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| Titel |
A comparison of measured HONO uptake and release with calculated source strengths in a heterogeneous forest environment |
| VerfasserIn |
Matthias Sörgel, Ivonne Trebs, Dianming Wu, Andreas Held |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250111599
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| Publikation (Nr.) |
 EGU/EGU2015-11735.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 (in the south-east of Germany) by applying a lift
system to move the sampling unit of the LOng Path Absorption Photometer (LOPAP) up and
down. For the forest floor, HONO was found to be predominantly deposited, whereas net
deposition was dominating in the clearing 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 and to the amount of HONO needed to sustain photolysis in the boundary
layer. 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. Therefore,
the semi-volatile weak acid HONO can serve as an example for the complexity of
exchange processes as multiple sources and sinks are coexisting, such as (photo-)
chemical formation, microbial formation, adsorption/desorption and (stomatal-) uptake. |
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