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| Titel |
Influence of meteorology and anthropogenic pollution on chemical flux divergence of the NO–NO2–O3 triad above and within a natural grassland canopy |
| VerfasserIn |
D. Plake, M. Sörgel, P. Stella, A. Held, I. Trebs |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 12, no. 4 ; Nr. 12, no. 4 (2015-02-17), S.945-959 |
| Datensatznummer |
250117817
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| Publikation (Nr.) |
 copernicus.org/bg-12-945-2015.pdf |
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| Zusammenfassung |
The detailed understanding of surface–atmosphere exchange fluxes of reactive
trace gases is a crucial precondition for reliable modelling of processes in
atmospheric chemistry. Plant canopies significantly impact the atmospheric
budget of trace gases. In the past, many studies focused on taller forest
canopies or crops, where the bulk plant material is concentrated in the
uppermost canopy layer. However, within grasslands, a land-cover class that
globally covers vast terrestrial areas, the canopy structure is fundamentally
different, as the main biomass is concentrated in the lowest part of the
canopy. This has obvious implications for aerodynamic in-canopy transport,
and consequently also impacts on global budgets of key species in atmospheric
chemistry such as nitric oxide (NO), nitrogen dioxide (NO2) and ozone
(O3).
This study presents for the first time a comprehensive data set of directly
measured in-canopy transport times and aerodynamic resistances, chemical
timescales, Damköhler numbers, trace gas and micrometeorological
measurements for a natural grassland canopy (canopy height = 0.6 m).
Special attention is paid to the impact of contrasting meteorological and air
chemical conditions on in-canopy transport and chemical flux divergence. Our
results show that the grassland canopy is decoupled throughout the day. In
the lowermost canopy layer, the measured transport times are fastest during
nighttime, which is due to convection during nighttime and a stable
stratification during daytime in this layer. The inverse was found in the
layers above. During periods of low wind speed and high NOx
(NO+NO2) levels, the effect of canopy decoupling on trace gas
transport was found to be especially distinct. The aerodynamic resistance in
the lowermost canopy layer (0.04–0.2 m) was around 1000 s m−1, which
is as high as values determined previously for the lowest metre of an
Amazonian rain forest canopy. The aerodynamic resistance representing the
bulk canopy was found to be more than 3–4 times higher than in forests.
Calculated Damköhler numbers (ratio of transport and chemical timescales)
suggest a strong flux divergence for the NO–NO2–O3 triad within the
canopy during daytime. During that time, the timescale of NO2 uptake by
plants ranged from 90 to 160 s and was the fastest relevant timescale, i.e.
faster than the reaction of NO and O3. Thus, our results reveal that
grassland canopies of similar structure exhibit a strong potential to retain
soil-emitted NO due to oxidation and subsequent uptake of NO2 by plants.
Furthermore, photo-chemical O3 production was observed above the canopy,
which was attributed to a deviation from the NO–NO2–O3
photostationary state by a surplus of NO2 due to oxidation of NO, by
e.g. peroxy radicals. The O3 production was one order of magnitude
higher during high NOx than during low NOx periods
and resulted in an underestimation of the O3 deposition flux measured
with the EC method. |
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