Articles | Volume 15, issue 11
Atmos. Chem. Phys., 15, 6283–6304, 2015
Atmos. Chem. Phys., 15, 6283–6304, 2015

Research article 09 Jun 2015

Research article | 09 Jun 2015

A large and ubiquitous source of atmospheric formic acid

D. B. Millet1, M. Baasandorj1, D. K. Farmer2, J. A. Thornton3, K. Baumann4, P. Brophy2, S. Chaliyakunnel1, J. A. de Gouw6,5, M. Graus*,6,5, L. Hu1,**, A. Koss6,5, B. H. Lee3, F. D. Lopez-Hilfiker3, J. A. Neuman6,5, F. Paulot7, J. Peischl6,5, I. B. Pollack6,5,***, T. B. Ryerson5, C. Warneke6,5, B. J. Williams8, and J. Xu9 D. B. Millet et al.
  • 1Department of Soil, Water, and Climate, University of Minnesota, Minneapolis–Saint Paul, MN 55108, USA
  • 2Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA
  • 3Department of Atmospheric Sciences, University of Washington, Seattle, WA 98195, USA
  • 4Atmospheric Research & Analysis Inc., Cary, NC 27513, USA
  • 5Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, CO 80305, USA
  • 6Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, USA
  • 7NOAA Geophysical Fluid Dynamics Laboratory, Princeton, NJ 08540, USA
  • 8Department of Energy, Environmental, and Chemical Engineering, Washington University in St. Louis, St. Louis, MO 63130, USA
  • 9Department of Physics and Atmospheric Science, Dalhousie University, Halifax, NS B3H 4R2, Canada
  • *now at: Institute of Meteorology and Geophysics, University of Innsbruck, 6020 Innsbruck, Austria
  • **now at: School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA
  • ***now at: Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523, USA

Abstract. Formic acid (HCOOH) is one of the most abundant acids in the atmosphere, with an important influence on precipitation chemistry and acidity. Here we employ a chemical transport model (GEOS-Chem CTM) to interpret recent airborne and ground-based measurements over the US Southeast in terms of the constraints they provide on HCOOH sources and sinks. Summertime boundary layer concentrations average several parts-per-billion, 2–3× larger than can be explained based on known production and loss pathways. This indicates one or more large missing HCOOH sources, and suggests either a key gap in current understanding of hydrocarbon oxidation or a large, unidentified, direct flux of HCOOH. Model-measurement comparisons implicate biogenic sources (e.g., isoprene oxidation) as the predominant HCOOH source. Resolving the unexplained boundary layer concentrations based (i) solely on isoprene oxidation would require a 3× increase in the model HCOOH yield, or (ii) solely on direct HCOOH emissions would require approximately a 25× increase in its biogenic flux. However, neither of these can explain the high HCOOH amounts seen in anthropogenic air masses and in the free troposphere. The overall indication is of a large biogenic source combined with ubiquitous chemical production of HCOOH across a range of precursors. Laboratory work is needed to better quantify the rates and mechanisms of carboxylic acid production from isoprene and other prevalent organics. Stabilized Criegee intermediates (SCIs) provide a large model source of HCOOH, while acetaldehyde tautomerization accounts for ~ 15% of the simulated global burden. Because carboxylic acids also react with SCIs and catalyze the reverse tautomerization reaction, HCOOH buffers against its own production by both of these pathways. Based on recent laboratory results, reaction between CH3O2 and OH could provide a major source of atmospheric HCOOH; however, including this chemistry degrades the model simulation of CH3OOH and NOx : CH3OOH. Developing better constraints on SCI and RO2 + OH chemistry is a high priority for future work. The model neither captures the large diurnal amplitude in HCOOH seen in surface air, nor its inverted vertical gradient at night. This implies a substantial bias in our current representation of deposition as modulated by boundary layer dynamics, and may indicate an HCOOH sink underestimate and thus an even larger missing source. A more robust treatment of surface deposition is a key need for improving simulations of HCOOH and related trace gases, and our understanding of their budgets.

Short summary
Formic acid (HCOOH) is an abundant atmospheric acid that affects precipitation chemistry and acidity. HCOOH measurements over the USA are 2-3× larger than can be explained by known sources and sinks, revealing a key gap in current understanding. Observations indicate a large biogenic source plus chemical production across a range of precursors. Model simulations cannot capture the HCOOH diurnal amplitude or nocturnal profile, implying a deposition bias and possibly even larger missing source.
Final-revised paper