Abstract
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.
Highlights
Surface uptake of reactive nitrogen provides a key pathway through which anthropogenic emissions may influence ecosystem vitality
This pattern is consistent with previous thermal decomposition (TD)-LIF measurements of total peroxynitrates ( PNs) at Blodgett Forest Research Station (BFRS), but summer 2007 mixing ratios of PNs and of peroxyacetyl nitrate (PAN)+peroxypropionyl nitrate (PPN)+MPAN are ∼30% lower than PNs measured in previous years (Murphy et al, 2007; Farmer and Cohen, 2008), consistent with lower average temperatures in summer 2007 (Day et al, 2008)
Analysis of PAN fluxes with a standard resistance model reveals that loss to the canopy is not restricted by turbulent or molecular diffusion, suggesting a surface-limited process; stomatal uptake is not sufficient to explain the magnitude of observed net fluxes, and evidence points toward a temperature-dependent process
Summary
Surface uptake of reactive nitrogen provides a key pathway through which anthropogenic emissions may influence ecosystem vitality. Carbon sequestration within boreal forests may be largely controlled by wet and dry deposition of atmospheric nitrogen, the bulk of which derives from anthropogenic activities (Magnani et al, 2007). As a primary sink for total atmospheric reactive nitrogen In the planetary boundary layer, NOx (≡NO+NO2) radicals undergo rapid (∼4 h) conversion into temporary or permanent reservoir species, including peroxy nitrates, alkyl nitrates, and nitric acid (Roberts, 1990; Murphy et al, 2007). Loss of NOy is believed to occur primarily via wet and dry deposition of nitric acid (HNO3), which is formed by gas-phase reaction of OH and NO2, as well as by reactions involving N2O5 and NO3. Dry deposition of HNO3 to vegetation is typically assumed to be limited by vertical mixing processes, giving rise
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