Articles | Volume 9, issue 2
Atmos. Chem. Phys., 9, 615–634, 2009

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

Atmos. Chem. Phys., 9, 615–634, 2009

  27 Jan 2009

27 Jan 2009

Eddy covariance fluxes of acyl peroxy nitrates (PAN, PPN and MPAN) above a Ponderosa pine forest

G. M. Wolfe1, J. A. Thornton2, R. L. N. Yatavelli2, M. McKay3, A. H. Goldstein3, B. LaFranchi4, K.-E. Min4, and R. C. Cohen4 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 Chemistry, University of California, Berkeley, CA, USA

Abstract. During the Biosphere Effects on AeRosols and Photochemistry EXperiment 2007 (BEARPEX-2007), we observed eddy covariance (EC) fluxes of speciated acyl peroxy nitrates (APNs), including peroxyacetyl nitrate (PAN), peroxypropionyl nitrate (PPN) and peroxymethacryloyl nitrate (MPAN), above a Ponderosa pine forest in the western Sierra Nevada. All APN fluxes are net downward during the day, with a median midday PAN exchange velocity of −0.3 cm s−1; nighttime storage-corrected APN EC fluxes are smaller than daytime fluxes but still downward. Analysis with a standard resistance model shows that loss of PAN to the canopy is not controlled by turbulent or molecular diffusion. Stomatal uptake can account for 25 to 50% of the observed downward PAN flux. Vertical gradients in the PAN thermal decomposition (TD) rate explain a similar fraction of the flux, suggesting that a significant portion of the PAN flux into the forest results from chemical processes in the canopy. The remaining "unidentified" portion of the net PAN flux (~15%) is ascribed to deposition or reactive uptake on non-stomatal surfaces (e.g. leaf cuticles or soil). Shifts in temperature, moisture and ecosystem activity during the summer – fall transition alter the relative contribution of stomatal uptake, non-stomatal uptake and thermochemical gradients to the net PAN flux. Daytime PAN and MPAN exchange velocities are a factor of 3 smaller than those of PPN during the first two weeks of the measurement period, consistent with strong intra-canopy chemical production of PAN and MPAN during this period. Depositional loss of APNs can be 3–21% of the gross gas-phase TD loss depending on temperature. As a source of nitrogen to the biosphere, PAN deposition represents approximately 4–19% of that due to dry deposition of nitric acid at this site.

Final-revised paper