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HCOOH in the Remote Atmosphere: Constraints from Atmospheric Tomography (ATom) Airborne Observations

Chen, Xin and Millet, Dylan B. and Neuman, J. Andrew and Veres, Patrick R. and Ray, Eric A. and Commane, Róisín and Daube, Bruce C. and McKain, Kathryn and Schwarz, Joshua P. and Katich, Joseph M. and Froyd, Karl D. and Schill, Gregory P. and Kim, Michelle J. and Crounse, John D. and Allen, Hannah M. and Apel, Eric C. and Hornbrook, Rebecca S. and Blake, Donald R. and Nault, Benjamin A. and Campuzano-Jost, Pedro and Jimenez, Jose L. and Dibb, Jack E. (2021) HCOOH in the Remote Atmosphere: Constraints from Atmospheric Tomography (ATom) Airborne Observations. ACS Earth and Space Chemistry, 5 (6). pp. 1436-1454. ISSN 2472-3452. PMCID PMC8216292. doi:10.1021/acsearthspacechem.1c00049.

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Formic acid (HCOOH) is an important component of atmospheric acidity but its budget is poorly understood, with prior observations implying substantial missing sources. Here, we combine pole-to-pole airborne observations from the Atmospheric Tomography Mission (ATom) with a chemical transport model (GEOS-Chem CTM) and back-trajectory analyses to provide the first global in situ characterization of HCOOH in the remote atmosphere. ATom reveals sub-100 ppt HCOOH concentrations over most of the remote oceans, punctuated by large enhancements associated with continental outflow. Enhancements correlate with known combustion tracers and trajectory-based fire influences. The GEOS-Chem model underpredicts these in-plume HCOOH enhancements, but elsewhere, we find no broad indication of a missing HCOOH source in the background free troposphere. We conclude that missing nonfire HCOOH precursors inferred previously are predominantly short-lived. We find indications of a wet scavenging underestimate in the model consistent with a positive HCOOH bias in the tropical upper troposphere. Observations reveal episodic evidence of ocean HCOOH uptake, which is well-captured by GEOS-Chem; however, despite its strong seawater undersaturation, HCOOH is not consistently depleted in the remote marine boundary layer. Over 50 fire and mixed plumes were intercepted during ATom with widely varying transit times and source regions. HCOOH:CO-normalized excess mixing ratios in these plumes range from 3.4 to >50 ppt/ppb CO and are often over an order of magnitude higher than expected primary emission ratios. HCOOH is thus a major reactive organic carbon reservoir in the aged plumes sampled during ATom, implying important missing pathways for in-plume HCOOH production.

Item Type:Article
Related URLs:
URLURL TypeDescription CentralArticle
Chen, Xin0000-0002-0952-0008
Millet, Dylan B.0000-0003-3076-125X
Ray, Eric A.0000-0001-8727-9849
Commane, Róisín0000-0003-1373-1550
McKain, Kathryn0000-0002-8323-5758
Froyd, Karl D.0000-0002-0797-6028
Schill, Gregory P.0000-0002-4084-0317
Kim, Michelle J.0000-0002-4922-4334
Crounse, John D.0000-0001-5443-729X
Allen, Hannah M.0000-0002-4218-5133
Apel, Eric C.0000-0001-9421-818X
Hornbrook, Rebecca S.0000-0002-6304-6554
Blake, Donald R.0000-0002-8283-5014
Nault, Benjamin A.0000-0001-9464-4787
Campuzano-Jost, Pedro0000-0003-3930-010X
Jimenez, Jose L.0000-0001-6203-1847
Dibb, Jack E.0000-0003-3096-7709
Additional Information:© 2021 American Chemical Society. Received: February 17, 2021; Revised: April 29, 2021; Accepted: April 30, 2021; Published: May 13, 2021. This research was primarily supported by the National Aeronautics and Space Administration (Grant NNX14AP89G). We thank Armin Wisthaler and Hanwant Singh for their support of this project. Computing resources were provided by the Minnesota Supercomputing Institute ( at the University of Minnesota. We acknowledge the contributions of Steven Wofsy (Harvard QCLS and ATom CO.X), Daniel Murphy (NOAA CSL PALMS), Paul Wennberg (CIT-CIMS), Bernadett Weinzierl (University of Vienna CAPS), Glenn Wolfe and Thomas Hanisco (NASA GSFC ISAF), Thomas Ryerson (NOAA CSL NO_yO₃), and Robert Talbot (NASA GTE MC/IC). We thank Gordon Novak, Jeffery Pierce, and Anna Hodshire for helpful discussions. ATom was funded by the NASA Earth Venture program through Grant NNX15AJ23G. CIRES researchers (J.A.N., E.A.R., J.M.K., K.D.F., and G.P.S.) acknowledge support from NOAA Cooperative Agreement NA17OAR4320101. NCAR researchers (E.C.A. and R.S.H.) acknowledge support from the National Center for Atmospheric Research, which is a major facility sponsored by the National Science Foundation under Cooperative Agreement 1852977. The CU Boulder HR-AMS team (B.A.N., P.C.-J., and J.L.J.) were supported by NASA Grants NNX15AH33A, 80NSSC19K0124, and 80NSSC18K0630. The authors declare no competing financial interest.
Funding AgencyGrant Number
National Oceanic and Atmospheric Administration (NOAA)NA17OAR4320101
National Center for Atmospheric Research (NCAR)UNSPECIFIED
Subject Keywords:formic acid, remote atmosphere, fire, deposition, Atmospheric Tomography Mission, iodide CIMS, chemical transport model, back trajectory
Issue or Number:6
PubMed Central ID:PMC8216292
Record Number:CaltechAUTHORS:20210518-081920794
Persistent URL:
Official Citation:HCOOH in the Remote Atmosphere: Constraints from Atmospheric Tomography (ATom) Airborne Observations. Xin Chen, Dylan B. Millet, J. Andrew Neuman, Patrick R. Veres, Eric A. Ray, Róisín Commane, Bruce C. Daube, Kathryn McKain, Joshua P. Schwarz, Joseph M. Katich, Karl D. Froyd, Gregory P. Schill, Michelle J. Kim, John D. Crounse, Hannah M. Allen, Eric C. Apel, Rebecca S. Hornbrook, Donald R. Blake, Benjamin A. Nault, Pedro Campuzano-Jost, Jose L. Jimenez, and Jack E. Dibb. ACS Earth and Space Chemistry 2021 5 (6), 1436-1454; DOI: 10.1021/acsearthspacechem.1c00049
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:109165
Deposited By: Tony Diaz
Deposited On:19 May 2021 18:57
Last Modified:12 Jul 2022 20:01

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