Surprisingly robust photochemistry in subarctic particles during winter: evidence from photooxidants

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This work is licensed under a Creative Commons Attribution 4.0 International License.

Authors

Laura M. D. Heinlein, University of California, Davis
Junwei He, University of California, Davis
Michael Oluwatoyin Sunday, University of California, Davis
Fangzhou Guo, University of Houston
James Campbell, University of Alaska, Fairbanks
Allison Moon, University of Washington
Sukriti Kapur, University of California, Irvine
Ting Fang, University of California, Irvine
Kasey Edwards, University of California, Irvine
Meeta Cesler-Maloney, University of Alaska
Alyssa J. Burns, University of California, Davis
Jack E. Dibb, University of New Hampshire, DurhamFollow
William Simpson, University of Alaska
Manabu Shiraiwa, University of California, Irvine
Becky Alexander, University of Washington
Jingqiu Mao, University of Alaska
James H. Flynn III, University of Houston
Jochen Stutz, University of California, Los Angeles
Cort Anastasio, University of California, Davis

Abstract

Subarctic cities notoriously experience severe winter pollution episodes with fine particle (PM2.5) concentrations above 35 µg m−3, the US Environmental Protection Agency (EPA) 24 h standard. While winter sources of primary particles in Fairbanks, Alaska, have been studied, the chemistry driving secondary particle formation is elusive. Biomass burning is a major source of wintertime primary particles, making the PM2.5 rich in light-absorbing brown carbon (BrC). When BrC absorbs sunlight, it produces photooxidants – reactive species potentially important for secondary sulfate and secondary organic aerosol formation – yet photooxidant measurements in high-latitude PM2.5 remain scarce. During the winter of 2022 Alaskan Layered Pollution And Chemical Analysis (ALPACA) field campaign in Fairbanks, we collected PM filters, extracted the filters into water, and exposed the extracts to simulated sunlight to characterize the production of three photooxidants: oxidizing triplet excited states of BrC, singlet molecular oxygen, and hydroxyl radical. Next, we used our measurements to model photooxidant production in highly concentrated aerosol liquid water. While conventional wisdom indicates photochemistry is limited during high-latitude winters, we find that BrC photochemistry is significant: we predict high triplet and singlet oxygen daytime particle concentrations up to and  M, respectively, with moderate hydroxyl radical concentrations up to  M. Although our modeling predicts that triplets account for 0.4 %–10 % of daytime secondary sulfate formation, particle photochemistry cumulatively dominates, generating 76 % of daytime secondary sulfate formation, largely due to in-particle hydrogen peroxide, which contributes 25 %–54 %. Finally, we estimate triplet production rates year-round, revealing the highest rates in late winter when Fairbanks experiences severe pollution and in summer when wildfires generate BrC.

Department

Earth Systems Research Center

Publication Date

8-29-2025

Journal Title

Atmospheric Chemistry and Physics

Publisher

EGU

Digital Object Identifier (DOI)

https://dx.doi.org/10.5194/acp-25-9561-2025

Document Type

Article

Recommended Citation

Heinlein, L. M. D., He, J., Sunday, M. O., Guo, F., Campbell, J., Moon, A., Kapur, S., Fang, T., Edwards, K., Cesler-Maloney, M., Burns, A. J., Dibb, J., Simpson, W., Shiraiwa, M., Alexander, B., Mao, J., Flynn III, J. H., Stutz, J., and Anastasio, C.: Surprisingly robust photochemistry in subarctic particles during winter: evidence from photooxidants, Atmos. Chem. Phys., 25, 9561–9581, https://doi.org/10.5194/acp-25-9561-2025, 2025.

Rights

© Author(s) 2025.

Comments

This is an open access article published by EGU in Atmospheric Chemistry and Physics in 2025, available online: https://dx.doi.org/10.5194/acp-25-9561-2025