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| Titel |
A comparison of measured HONO uptake and release with calculated source strengths in a heterogeneous forest environment |
| VerfasserIn |
M. Sörgel, I. Trebs, D. Wu, A. Held |
| 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 ; 15, no. 16 ; Nr. 15, no. 16 (2015-08-20), S.9237-9251 |
| Datensatznummer |
250119978
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| Publikation (Nr.) |
 copernicus.org/acp-15-9237-2015.pdf |
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| Zusammenfassung |
Vertical mixing ratio profiles of nitrous acid (HONO) were measured in a
clearing and on the forest floor in a rural forest environment. For the
forest floor, HONO was found to predominantly deposit, whereas for the clearing, net
deposition dominated only during nighttime and net
emissions were observed during daytime. For selected days, net fluxes of
HONO were calculated from the measured profiles using the aerodynamic
gradient method. The emission fluxes were in the range of 0.02 to 0.07 nmol m−2 s−1 and thus were in the lower range of previous
observations. These fluxes were compared to the strengths of postulated HONO
sources. Laboratory measurements of different soil samples from both sites
revealed an upper limit for soil biogenic HONO emission fluxes of 0.025 nmol m−2 s−1. HONO formation by light-induced NO2 conversion was
calculated to be below 0.03 nmol m−2 s−1 for the investigated
days, which is comparable to the potential soil fluxes. Due to light
saturation at low irradiance, this reaction pathway was largely found to be
independent of light intensity, i.e. it was only dependent on ambient
NO2.
We used three different approaches based on measured leaf nitrate loadings
for calculating HONO formation from HNO3 photolysis. While the first
two approaches based on empirical HONO formation rates yielded values in the
same order of magnitude as the estimated fluxes, the third approach based on
available kinetic data of the postulated pathway failed to produce
noticeable amounts of HONO. Estimates based on reported cross sections of
adsorbed HNO3 indicate that the lifetime of adsorbed HNO3 was
only about 15 min, which would imply a substantial renoxification. Although
the photolysis of HNO3 was significantly enhanced at the surface, the
subsequent light-induced conversion of the photolysis product NO2 did
not produce considerable amounts of HONO. Consequently, this reaction might
occur via an alternative mechanism.
By explicitly calculating HONO formation based on available kinetic data
and simple parameterizations, we showed that (a) for low NOx the light-induced conversion of NO2 on humic acids is already light saturated
by the early morning, (b) HONO formation from photolysis of adsorbed HNO3
appears to proceed via an alternative mechanism and (c) estimates of HONO
emissions from soil are very sensitive to mass transfer and acidic soils do
not necessarily favour HONO emissions. |
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