Articles | Volume 15, issue 4
Atmos. Chem. Phys., 15, 1975–1993, 2015

Special issue: Uintah Basin Winter Ozone Studies (ACP/AMT inter-journal SI)

Atmos. Chem. Phys., 15, 1975–1993, 2015

Research article 24 Feb 2015

Research article | 24 Feb 2015

Investigation of secondary formation of formic acid: urban environment vs. oil and gas producing region

B. Yuan1,2, P. R. Veres1,2, C. Warneke1,2, J. M. Roberts1, J. B. Gilman1,2, A. Koss1,2, P. M. Edwards1,2,*, M. Graus1,2,**, W. C. Kuster1,2, S.-M. Li3, R. J. Wild1,2, S. S. Brown1, W. P. Dubé1,2, B. M. Lerner1,2, E. J. Williams1, J. E. Johnson4,5, P. K. Quinn5, T. S. Bates4,5, B. Lefer6, P. L. Hayes2,7,***, J. L. Jimenez2,7, R. J. Weber8, R. Zamora1, B. Ervens1,2, D. B. Millet9, B. Rappenglück6, and J. A. de Gouw1,2,7 B. Yuan et al.
  • 1Chemical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO, USA
  • 2Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder, Boulder, CO, USA
  • 3Environment Canada, Science and Technology Branch, Toronto, ON, Canada
  • 4Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, WA, USA
  • 5NOAA Pacific Marine Environmental Laboratory (PMEL), Seattle, WA, USA
  • 6Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX, USA
  • 7Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, CO, USA
  • 8School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA
  • 9Department of Soil, Water and Climate, University of Minnesota, St. Paul, Minnesota, USA
  • *now at: Department of Chemistry, University of York, York, UK
  • **now at: Institute of Meteorology and Geophysics, University of Innsbruck, Innsbruck, Austria
  • ***now at: Université de Montréal, Department of Chemistry, Montreal, QC, Canada

Abstract. Formic acid (HCOOH) is one of the most abundant carboxylic acids in the atmosphere. However, current photochemical models cannot fully explain observed concentrations and in particular secondary formation of formic acid across various environments. In this work, formic acid measurements made at an urban receptor site (Pasadena) in June–July 2010 during CalNex (California Research at the Nexus of Air Quality and Climate Change) and a site in an oil and gas producing region (Uintah Basin) in January–February 2013 during UBWOS 2013 (Uintah Basin Winter Ozone Studies) will be discussed. Although the VOC (volatile organic compounds) compositions differed dramatically at the two sites, measured formic acid concentrations were comparable: 2.3 ± 1.3 in UBWOS 2013 and 2.0 ± 1.0 ppb in CalNex. We determine that concentrations of formic acid at both sites were dominated by secondary formation (> 99%). A constrained box model using the Master Chemical Mechanism (MCM v3.2) underestimates the measured formic acid concentrations drastically at both sites (by a factor of > 10). Compared to the original MCM model that includes only ozonolysis of unsaturated organic compounds and OH oxidation of acetylene, when we updated yields of ozonolysis of alkenes and included OH oxidation of isoprene, vinyl alcohol chemistry, reaction of formaldehyde with HO2, oxidation of aromatics, and reaction of CH3O2 with OH, the model predictions for formic acid were improved by a factor of 6.4 in UBWOS 2013 and 4.5 in CalNex, respectively. A comparison of measured and modeled HCOOH/acetone ratios is used to evaluate the model performance for formic acid. We conclude that the modified chemical mechanism can explain 19 and 45% of secondary formation of formic acid in UBWOS 2013 and CalNex, respectively. The contributions from aqueous reactions in aerosol and heterogeneous reactions on aerosol surface to formic acid are estimated to be 0–6 and 0–5% in UBWOS 2013 and CalNex, respectively. We observe that air–snow exchange processes and morning fog events may also contribute to ambient formic acid concentrations during UBWOS 2013 (~ 20% in total). In total, 53–59 in UBWOS 2013 and 50–55% in CalNex of secondary formation of formic acid remains unexplained. More work on formic acid formation pathways is needed to reduce the uncertainties in the sources and budget of formic acid and to narrow the gaps between measurements and model results.

Short summary
In this work, secondary formation of formic acid at an urban site and a site in an oil and gas production region is studied. We investigated various gas phase formation pathways of formic acid, including those recently proposed, using a box model. The contributions from aerosol-related processes, fog events and air-snow exchange to formic acid are also quantified.
Final-revised paper