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\begin{document}

\title{Neutral tropical African CO2 exchange estimated from aircraft and
satellite observations}



\author[1]{Benjamin Gaubert}%
\author[2]{Britton B. Stephens}%
\author[3]{David F. Baker}%
\author[4]{Sourish Basu}%
\author[5]{Michael Bertolacci}%
\author[6]{Kevin W. Bowman}%
\author[2]{Rebecca R Buchholz}%
\author[7]{Abhishek Chatterjee}%
\author[8]{Frederic Chevallier}%
\author[9]{Roisin Commane}%
\author[10]{Noel Cressie}%
\author[11]{Feng Deng}%
\author[3]{Nicole Jacobs}%
\author[12]{Matthew S. Johnson}%
\author[13]{Shamil Maksyutov}%
\author[14]{Kathryn McKain}%
\author[15]{Junjie Liu}%
\author[16]{Zhiqiang Liu}%
\author[17]{Eric Morgan}%
\author[3]{Christopher O'Dell}%
\author[18]{Sajeev Philip}%
\author[19]{Eric A Ray}%
\author[15]{David Schimel}%
\author[3]{Andrew E. Schuh}%
\author[3]{Thomas E. Taylor}%
\author[20]{Brad Weir}%
\author[21]{Dave van Wees}%
\author[22]{Steven C. C. Wofsy}%
\author[5]{Andrew Zammit-Mangion}%
\author[23]{Ning Zeng}%
\affil[1]{National Center for Atmospheric Research (NCAR)}%
\affil[2]{National Center for Atmospheric Research (UCAR)}%
\affil[3]{Colorado State University}%
\affil[4]{NASA GSFC / GMAO NASA Goddard Space Flight Center 8800 Greenbelt Road Code 610.1, Bldg 33, Rm G110 Greenbelt MD 20771 USA}%
\affil[5]{University of Wollongong}%
\affil[6]{Jet Propulsion Lab (NASA)}%
\affil[7]{NASA Jet Propulsion Laboratory}%
\affil[8]{Laboratoire des Sciences du Climat et de l'Environnement (LSCE)}%
\affil[9]{Columbia University}%
\affil[10]{NIASRA}%
\affil[11]{University of Toronto}%
\affil[12]{NASA Ames Research Center}%
\affil[13]{National Institute for Environmental Studies}%
\affil[14]{National Oceanic and Atmospheric Administration (NOAA)}%
\affil[15]{Jet Propulsion Laboratory}%
\affil[16]{Institute of Atmospheric Physics}%
\affil[17]{University of California, San Diego}%
\affil[18]{Indian Institute of Technology (IIT) Delhi}%
\affil[19]{NOAA Chemical Sciences Laboratory}%
\affil[20]{USRA / NASA Goddard}%
\affil[21]{Vrije Universiteit Amsterdam}%
\affil[22]{Harvard University}%
\affil[23]{University of Maryland, College Park}%


\vspace{-1em}



  
  \date{May 24, 2024}


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\selectlanguage{english}
\begin{abstract}
Tropical lands play an important role in the global carbon cycle yet
their contribution remains uncertain owing to sparse observations.
Satellite observations of atmospheric carbon dioxide
(CO\textsubscript{2}) have greatly increased spatial coverage over
tropical regions, providing the potential for improved estimates of
terrestrial fluxes. Despite this advancement, the spread among
satellite-based and in-situ atmospheric CO\textsubscript{2} flux
inversions over northern tropical Africa (NTA), spanning 0-24*N, remains
large. Satellite-based estimates of an annual source of 0.8-1.45 PgC
yr\textsuperscript{-1} challenge our understanding of tropical and
global carbon cycling. Here, we compare posterior mole fractions from
the suite of inversions participating in the Orbiting Carbon Observatory
2 (OCO-2) Version 10 Model Intercomparison Project (v10 MIP) with
independent in-situ airborne observations made over the tropical
Atlantic Ocean by the NASA Atmospheric Tomography (ATom) mission during
four seasons. We develop emergent constraints on tropical African
CO\textsubscript{2} fluxes using flux-concentration relationships
defined by the model suite. We find an annual flux of 0.14 \selectlanguage{ngerman}± 0.39 PgC
yr\selectlanguage{english}\textsuperscript{-1} (mean and standard deviation) for NTA, 2016-2018.
The satellite-based flux bias suggests a potential positive
concentration bias in OCO-2 B10 and earlier version retrievals over land
in NTA during the dry season. Nevertheless, the OCO-2 observations
provide improved flux estimates relative to the in situ observing
network at other times of year, indicating stronger uptake in NTA during
the wet season than the in-situ inversion estimates.%
\end{abstract}\selectlanguage{ngerman}%



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