Complex Effects of Reduced Mobile Source Emissions on Submicron Particulate Matter Concentrations in Los Angeles
Creators
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Schulze, Benjamin C.1
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Kenseth, Christopher M.1
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Ward, Ryan X.1
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Pennington, Elyse A.1
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Seltzer, Karl M.2
- Van Rooy, Paul3
- Tasnia, Afsara3
- Barletta, Barbara4
- Meinardi, Simone4
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Ehrenfels, Melissa A.5, 6
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Jensen, Andrew R.5, 6
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Huang, Yuanlong1
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Parker, Harrison A.1
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Hasheminassab, Sina1
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Day, Douglas A.5, 6
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Campuzano-Jost, Pedro5, 6
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de Gouw, Joost5, 6
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Jimenez, Jose L.5, 6
- Blake, Donald R.4
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Barsanti, Kelley C.3
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Pye, Havala O. T.2
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Crounse, John D.1
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Wennberg, Paul O.1
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Seinfeld, John H.1
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1.
California Institute of Technology
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2.
Environmental Protection Agency
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3.
University of California, Riverside
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4.
University of California, Irvine
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5.
Cooperative Institute for Research in Environmental Sciences
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6.
University of Colorado Boulder
Abstract
Despite considerable reductions in mobile source emissions, annual average aerosol concentrations measured in Los Angeles using Federal Reference Methods (FRM) have not appreciably declined over the past decade. Here, we use submicron aerosol measurements and zero-dimensional modeling to quantify the impacts of these emission reductions on aerosol formation in Pasadena, CA, during the late spring and summer of 2022. Reductions in secondary organic aerosol (SOA) concentrations expected from reduced mobile source emissions appear to have been largely offset by increases in hydroxyl radical concentrations, an indirect effect of reduced nitrogen oxide (NOx) emissions. As a result, while the predicted contribution of mobile sources to the SOA burden has declined from ∼50% in 2010 to only ∼25% in 2022, concentrations of locally formed SOA have remained relatively constant. In contrast, reductions in mobile source NOx emissions have likely reduced overnight production of nitric acid and ammonium nitrate (AN) aerosol. We provide indirect evidence that FRM measurements may have failed to capture the reduction in AN since 2010 due to the evaporation of semivolatile species from FRM filter samples. Our results suggest that given the effectiveness of historical regulatory efforts aimed at mobile sources, and on-road sources in particular, additional reductions in submicron aerosol concentrations in Los Angeles will likely require increased focus on abating emissions from nonroad and area sources.
Copyright and License
© 2025 American Chemical Society.
Acknowledgement
We thank Nga Lee Ng, Brian McDonald, Carsten Warneke, and Michael Kleeman for helpful discussions. We also thank the Caltech Facilities Department, and in particular Raul Turcios, for their help during the campaign. This research was funded by the NOAA Climate Program Office’s Oceanic and Atmospheric Research Program (Grants NA21OAR4310224 and NA21OAR4310222) and California Air Resources Board (Contract No. 21RD017). The University of Colorado groups were supported by NSF AGS (Grant 2206655), NASA (Grants 80NSSC21K1451 and 80NSSC23K0828), and a CIRES Innovative Research Project. We thank the Resnick Sustainability Institute for funding the criteria pollutant instrumentation on the Caltech campus. The views expressed in this article are those of the authors and do not necessarily reflect the views or policies of the U.S. EPA. NOAA, or the California Air Resources Board.
Supplemental Material
Further descriptions of analytical methods, statistical analyses, and model architectures and parametrizations and of analyses that support the primary arguments, along with associated figures (PDF)
Files
ea5c00199_si_001.pdf
Additional details
Funding
- National Oceanic and Atmospheric Administration
- NA21OAR4310224
- National Oceanic and Atmospheric Administration
- NA21OAR4310222
- California Air Resources Board
- 21RD017
- National Science Foundation
- 2206655
- National Aeronautics and Space Administration
- 80NSSC21K1451
- National Aeronautics and Space Administration
- 80NSSC23K0828
- Cooperative Institute for Research in Environmental Sciences
Dates
- Submitted
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2025-06-17
- Accepted
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2025-12-05
- Available
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2025-12-31Published online