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
Received: 24 Aug 2010 – Published in Atmos. Chem. Phys. Discuss.: 20 Sep 2010
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.
Revised: 29 Jan 2011 – Accepted: 01 Feb 2011 – Published: 15 Feb 2011
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.
Wolfe, G. M., Thornton, J. A., Bouvier-Brown, N. C., Goldstein, A. H., Park, J.-H., McKay, M., Matross, D. M., Mao, J., Brune, W. H., LaFranchi, B. W., Browne, E. C., Min, K.-E., Wooldridge, P. J., Cohen, R. C., Crounse, J. D., Faloona, I. C., Gilman, J. B., Kuster, W. C., de Gouw, J. A., Huisman, A., and Keutsch, F. N.: The Chemistry of Atmosphere-Forest Exchange (CAFE) Model – Part 2: Application to BEARPEX-2007 observations, Atmos. Chem. Phys., 11, 1269-1294, doi:10.5194/acp-11-1269-2011, 2011.