Articles | Volume 11, issue 3
Atmos. Chem. Phys., 11, 1269–1294, 2011

Special issue: Biosphere Effects on Aerosols and Photochemistry Experiment:...

Atmos. Chem. Phys., 11, 1269–1294, 2011

Research article 15 Feb 2011

Research article | 15 Feb 2011

The Chemistry of Atmosphere-Forest Exchange (CAFE) Model – Part 2: Application to BEARPEX-2007 observations

G. M. Wolfe1,2, J. A. Thornton2, N. C. Bouvier-Brown3,*, A. H. Goldstein3, J.-H. Park3, M. McKay3,**, D. M. Matross3,***, J. Mao4,****, W. H. Brune4, B. W. LaFranchi5,*****, E. C. Browne5, K.-E. Min5, P. J. Wooldridge5, R. C. Cohen5, J. D. Crounse6, I. C. Faloona7, J. B. Gilman8,9, W. C. Kuster8, J. A. de Gouw8,9, A. Huisman10, and F. N. Keutsch10 G. M. Wolfe et al.
  • 1Department of Chemistry, University of Washington, Seattle, WA, USA
  • 2Department of Atmospheric Sciences, University of Washington, Seattle, WA, USA
  • 3Department of Environmental Science, Policy, and Management, University of California, Berkeley, CA, USA
  • 4Department of Meteorology, Pennsylvania State University, University Park, PA, USA
  • 5Department of Chemistry, University of California, Berkeley, CA, USA
  • 6Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA
  • 7Department of Land, Air and Water Resources, University of California, Davis, CA, USA
  • 8NOAA Earth System Research Laboratory, Boulder, CO, USA
  • 9Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA
  • 10Department of Chemistry, University of Wisconsin, Madison, WI, USA
  • *now at: Chemistry and Biochemistry Department, Loyola Marymount University, Los Angeles, CA, USA
  • **now at: California Air Resources Board, Sacramento, CA, USA
  • ***now at: KEMA, Inc., Oakland, CA, USA
  • ****now at: School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA
  • *****now at: Center for Accelerator Mass Spectrometry (CAMS), Lawrence Livermore National Lab, Livermore, CA, USA

Abstract. In a companion paper, we introduced the Chemistry of Atmosphere-Forest Exchange (CAFE) model, a vertically-resolved 1-D chemical transport model designed to probe the details of near-surface reactive gas exchange. Here, we apply CAFE to noontime observations from the 2007 Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX-2007). In this work we evaluate the CAFE modeling approach, demonstrate the significance of in-canopy chemistry for forest-atmosphere exchange and identify key shortcomings in the current understanding of intra-canopy processes.

CAFE generally reproduces BEARPEX-2007 observations but requires an enhanced radical recycling mechanism to overcome a factor of 6 underestimate of hydroxyl (OH) concentrations observed during a warm (~29 °C) period. Modeled fluxes of acyl peroxy nitrates (APN) are quite sensitive to gradients in chemical production and loss, demonstrating that chemistry may perturb forest-atmosphere exchange even when the chemical timescale is long relative to the canopy mixing timescale. The model underestimates peroxy acetyl nitrate (PAN) fluxes by 50% and the exchange velocity by nearly a factor of three under warmer conditions, suggesting that near-surface APN sinks are underestimated relative to the sources. Nitric acid typically dominates gross dry N deposition at this site, though other reactive nitrogen (NOy) species can comprise up to 28% of the N deposition budget under cooler conditions. Upward NO2 fluxes cause the net above-canopy NOy flux to be ~30% lower than the gross depositional flux. CAFE under-predicts ozone fluxes and exchange velocities by ~20%. Large uncertainty in the parameterization of cuticular and ground deposition precludes conclusive attribution of non-stomatal fluxes to chemistry or surface uptake. Model-measurement comparisons of vertical concentration gradients for several emitted species suggests that the lower canopy airspace may be only weakly coupled with the upper canopy. Future efforts to model forest-atmosphere exchange will require a more mechanistic understanding of non-stomatal deposition and a more thorough characterization of in-canopy mixing processes.

Final-revised paper