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.
Bibliographical noteFunding Information:
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 ( https://www.msi.umn.edu ) 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 NOO), 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. y 3
© 2021 American Chemical Society.
- Atmospheric Tomography Mission
- back trajectory
- chemical transport model
- formic acid
- iodide CIMS
- remote atmosphere