Preprints
https://doi.org/10.5194/ar-2026-12
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/ar-2026-12
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
20 Mar 2026
| 20 Mar 2026
Status: a revised version of this preprint was accepted for the journal AR and is expected to appear here in due course.
Measurements and modeling of urban secondary organic aerosol formation potential as a function of precursor volatility class in the Los Angeles area during summer 2022
Abstract. Urban secondary organic aerosol (SOA) contributes to degraded air quality which can affect human health. Improvements in Los Angeles (LA), CA air quality have mainly plateaued since 2010. In summer 2022, measurements were made to quantify the SOA formation potential (SOA-FP) in ambient LA air. Two oxidation flow reactors (OFRs) ingested ambient air: one was equipped with an electrically conductive polymer inlet that denuded lower volatility species, and the other was run without an inlet. This allowed the separate quantification of SOA-FP from higher vs. lower volatility precursors. To our knowledge these are the first direct measurements of these fractions. Measured ambient SOA was similar and total SOA-FP was lower in 2022 vs. 2010, consistent with higher ambient OH causing greater consumptions of SOA precursors in 2022. The dual-OFR measurements suggest ~31 % of the total SOA-FP is due to compounds with volatilities in the SVOC and lower IVOC ranges. Results are compared to two box models: one based on CRACMM and the other adapted from a recent Caltech publication. CRACMM predicted ambient OA well but underpredicted SOA-FP by about a factor of 2, while the Caltech model underestimated OA and overpredicted SOA-FP by a factor of 2.5. Our study finds terpenoids contribute to, but do not dominate, SOA-FP.
How to cite. Ehrenfels, M. A., Schulze, B. C., Jensen, A. R., Tasnia, A., Day, D. A., Campuzano-Jost, P., Handschy, A. V., Schueneman, M. K., Yun, S., Kim, D., Sueper, D., Pye, H. O. T., Murphy, B. N., Skipper, T. N., Barsanti, K. C., de Gouw, J. A., and Jimenez, J. L.: Measurements and modeling of urban secondary organic aerosol formation potential as a function of precursor volatility class in the Los Angeles area during summer 2022, Aerosol Research Discuss. [preprint], https://doi.org/10.5194/ar-2026-12, in review, 2026.
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Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor
| : Report abuse
- RC1: 'Comment on ar-2026-12', Anonymous Referee #1, 21 Jun 2026
- RC2: 'Comment on ar-2026-12', Anonymous Referee #2, 23 Jun 2026
-
AC1: 'Comment on ar-2026-12', Melissa A. Ehrenfels, 25 Sep 2026
The comment was uploaded in the form of a supplement: https://ar.copernicus.org/preprints/ar-2026-12/ar-2026-12-AC1-supplement.pdf
Status: closed
Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor
| : Report abuse
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RC1: 'Comment on ar-2026-12', Anonymous Referee #1, 21 Jun 2026
This manuscript presents measurements of secondary organic aerosol formation potential (SOA-FP) in the Los Angeles basin during summer 2022 using a novel dual oxidation flow reactor (OFR) system. By employing an electrically conductive perfluoroalkoxy alkane inlet upstream of one OFR, the authors separate ambient precursors according to volatility and experimentally quantify the relative contributions of higher- and lower-volatility compounds to SOA formation. The measurements are further compared with predictions from two box models.
Overall, this is a well-written, carefully executed, and comprehensive study. The experimental approach is novel and represents an important advance in the direct characterization of SOA precursor volatility classes in ambient air. The manuscript is clearly structured, the methodology is described in sufficient detail, and the interpretation is generally well supported by the observations. I believe that this work is suitable for publication after the authors address the comments below.
- The manuscript discusses SOA formation primarily in terms of volatility classes. However, recent laboratory and field studies have demonstrated that rapid autoxidation and HOM formation can substantially enhance SOA yields even for relatively volatile precursors, especially monoterpenes and aromatic compounds. Given that OFRs can efficiently generate HOMs under low-NO conditions, could the authors comment on how autoxidation chemistry may influence the inferred SOA-FP distribution across volatility classes? This may be particularly relevant when comparing the measurements with mechanisms such as CRACMM that contain simplified autoxidation treatments.
- The manuscript attributes the lower SOA formation potential observed in 2022 relative to CalNex 2010 to a combination of reduced precursor concentrations and enhanced ambient OH exposure, which presumably leads to greater consumption of SOA precursors prior to sampling. However, increased atmospheric oxidation can also generate lower-volatility products that remain available for further SOA formation in the OFR. Could the authors discuss how multigenerational oxidation influences the interpretation of SOA-FP? Specifically, does increased photochemical age necessarily imply lower SOA-FP, or could aging shift the precursor distribution toward more oxygenated, lower-volatility species that are not fully captured by the present framework?
- One important aspect of the experimental design that may influence the interpretation of the results is the performance of the c-PFA inlet as a volatility filter. While the authors characterize the transmission efficiency, it remains unclear whether multifunctional compounds with moderate volatility and strong polarity may experience enhanced wall interactions beyond those predicted solely from volatility. Previous studies have shown substantial wall losses of oxygenated organics in sampling lines (e.g., Pagonis et al., 2017; Deming et al., 2019). Could the authors discuss whether differential losses of highly functionalized species could bias the inferred partitioning between higher and lower-volatility precursor classes?
- The comparison between model performance and observations provides useful insight. This study shows that CRACMM reproduces ambient OA reasonably well but substantially underpredicts SOA-FP, while the Caltech model exhibits the opposite behavior (underestimates OA and overestimates SOA-FP). Although the model–measurement discrepancies are clearly presented, it remains somewhat unclear which processes are primarily responsible for these biases. Could the authors further discuss whether the disagreement mainly arises from uncertainties in emissions, gas-phase oxidation chemistry, volatility distributions, or gas-particle partitioning or some other factors? Such discussion would really strengthen the implications for future model development.
