Observations of HNO3, ΣAN, ΣPN and NO2 fluxes: evidence for rapid HOx chemistry within a pine forest canopy

Other literature type English OPEN
Farmer, D. K. ; Cohen, R. C. (2008)

Measurements of exchange of reactive nitrogen oxides between the atmosphere and a ponderosa pine forest in the Sierra Nevada Mountains are reported. During winter, we observe upward fluxes of NO<sub>2</sub>, and downward fluxes of total peroxy and peroxy acyl nitrates (&Sigma;PNs), total gas and particle phase alkyl and multifunctional alkyl nitrates (&Sigma;ANs<sub>(g+p)</sub>), and the sum of gaseous HNO<sub>3</sub> and semi-volatile NO<sub>3</sub><sup>&minus;</sup> particles (HNO<sub>3(g+p)</sub>). We use calculations of the vertical profile and flux of NO, partially constrained by observations, to show that net midday &Sigma;NO<sub>yi</sub> fluxes in winter are –4.9 ppt m s<sup>&minus;1</sup>. The signs and magnitudes of these wintertime individual and &Sigma;NO<sub>yi</sub> fluxes are in the range of prior measurements. In contrast, during summer, we observe downward fluxes only of &Sigma;ANs<sub>(g+p)</sub>, and upward fluxes of HNO<sub>3(g+p)</sub>, &Sigma;PNs and NO<sub>2</sub> with signs and magnitudes that are unlike most, if not all, previous observations and analyses of fluxes of individual nitrogen oxides. The results imply that the mechanisms contributing to NO<sub>y</sub> fluxes, at least at this site, are much more complex than previously recognized. We show that the observations of upward fluxes of HNO<sub>3(g+p)</sub> and &sigma;PNs during summer are consistent with oxidation of NO<sub>2</sub> and acetaldehyde by an OH x residence time of 1.1&times;10<sup>10</sup> molec OH cm<sup>&minus;3</sup> s, corresponding to 3 to 16&times;10<sup>7</sup> molecules cm<sup>&minus;3</sup> OH within the forest canopy for a 420 to 70 s canopy residence time. We show that &Sigma;AN<sub>(g+p)</sub> fluxes are consistent with this range in OH if the reaction of OH with &Sigma;ANs produces either HNO<sub>3</sub> or NO<sub>2</sub> with a 6–30% yield. Calculations of NO fluxes constrained by the NO<sub>2</sub> observations and the inferred OH indicate that NO<sub>x</sub> fluxes are downward into the canopy because of the substantial conversion of NO<sub>x</sub> to HNO<sub>3</sub> and &sigma;PNs in the canopy. Even so, we derive that NO<sub>x</sub> emission fluxes of ~15 ng(N) m<sup>&minus;2</sup> s<sup>&minus;1</sup> at midday during summer are required to balance the NO<sub>x</sub> and NO<sub>y</sub> flux budgets. These fluxes are partly explained by estimates of soil emissions (estimated to be between 3 and 6 ng(N) m<sup>&minus;2</sup> s<sup>-1</sup>). One possibility for the remainder of the NO<sub>x</sub> source is large HONO emissions. Alternatively, the 15 ng(N) m<sup>&minus;2</sup> s<sup>&minus;1</sup> emission estimate may be too large, and the budget balanced if the deposition of HNO<sub>3</sub> and &sigma;PNs is slower than we estimate, if there are large errors in either our understanding of peroxy radical chemistry, or our assumptions that the budget is required to balance because the fluxes do not obey similarity theory.
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