scholarly journals FLUXNET-CH<sub>4</sub>: a global, multi-ecosystem dataset and analysis of methane seasonality from freshwater wetlands

2021 ◽  
Vol 13 (7) ◽  
pp. 3607-3689
Author(s):  
Kyle B. Delwiche ◽  
Sara Helen Knox ◽  
Avni Malhotra ◽  
Etienne Fluet-Chouinard ◽  
Gavin McNicol ◽  
...  
Keyword(s):  
Freshwater Wetlands ◽  
Added Value ◽  
Freshwater Wetland ◽  
High Temporal Resolution ◽  
Soil Warming ◽  
Ch4 Flux ◽  
Ch4 Emissions ◽  
Individual Site ◽  
Flux Measurements ◽  
Peak Summer

Abstract. Methane (CH4) emissions from natural landscapes constitute roughly half of global CH4 contributions to the atmosphere, yet large uncertainties remain in the absolute magnitude and the seasonality of emission quantities and drivers. Eddy covariance (EC) measurements of CH4 flux are ideal for constraining ecosystem-scale CH4 emissions due to quasi-continuous and high-temporal-resolution CH4 flux measurements, coincident carbon dioxide, water, and energy flux measurements, lack of ecosystem disturbance, and increased availability of datasets over the last decade. Here, we (1) describe the newly published dataset, FLUXNET-CH4 Version 1.0, the first open-source global dataset of CH4 EC measurements (available at https://fluxnet.org/data/fluxnet-ch4-community-product/, last access: 7 April 2021). FLUXNET-CH4 includes half-hourly and daily gap-filled and non-gap-filled aggregated CH4 fluxes and meteorological data from 79 sites globally: 42 freshwater wetlands, 6 brackish and saline wetlands, 7 formerly drained ecosystems, 7 rice paddy sites, 2 lakes, and 15 uplands. Then, we (2) evaluate FLUXNET-CH4 representativeness for freshwater wetland coverage globally because the majority of sites in FLUXNET-CH4 Version 1.0 are freshwater wetlands which are a substantial source of total atmospheric CH4 emissions; and (3) we provide the first global estimates of the seasonal variability and seasonality predictors of freshwater wetland CH4 fluxes. Our representativeness analysis suggests that the freshwater wetland sites in the dataset cover global wetland bioclimatic attributes (encompassing energy, moisture, and vegetation-related parameters) in arctic, boreal, and temperate regions but only sparsely cover humid tropical regions. Seasonality metrics of wetland CH4 emissions vary considerably across latitudinal bands. In freshwater wetlands (except those between 20∘ S to 20∘ N) the spring onset of elevated CH4 emissions starts 3 d earlier, and the CH4 emission season lasts 4 d longer, for each degree Celsius increase in mean annual air temperature. On average, the spring onset of increasing CH4 emissions lags behind soil warming by 1 month, with very few sites experiencing increased CH4 emissions prior to the onset of soil warming. In contrast, roughly half of these sites experience the spring onset of rising CH4 emissions prior to the spring increase in gross primary productivity (GPP). The timing of peak summer CH4 emissions does not correlate with the timing for either peak summer temperature or peak GPP. Our results provide seasonality parameters for CH4 modeling and highlight seasonality metrics that cannot be predicted by temperature or GPP (i.e., seasonality of CH4 peak). FLUXNET-CH4 is a powerful new resource for diagnosing and understanding the role of terrestrial ecosystems and climate drivers in the global CH4 cycle, and future additions of sites in tropical ecosystems and site years of data collection will provide added value to this database. All seasonality parameters are available at https://doi.org/10.5281/zenodo.4672601 (Delwiche et al., 2021). Additionally, raw FLUXNET-CH4 data used to extract seasonality parameters can be downloaded from https://fluxnet.org/data/fluxnet-ch4-community-product/ (last access: 7 April 2021), and a complete list of the 79 individual site data DOIs is provided in Table 2 of this paper.

scholarly journals FLUXNET-CH4: A global, multi-ecosystem dataset and analysis of methane seasonality from freshwater wetlands

2021 ◽  
Author(s):  
Kyle B. Delwiche ◽  
Sara Helen Knox ◽  
Avni Malhotra ◽  
Etienne Fluet-Chouinard ◽  
Gavin McNicol ◽  
...  
Keyword(s):  
Meteorological Data ◽  
Freshwater Wetlands ◽  
Data Availability ◽  
Freshwater Wetland ◽  
High Temporal Resolution ◽  
Soil Warming ◽  
Ch4 Flux ◽  
Ch4 Emissions ◽  
Individual Site ◽  
Peak Summer