- The present study attributes only a modest fraction of SOA-FP to terpenoids. However, because biogenic oxidation products can undergo extensive multigenerational oxidation and partition into lower-volatility bins, is it possible that some SOA-FP attributed to lower-volatility precursors originated from earlier terpenoid oxidation? How does the volatility-based framework distinguish between primary terpenoid precursors and their aged oxidation products? Also, the OFRs were operated in OFR185 mode at approximately two equivalent days of OH exposure under low-NO conditions. Since terpenoid oxidation chemistry and SOA yields are strongly dependent on NOx, it would be useful to discuss in more detail how representative the inferred terpenoid SOA-FP is of summertime atmospheric conditions in Los Angeles, where oxidation frequently occurs under moderate-to-high NOx conditions. Could the OFR operating conditions systematically bias the estimated contribution of terpenoids?
Citation: https://doi.org/10.5194/ar-2026-12-RC1 - RC2: 'Comment on ar-2026-12', Anonymous Referee #2, 23 Jun 2026
-
AC1: 'Comment on ar-2026-12', Melissa A. Ehrenfels, 25 Sep 2026
The comment was uploaded in the form of a supplement: https://ar.copernicus.org/preprints/ar-2026-12/ar-2026-12-AC1-supplement.pdf
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Melissa A. Ehrenfels
Department of Chemistry, University of Colorado Boulder, Boulder, CO 80309, USA
Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309, USA
Benjamin C. Schulze
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
currently at: Bain & Company, Denver, CO 80202, USA
Andrew R. Jensen
Department of Chemistry, University of Colorado Boulder, Boulder, CO 80309, USA
Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309, USA
currently at: Department of Chemistry and Biochemistry, Kalamazoo College, Kalamazoo, MI 49006, USA
Afsara Tasnia
Department of Chemical & Environmental Engineering, Center for Environmental Research & Technology, University of California-Riverside, Riverside, CA 92521, USA
currently at: South Coast Air Quality Management District, Diamond Bar, CA 91765, USA
Douglas A. Day
Department of Chemistry, University of Colorado Boulder, Boulder, CO 80309, USA
Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309, USA
Pedro Campuzano-Jost
Department of Chemistry, University of Colorado Boulder, Boulder, CO 80309, USA
Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309, USA
Anne V. Handschy
Department of Chemistry, University of Colorado Boulder, Boulder, CO 80309, USA
Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309, USA
Melinda K. Schueneman
Department of Chemistry, University of Colorado Boulder, Boulder, CO 80309, USA
Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309, USA
currently at: Office of Applied Science and Environmental Solutions, U.S. Environmental Protection Agency, Research Triangle Park, NC 27709, USA
Seonsik Yun
Department of Chemistry, University of Colorado Boulder, Boulder, CO 80309, USA
Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309, USA
Dongwook Kim
Department of Chemistry, University of Colorado Boulder, Boulder, CO 80309, USA
Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309, USA
Donna Sueper
Department of Chemistry, University of Colorado Boulder, Boulder, CO 80309, USA
Aerodyne Research Inc., Billerica, MA 01821, USA
Havala O. T. Pye
Office of Research and Development, U.S. Environmental Protection Agency, Research Triangle Park, NC 27709, USA
currently at: Office of Applied Science and Environmental Solutions, U.S. Environmental Protection Agency, Research Triangle Park, NC 27709, USA
Benjamin N. Murphy
Office of Research and Development, U.S. Environmental Protection Agency, Research Triangle Park, NC 27709, USA
currently at: Norwegian Meteorological Institute, 0371 Oslo, Norway
T. Nash Skipper
Office of Research and Development, U.S. Environmental Protection Agency, Research Triangle Park, NC 27709, USA
currently at: Office of Air and Radiation, U.S. Environmental Protection Agency, Research Triangle Park, NC 27709, USA
Kelley C. Barsanti
Department of Chemical & Environmental Engineering, Center for Environmental Research & Technology, University of California-Riverside, Riverside, CA 92521, USA
currently at: NSF National Center for Atmospheric Research, Boulder, CO 80305, USA
Joost A. de Gouw
Department of Chemistry, University of Colorado Boulder, Boulder, CO 80309, USA
Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309, USA
Jose L. Jimenez
CORRESPONDING AUTHOR
Department of Chemistry, University of Colorado Boulder, Boulder, CO 80309, USA
Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309, USA
Short summary
Los Angeles, CA suffers from poor air quality, partially due to secondary aerosol formation. The identity and sources of the most important gas phase precursors is a complex and unresolved issue. For the first time, we separated ambient gas-phase species by volatility, then measured their potential to form aerosols. We found that higher-volatility emissions are responsible for 2/3 of the aerosol formation, and lower volatility ones for 1/3. We evaluate two box models against our measurements.
Los Angeles, CA suffers from poor air quality, partially due to secondary aerosol formation. The...
Altmetrics
This manuscript presents measurements of secondary organic aerosol formation potential (SOA-FP) in the Los Angeles basin during summer 2022 using a novel dual oxidation flow reactor (OFR) system. By employing an electrically conductive perfluoroalkoxy alkane inlet upstream of one OFR, the authors separate ambient precursors according to volatility and experimentally quantify the relative contributions of higher- and lower-volatility compounds to SOA formation. The measurements are further compared with predictions from two box models.
Overall, this is a well-written, carefully executed, and comprehensive study. The experimental approach is novel and represents an important advance in the direct characterization of SOA precursor volatility classes in ambient air. The manuscript is clearly structured, the methodology is described in sufficient detail, and the interpretation is generally well supported by the observations. I believe that this work is suitable for publication after the authors address the comments below.