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During the Biosphere Effects onAeRosols and  PhotochemistryEXperiment 2007 (BEARPEX-2007), we observed eddy covariance (EC) fluxes ofspeciated acyl peroxy nitrates (APNs), including peroxyacetyl nitrate (PAN),peroxypropionyl nitrate (PPN) and peroxymethacryloyl nitrate (MPAN), above aPonderosa pine forest in the western Sierra Nevada. All APN fluxes are netdownward during the day, with a median midday PAN exchange velocity of −0.3 cm s<sup>−1</sup>;nighttime storage-corrected APN EC fluxes are smaller thandaytime fluxes but still downward. Analysis with a standard resistance modelshows that loss of PAN to the canopy is not controlled by turbulent ormolecular diffusion. Stomatal uptake can account for 25 to 50% of theobserved downward PAN flux. Vertical gradients in the PAN thermaldecomposition (TD) rate explain a similar fraction of the flux, suggestingthat a significant portion of the PAN flux into the forest results fromchemical processes in the canopy. The remaining "unidentified" portion ofthe net PAN flux (~15%) is ascribed to deposition or reactiveuptake on non-stomatal surfaces (e.g. leaf cuticles or soil). Shifts intemperature, moisture and ecosystem activity during the summer – falltransition alter the relative contribution of stomatal uptake, non-stomataluptake and thermochemical gradients to the net PAN flux. Daytime PAN andMPAN exchange velocities are a factor of 3 smaller than those of PPN duringthe first two weeks of the measurement period, consistent with strongintra-canopy chemical production of PAN and MPAN during this period.Depositional loss of APNs can be 3–21% of the gross gas-phase TD lossdepending on temperature. As a source of nitrogen to the biosphere, PANdeposition represents approximately 4–19% of that due to drydeposition of nitric acid at this site

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