Published September 15, 2023 | Published
Journal Article Open

OH, HO₂, and RO₂ radical chemistry in a rural forest environment: measurements, model comparisons, and evidence of a missing radical sink

  • 1. ROR icon Indiana University Bloomington
  • 2. ROR icon California Air Resources Board
  • 3. ROR icon Colorado Department of Public Health and Environment
  • 4. ROR icon University of Massachusetts Amherst
  • 5. ROR icon University of California, Berkeley
  • 6. ROR icon Smith College
  • 7. ROR icon Drexel University
  • 8. ROR icon University of Maryland, Baltimore County
  • 9. ROR icon University of Minnesota
  • 10. ROR icon Forschungszentrum Jülich
  • 11. ROR icon National Center for Atmospheric Research
  • 12. ROR icon University of Lille
  • 13. ROR icon University of Houston
  • 14. ROR icon Harvard University
  • 15. ROR icon Georgia Institute of Technology
  • 16. Boulder A.I.R. LLC, Boulder, CO 80305, USA
  • 17. ROR icon Institute of Arctic and Alpine Research
  • 18. ROR icon California Institute of Technology
  • 19. ROR icon Purdue University West Lafayette
  • 20. ROR icon University of California, San Diego
  • 21. ROR icon Stony Brook University
  • 22. ROR icon Western Michigan University

Abstract

The hydroxyl (OH), hydroperoxy (HO2), and organic peroxy (RO2) radicals play important roles in atmospheric chemistry. In the presence of nitrogen oxides (NOx), reactions between OH and volatile organic compounds (VOCs) can initiate a radical propagation cycle that leads to the production of ozone and secondary organic aerosols. Previous measurements of these radicals under low-NOx conditions in forested environments characterized by emissions of biogenic VOCs, including isoprene and monoterpenes, have shown discrepancies with modeled concentrations.

During the summer of 2016, OH, HO2, and RO2 radical concentrations were measured as part of the Program for Research on Oxidants: Photochemistry, Emissions, and Transport – Atmospheric Measurements of Oxidants in Summer (PROPHET-AMOS) campaign in a midlatitude deciduous broadleaf forest. Measurements of OH and HO2 were made by laser-induced fluorescence–fluorescence assay by gas expansion (LIF-FAGE) techniques, and total peroxy radical (XO2) mixing ratios were measured by the Ethane CHemical AMPlifier (ECHAMP) instrument. Supporting measurements of photolysis frequencies, VOCs, NOx, O3, and meteorological data were used to constrain a zero-dimensional box model utilizing either the Regional Atmospheric Chemical Mechanism (RACM2) or the Master Chemical Mechanism (MCM). Model simulations tested the influence of HOx regeneration reactions within the isoprene oxidation scheme from the Leuven Isoprene Mechanism (LIM1). On average, the LIM1 models overestimated daytime maximum measurements by approximately 40 % for OH, 65 % for HO2, and more than a factor of 2 for XO2. Modeled XO2 mixing ratios were also significantly higher than measured at night. Addition of RO2+ RO2 accretion reactions for terpene-derived RO2 radicals to the model can partially explain the discrepancy between measurements and modeled peroxy radical concentrations at night but cannot explain the daytime discrepancies when OH reactivity is dominated by isoprene. The models also overestimated measured concentrations of isoprene-derived hydroxyhydroperoxides (ISOPOOH) by a factor of 10 during the daytime, consistent with the model overestimation of peroxy radical concentrations. Constraining the model to the measured concentration of peroxy radicals improves the agreement with the measured ISOPOOH concentrations, suggesting that the measured radical concentrations are more consistent with the measured ISOPOOH concentrations. These results suggest that the models may be missing an important daytime radical sink and could be overestimating the rate of ozone and secondary product formation in this forest.

Copyright and License

© Author(s) 2023. This work is distributed under the Creative Commons Attribution 4.0 License.

Published by Copernicus Publications on behalf of the European Geosciences Union.

Acknowledgement

We thank Joe Sakowski for his assistance with the OH, HO2, and OH reactivity measurements. We also thank Krystal Vasquez, Eric Praske, John Crounse, and Paul Wennberg for their hard effort obtaining the ISOPOOH measurements; Deedee Montzka for assistance in obtaining the NOx measurements; all PROPHET-AMOS participants for making this work possible; and the University of Michigan Biological Station for hosting the field study.

Funding

This research has been supported by the National Science Foundation (grant nos. AGS-1440834, AGS-1827450, AGS-1443842, AGS-1719918, AGS-1561755, AGS-1643306, AGS-1932771, and AGS-1428257).

Contributions

BB, ML, YW, PR, and PSS were responsible for the LIF-FAGE measurements of OH, HO2, and OH reactivity. BD, MDR, DCA, and EW were responsible for the ECHAMP measurements of XO2. HDA and DBM were responsible for the PTR-MS measurements of VOCs and OVOCs. SD and TL were responsible for the GC measurements of VOCs and OVOCs. AW, GT, JO, and DM were responsible for the measurements of NO and NO2. WW and DH were responsible for the measurements of O3 at the Ameriflux tower. JF, ME, and SA were responsible for the measurements of photolysis frequencies and CO. JR, JS, and FK were responsible for the measurements of formaldehyde. HMA was responsible for the measurements of ISOPOOH. JHS and PBS were responsible for the IHN measurements. SB was responsible for coordination and preparation of the PROPHET site. BB, ML, YW, PR, and PSS conducted the analysis and photochemical modeling and wrote the paper with feedback from all co-authors.

Data Availability

Data presented in this study can be obtained from the authors upon request (pstevens@indiana.edu).

Supplemental Material

The supplement related to this article is available online at: https://doi.org/10.5194/acp-23-10287-2023-supplement.

Conflict of Interest

At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics. The peer-review process was guided by an independent editor, and the authors also have no other competing interests to declare.

Additional Information

This paper was edited by Lisa Whalley and reviewed by two anonymous referees.

Files

acp-23-10287-2023.pdf
Files (6.9 MB)
Name Size Download all
md5:d73ee31934fcd44d11db3abbdaca4694
954.0 kB Preview Download
md5:83a79030ff300f00f2132f6d1a590e5d
5.9 MB Preview Download

Additional details

Created:
April 7, 2025
Modified:
April 7, 2025