Has version
Journal Article: PMC12094242 (PMCID)
Is supplemented by
Supplemental Material: https://www.science.org/doi/suppl/10.1126/sciadv.adt8957/suppl_file/sciadv.adt8957_sm.pdf (URL)

Funding

Resnick Sustainability Institute
Alexander S. Onassis Foundation
F ΖT 006-1/2023-2024
National Science Foundation
AGS-2131914
National Oceanic and Atmospheric Administration
NA21OAR4310224
California Air Resources Board
21RD017
California Air Resources Board
22RD015

Dates

Accepted
2025-04-17
Available
2025-05-21
Published

Caltech Custom Metadata

Caltech groups
Division of Chemistry and Chemical Engineering (CCE), Division of Engineering and Applied Science (EAS), Division of Geological and Planetary Sciences (GPS), Resnick Sustainability Institute
Publication Status
Published