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Measurements of exchange of reactive nitrogen oxides between the atmosphereand a ponderosa pine forest in the Sierra Nevada Mountains are reported.During winter, we observe upward fluxes of NO2, and downward fluxes oftotal peroxy and peroxy acyl nitrates (ΣPNs), total gas and particlephase alkyl and multifunctional alkyl nitrates (ΣANs(g+p)), andthe sum of gaseous HNO3 and semi-volatile NO3− particles(HNO3(g+p)). We use calculations of the vertical profile and flux ofNO, partially constrained by observations, to show that net midday ΣNOyi fluxes in winter are –4.9 ppt m s−1. The signs andmagnitudes of these wintertime individual and ΣNOyi fluxes arein the range of prior measurements. In contrast, during summer, we observedownward fluxes only of ΣANs(g+p), and upward fluxes ofHNO3(g+p), ΣPNs and NO2 with signs and magnitudes that areunlike most, if not all, previous observations and analyses of fluxes ofindividual nitrogen oxides. The results imply that the mechanismscontributing to NOy fluxes, at least at this site, are much morecomplex than previously recognized. We show that the observations of upwardfluxes of HNO3(g+p) and σPNs during summer are consistent withoxidation of NO2 and acetaldehyde by an OH x residence time of1.1×1010 molec OH cm−3 s, corresponding to 3 to16×107 molecules cm−3 OH within the forest canopy for a 420 to70 scanopy residence time. We show that ΣAN(g+p) fluxes areconsistent with this range in OH if the reaction of OH with ΣANsproduces either HNO3 or NO2 with a 6–30% yield. Calculations ofNO fluxes constrained by the NO2 observations and the inferred OHindicate that NOx fluxes are downward into the canopy because of thesubstantial conversion of NOx to HNO3 and σPNs in thecanopy. Even so, we derive that NOx emission fluxes of~15 ng(N) m−2 s−1 at midday during summer are required to balance theNOx and NOy flux budgets. These fluxes are partly explained byestimates of soil emissions (estimated to be between 3 and6 ng(N) m−2 s-1). One possibility for the remainder of the NOx source is largeHONO emissions. Alternatively, the 15 ng(N) m−2 s−1 emissionestimate may be too large, and the budget balanced if the deposition ofHNO3 and σPNs is slower than we estimate, if there are largeerrors in either our understanding of peroxy radical chemistry, or ourassumptions that the budget is required to balance because the fluxes do notobey similarity theory

atmospheric chemistry and physics

Observations of HNO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;, ΣAN, ΣPN and NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fluxes: evidence for rapid HO&amp;lt;sub&amp;gt;x&amp;lt;/sub&amp;gt; chemistry within a pine forest canopy