OH, HO2, and RO2 radical chemistry in a rural forest environment: measurements, model comparisons, and evidence of a missing radical sink
Authors: Bottorff Brandon , Lew Michelle M. , Woo Youngjun , Rickly Pamela , Rollings Matthew D. , Deming Benjamin , Anderson Daniel C. , Wood Ezra , Alwe Hariprasad D. , Millet Dylan B. , Weinheimer Andrew , Tyndall Geoff , Ortega John , Dusanter Sebastien , Leonardis Thierry , Flynn James , Erickson Matt , Alvarez Sergio , Rivera-Rios Jean C. , Shutter Joshua D. , Keutsch Frank , Helmig Detlev , Wang Wei , Allen Hannah M. , Slade Johnathan H. , Shepson Paul B. , Bertman Steven , Stevens Philip S. 
Published: September 2023

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.