OH, HO₂, and RO₂ radical chemistry in a rural forest environment: Measurements, model comparisons, and evidence of a missing radical sink
|Lew, Michelle M.
|Authors: - Brandon Bottorff, Michelle M. Lew, Youngjun Woo, Pamela Rickly, Matthew D. Rollings, Benjamin Demming, Daniel C. Anderson, Ezra Wood, Hariprasad D. Alwe, Dylan B. Millet , AndrewWeinheimer, Geoff Tyndall, John Ortega, Sebastien Dusanter, Thierry Leonardis, James Flynn, MattErickson, Sergio Alvare, Jean C. Rivera-Rios, Joshua D. Shutter, Frank Keutsch, Detlev Helmig,Wei Wang, Hannah M. Allen, Steven Bertman, and Philip S. Stevens
|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 mid-latitude deciduous broadleaf forest. Measurements of OH and HO₂ were made by laser-induced fluorescence – fluorescence assay by gas expansion techniques (LIF-FAGE) and total peroxy radical (XO₂) mixing ratios were measured by an ethane chemical amplification (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 two for XO₂. Modelled 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 modelled 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 ten 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.
|This study was supported by the National Science Foundation, grants AGS-1440834 and AGS-1827450 to Indiana University, grant AGS-1443842 to the University of Massachusetts Amherst, grant AGS-1719918 to Drexel University, grant AGS-1561755 to the University of Colorado, grant AGS-1643306 to Harvard University, and grants AGS-1932771 and AGS-1428257 to the University of Minnesota. 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, and all PROPHET-AMOS participants for making this work possible and the University of Michigan Biological Station for hosting the field study
|Bottorff, B.,et al. "OH, HO₂, and RO₂ radical chemistry in a rural forest environment: Measurements, model comparisons, and evidence of a missing radical sink" EGUsphere [preprint] (24 April, 2023). https://doi.org/10.5194/egusphere-2023-790, 2023.
|The University of Maryland, Baltimore County (UMBC)
|UMBC GESTAR II Collection
|UMBC Faculty Collection
|This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.
|Public Domain Mark 1.0
|OH, HO₂, and RO₂ radical chemistry in a rural forest environment: Measurements, model comparisons, and evidence of a missing radical sink
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