| Summary: | Methane (CH4 ) emissions from biogenic sources,such as Arctic permafrost wetlands, are associated with largeuncertainties because of the high variability of fluxes in bothspace and time. This variability poses a challenge to monitoringCH4 fluxes with the eddy covariance (EC) technique, becausethis approach requires stationary signals from spatiallyhomogeneous sources. Episodic outbursts of CH4 emissions,i.e. triggered by spontaneous outgassing of bubbles or ventingof methane-rich air from lower levels due to shifts in atmosphericconditions, are particularly challenging to quantify.Such events typically last for only a few minutes, whichis much shorter than the common averaging interval for EC(30 min). The steady-state assumption is jeopardised, whichpotentially leads to a non-negligible bias in the CH4 flux. Based on data from Chersky, NE Siberia, we tested and evaluateda flux calculation method based on wavelet analysis,which, in contrast to regular EC data processing, does not requiresteady-state conditions and is allowed to obtain fluxesover averaging periods as short as 1 min. Statistics on meteorologicalconditions before, during, and after the detectedevents revealed that it is atmospheric mixing that triggeredsuch events rather than CH4 emission from the soil. By investigatingindividual events in more detail, we identified apotential influence of various mesoscale processes like gravitywaves, low-level jets, weather fronts passing the site, andcold-air advection from a nearby mountain ridge as the dominatingprocesses. The occurrence of extreme CH4 flux eventsover the summer season followed a seasonal course with amaximum in early August, which is strongly correlated withthe maximum soil temperature. Overall, our findings demonstratethat wavelet analysis is a powerful method for resolvinghighly variable flux events on the order of minutes, andcan therefore support the evaluation of EC flux data qualityunder non-steady-state conditions. Funding information: This work has been supported by the European ...
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