Eddy covariance fluxes of formic acid, HCOOH, were measured over a boreal forest canopy in spring/summer 2014. The HCOOH fluxes were bidirectional but mostly upward during daytime, in contrast to studies elsewhere that reported mostly downward fluxes. Downward flux episodes were explained well by modeled dry deposition rates. The sum of net observed flux and modeled dry deposition yields an upward “gross flux” of HCOOH, which could not be quantitatively explained by literature estimates of direct vegetative/soil emissions nor by efficient chemical production from other volatile organic compounds, suggesting missing or greatly underestimated HCOOH sources in the boreal ecosystem. We implemented a vegetative HCOOH source into the GEOS-Chem chemical transport model to match our derived gross flux and evaluated the updated model against airborne and spaceborne observations. Model biases in the boundary layer were substantially reduced based on this revised treatment, but biases in the free troposphere remain unexplained.
Bibliographical noteFunding Information:
We thank T. Vesala, P. Kolari, P. Keronen, E. Siivola, M. Kajos, and A. Manninen at U. Helsinki for helpful discussions and model and measurement data related to SMEAR II. We also thank J. de Gouw (NOAA ESRL), and the SENEX and TES science teams for providing observations, and P. Punttila (Ymp?rist?) and D.M. Sorger (NC State) for entomological insights. The University of Washington participated in the BAECC campaign with funds from the U.S. Department of Energy (DE-SC0006867). S. Schobesberger acknowledges support from the European Commission (OXFLUX, project 701958), D. Taipale from the European Regional Development Fund (Centre of Excellence EcolChange), and D. B. M. from NSF CAREER (1148951) and the Minnesota Supercomputing Institute. We thank K. Cady-Pereira (AER), M. Shephard (Environment Canada), and M. Luo (JPL) for developing TES HCOOH measurements, publicly available at http://tes.jpl.nasa.gov/data/. GEOS-Chem model code is available at www.geos-chem.org. SOSAA model output, the high-frequency HCOOH mixing ratio measurements by CIMS, and anemometer wind measurements are available at http://hdl.handle.net/1773/36867.
- biogenic emissions
- boreal forest
- chemical ionization mass spectrometry
- eddy covariance fluxes
- formic acid