Characterisation of short-term extreme methane fluxes related to non-turbulent mixing above an Arctic permafrost ecosystem
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Published:2019-04-01
Issue:6
Volume:19
Page:4041-4059
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ISSN:1680-7324
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Container-title:Atmospheric Chemistry and Physics
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language:en
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Short-container-title:Atmos. Chem. Phys.
Author:
Schaller Carsten, Kittler Fanny, Foken ThomasORCID, Göckede MathiasORCID
Abstract
Abstract. Methane (CH4) emissions from biogenic sources,
such as Arctic permafrost wetlands, are associated with large uncertainties
because of the high variability of fluxes in both space and time. This
variability poses a challenge to monitoring CH4 fluxes with the eddy
covariance (EC) technique, because this approach requires stationary signals
from spatially homogeneous sources. Episodic outbursts of CH4
emissions, i.e. triggered by spontaneous outgassing of bubbles or venting of
methane-rich air from lower levels due to shifts in atmospheric conditions,
are particularly challenging to quantify. Such events typically last for only
a few minutes, which is much shorter than the common averaging interval for
EC (30 min). The steady-state assumption is jeopardised, which potentially
leads to a non-negligible bias in the CH4 flux. Based on data from
Chersky, NE Siberia, we tested and evaluated a flux calculation method based
on wavelet analysis, which, in contrast to regular EC data processing, does
not require steady-state conditions and is allowed to obtain fluxes over
averaging periods as short as 1 min. Statistics on meteorological conditions
before, during, and after the detected events revealed that it is atmospheric
mixing that triggered such events rather than CH4 emission from the
soil. By investigating individual events in more detail, we identified a
potential influence of various mesoscale processes like gravity waves,
low-level jets, weather fronts passing the site, and cold-air advection from
a nearby mountain ridge as the dominating processes. The occurrence of
extreme CH4 flux events over the summer season followed a seasonal
course with a maximum in early August, which is strongly correlated with the
maximum soil temperature. Overall, our findings demonstrate that wavelet
analysis is a powerful method for resolving highly variable flux events on
the order of minutes, and can therefore support the evaluation of EC flux
data quality under non-steady-state conditions.
Funder
Bundesministerium für Bildung und Forschung European Commission AXA Research Fund
Publisher
Copernicus GmbH
Subject
Atmospheric Science
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