Constraining remote oxidation capacity with ATom observations
Authors: Travis Katherine R. , Heald Colette L. , Allen Hannah M. , Apel Eric C. , Arnold Stephen R. , Blake Donald R. , Brune William H. , Chen Xin , Commane Róisín , Crounse John D. , Daube Bruce C. , Diskin Glenn S. , Elkins James W. , Evans Mathew J. , Hall Samuel R. , Hintsa Eric J. , Hornbrook Rebecca S. , Kasibhatla Prasad S. , Kim Michelle J. , Luo Gan , McKain Kathryn , Millet Dylan B. , Moore Fred L. , Peischl Jeffrey , Ryerson Thomas B. , Sherwen Tomás , Thames Alexander B. , Ullmann Kirk , Wang Xuan , Wennberg Paul O. , Wolfe Glenn M. , Yu Fangqun 
Published: July 2020

Abstract. The global oxidation capacity, defined as the tropospheric mean concentration
of the hydroxyl radical (OH), controls the lifetime of reactive trace gases
in the atmosphere such as methane and carbon monoxide (CO). Models tend to
underestimate the methane lifetime and CO concentrations throughout the
troposphere, which is consistent with excessive OH. Approximately half of the
oxidation of methane and non-methane volatile organic compounds (VOCs) is
thought to occur over the oceans where oxidant chemistry has received little validation due to a lack of observational constraints. We use observations
from the first two deployments of the NASA ATom aircraft campaign during
July–August 2016 and January–February 2017 to evaluate the oxidation
capacity over the remote oceans and its representation by the GEOS-Chem
chemical transport model. The model successfully simulates the magnitude and
vertical profile of remote OH within the measurement uncertainties.
Comparisons against the drivers of OH production (water vapor, ozone, and
NOy concentrations, ozone photolysis frequencies) also show
minimal bias, with the exception of wintertime NOy. The severe model overestimate of NOy during this period may
indicate insufficient wet scavenging and/or missing loss on sea-salt
aerosols. Large uncertainties in these processes require further study to
improve simulated NOy partitioning and removal in the
troposphere, but preliminary tests suggest that their overall impact could marginally reduce the model bias in tropospheric OH. During the ATom-1
deployment, OH reactivity (OHR) below 3 km is significantly enhanced, and
this is not captured by the sum of its measured components
(cOHRobs) or by the model (cOHRmod). This enhancement
could suggest missing reactive VOCs but cannot be explained by a
comprehensive simulation of both biotic and abiotic ocean sources of VOCs.
Additional sources of VOC reactivity in this region are difficult to
reconcile with the full suite of ATom measurement constraints. The model
generally reproduces the magnitude and seasonality of cOHRobs but
underestimates the contribution of oxygenated VOCs, mainly acetaldehyde,
which is severely underestimated throughout the troposphere despite its
calculated lifetime of less than a day. Missing model acetaldehyde in
previous studies was attributed to measurement uncertainties that have been
largely resolved. Observations of peroxyacetic acid (PAA) provide new support
for remote levels of acetaldehyde. The underestimate in both model
acetaldehyde and PAA is present throughout the year in both hemispheres and
peaks during Northern Hemisphere summer. The addition of ocean sources of
VOCs in the model increases cOHRmod by 3 % to 9 % and
improves model–measurement agreement for acetaldehyde, particularly in winter, but cannot resolve the model summertime bias. Doing so would require
100 Tg yr−1 of a long-lived unknown precursor throughout the year with
significant additional emissions in the Northern Hemisphere summer. Improving
the model bias for remote acetaldehyde and PAA is unlikely to fully resolve
previously reported model global biases in OH and methane lifetime, suggesting that future work should examine the sources and sinks of OH over land.