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| Titel |
Observations of HNO3, ΣAN, ΣPN and NO2 fluxes: evidence for rapid HOx chemistry within a pine forest canopy |
| VerfasserIn |
D. K. Farmer, R. C. Cohen |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 8, no. 14 ; Nr. 8, no. 14 (2008-07-22), S.3899-3917 |
| Datensatznummer |
250006293
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| Publikation (Nr.) |
 copernicus.org/acp-8-3899-2008.pdf |
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| Zusammenfassung |
| 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 NO2, and downward fluxes of
total peroxy and peroxy acyl nitrates (ΣPNs), total gas and particle
phase alkyl and multifunctional alkyl nitrates (ΣANs(g+p)), and
the sum of gaseous HNO3 and semi-volatile NO3− particles
(HNO3(g+p)). We use calculations of the vertical profile and flux of
NO, partially constrained by observations, to show that net midday ΣNOyi fluxes in winter are –4.9 ppt m s−1. The signs and
magnitudes of these wintertime individual and ΣNOyi fluxes are
in the range of prior measurements. In contrast, during summer, we observe
downward fluxes only of ΣANs(g+p), and upward fluxes of
HNO3(g+p), ΣPNs and NO2 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 NOy fluxes, at least at this site, are much more
complex than previously recognized. We show that the observations of upward
fluxes of HNO3(g+p) and σPNs during summer are consistent with
oxidation of NO2 and acetaldehyde by an OH x residence time of
1.1×1010 molec OH cm−3 s, corresponding to 3 to
16×107 molecules cm−3 OH within the forest canopy for a 420 to
70 s
canopy residence time. We show that ΣAN(g+p) fluxes are
consistent with this range in OH if the reaction of OH with ΣANs
produces either HNO3 or NO2 with a 6–30% yield. Calculations of
NO fluxes constrained by the NO2 observations and the inferred OH
indicate that NOx fluxes are downward into the canopy because of the
substantial conversion of NOx to HNO3 and σPNs in the
canopy. Even so, we derive that NOx emission fluxes of
~15 ng(N) m−2 s−1 at midday during summer are required to balance the
NOx and NOy flux budgets. These fluxes are partly explained by
estimates of soil emissions (estimated to be between 3 and
6 ng(N) m−2 s-1). One possibility for the remainder of the NOx source is large
HONO emissions. Alternatively, the 15 ng(N) m−2 s−1 emission
estimate may be too large, and the budget balanced if the deposition of
HNO3 and σPNs 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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