The chemistry of atmosphere-forest exchange (CAFE) model-part 2: Application to BEARPEX-2007 observations

G. M. Wolfe, J. A. Thornton, N. C. Bouvier-Brown, A. H. Goldstein, J. H. Park, M. McKay, D. M. Matross, J. Mao, W. H. Brune, B. W. LaFranchi, E. C. Browne, K. E. Min, P. J. Wooldridge, R. C. Cohen, J. D. Crounse, I. C. Faloona, J. B. Gilman, W. C. Kuster, J. A. De Gouw, A. HuismanF. N. Keutsch

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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.

Original languageEnglish (US)
Pages (from-to)1269-1294
Number of pages26
JournalAtmospheric Chemistry and Physics
Volume11
Issue number3
DOIs
StatePublished - 2011

All Science Journal Classification (ASJC) codes

  • Atmospheric Science

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    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., ... Keutsch, F. N. (2011). The chemistry of atmosphere-forest exchange (CAFE) model-part 2: Application to BEARPEX-2007 observations. Atmospheric Chemistry and Physics, 11(3), 1269-1294. https://doi.org/10.5194/acp-11-1269-2011