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\begin{document}

\title{Investigating the Understanding of Oxidation Chemistry Using 20 Years of
Airborne OH and HO2 Observations}



\author[1,1]{David O Miller}%
\author[1,1]{William H. Brune}%
\affil[1]{Pennsylvania State University}%


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  \date{May 26, 2024}


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\begin{abstract}
Hydroxyl (OH) and hydroperoxyl (HO\textsubscript{2}) drive the
atmosphere's oxidation of gases emitted from Earth's surface and the
formation and aging of aerosol particles. Thus, understanding OH and
HO\textsubscript{2} chemistry is essential for examining the impact of
human activity on future atmospheric composition and climate. Using the
OH and HO\textsubscript{2} dataset collected with the Penn State
Airborne Tropospheric Hydrogen Oxides Sensor (ATHOS) during nine
aircraft missions over the past 20 years, we compare observed OH and
HO\textsubscript{2} to that modeled using the same near-explicit
photochemical box model. In general, the agreement is well within the
uncertainties of the observations and models, even when the model is
constrained only with a common data set of simultaneous measurements.
However, in regions influenced by anthropogenic or biogenic volatile
organic compounds, the model chemical mechanism and size of the data set
of constraining measurements do matter. In cleaner regions, the
differences between observed and modeled OH and HO\textsubscript{2}
found in previous studies generally remain and do not appear to be
systematic, indicating that the differences are driven by measurement
issues for ATHOS and/or other instruments. Thus, these comparisons
indicate that the oxidation chemistry in most of the free troposphere is
understood to as well as current measurements can determine. The focus
of future research needs to be on regions rich in volatile organic
compounds, where observed-to-modeled differences are more persistent,
and on improving measurement consistency.%
\end{abstract}%



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