Abstract. Methane (CH4) emissions from natural landscapes constitute roughly half of global CH4 contributions to the atmosphere, yet large uncertainties remain in the absolute magnitude and the seasonality of emission quantities and drivers. Eddy covariance (EC) measurements of CH4 flux are ideal for constraining ecosystem-scale CH4 emissions, including their seasonality, due to quasi-continuous and high temporal resolution of flux measurements, coincident measurements of carbon, water, and energy fluxes, lack of ecosystem disturbance, and increased availability of datasets over the last decade. Here, we 1) describe the newly published dataset, FLUXNET-CH4 Version 1.0, the first global dataset of CH4 EC measurements (available at https://fluxnet.org/data/fluxnet-ch4- community-product/). FLUXNET-CH4 includes half-hourly and daily gap-filled and non gap-filled aggregated CH4 fluxes and meteorological data from 79 sites globally: 42 freshwater wetlands, 6 brackish and saline wetlands, 7 formerly drained ecosystems, 7 rice paddy sites, 2 lakes, and 15 uplands. Then, we 2) evaluate FLUXNET-CH4 representativeness for freshwater wetland coverage globally, because the majority of sites in FLUXNET-CH4 Version 1.0 are freshwater wetlands and because freshwater wetlands are a substantial source of total atmospheric CH4 emissions; and 3) provide the first global estimates of the seasonal variability and seasonality predictors of freshwater wetland CH4 fluxes. Our representativeness analysis suggests that the freshwater wetland sites in the dataset cover global wetland bioclimatic attributes (encompassing energy, moisture, and vegetation-related parameters) in arctic, boreal, and temperate regions, but only sparsely cover humid tropical regions. Seasonality metrics of wetland CH4 emissions vary considerably across latitudinal bands. In freshwater wetlands (except those between 20° S to 20° N) the spring onset of elevated CH4 emissions starts three days earlier, and the CH4 emission season lasts 4 days longer, for each degree C increase in mean annual air temperature. On average, the onset of increasing CH4 emissions lags soil warming by one month, with very few sites experiencing increased CH4 emissions prior to the onset of soil warming. In contrast, roughly half of these sites experience the spring onset of rising CH4 emissions prior to the spring increase in gross primary productivity (GPP). The timing of peak summer CH4 emissions does not correlate with the timing for either peak summer temperature or peak GPP. Our results provide seasonality parameters for CH4 modeling, and highlight seasonality metrics that cannot be predicted by temperature or GPP (i.e., seasonality of CH4 peak). The FLUXNET-CH4 dataset provides an open-access resource for CH4 flux synthesis, has a range of applications, and is unique in that it includes coupled measurements of important CH4 drivers such as GPP and temperature. Although FLUXNET-CH4 could certainly be improved by adding more sites in tropical ecosystems and by increasing the number of site-years at existing sites, it is a powerful new resource for diagnosing and understanding the role of terrestrial ecosystems and climate drivers in the global CH4 cycle. All seasonality parameters are available at https://doi.org/10.5281/zenodo.4408468. Additionally, raw FLUXNET-CH4 data used to extract seasonality parameters can be downloaded from https://fluxnet.org/data/fluxnet-ch4-community-product/, and a complete list of the 79 individual site data DOIs is provided in Table 2 in the Data Availability section of this document.

scholarly journals Rhizosphere to the atmosphere: contrasting methane pathways, fluxes and geochemical drivers across the terrestrial-aquatic wetland boundary

2019 ◽  
Author(s):  
Luke C. Jeffrey ◽  
Damien T. Maher ◽  
Scott Johnston ◽  
Kylie Maguire ◽  
Andrew D. L. Steven ◽  
...  
Keyword(s):  
Freshwater Wetlands ◽  
Sediment Geochemistry ◽  
Emission Rates ◽  
Redox Potentials ◽  
Habitat Variability ◽  
Soil Redox Potential ◽  
Ch4 Flux ◽  
Soil Redox ◽  
Ch4 Emissions ◽  
Seasonal Wetland

Abstract. Although wetlands represent the largest natural source of atmospheric CH4, large uncertainties remain regarding the global CH4 flux. Wetland hydrological oscillations contribute to this uncertainty, dramatically altering wetland area, water table height, soil redox potentials and CH4 emissions. This study compares both terrestrial and aquatic CH4 fluxes over two distinct seasons in both permanent and seasonal remediated freshwater wetlands in subtropical Australia. We account for aquatic CH4 diffusion and ebullition rates, and plant-mediated CH4 fluxes from three distinct vegetation communities, thereby examining seasonal, diurnal and intra-habitat variability. CH4 emission rates were related to underlying sediment geochemistry. For example, distinct negative relationships between Fe(III) and SO42− and CH4 fluxes were observed, whereas distinct positive trends occurred between CH4 emissions and Fe(II) / AVS, where sediment Fe(III) and SO42− were depleted. The highest CH4 emissions of the seasonal wetland were measured during flooded conditions and always during daylight hours, which is consistent with soil redox potential and temperature being important co-drivers of CH4 flux. The highest CH4 fluxes were consistently emitted from the permanent wetland (1.5 to 10.5 mmol m−2 d−1), followed by the Phragmites australis community within the seasonal wetland (0.8 to 2.3 mmol m−2 d−1), whilst the lowest CH4 fluxes came from a region of forested Juncus sp. (−0.01 to 0.1 mmol m−2 d−1) which also corresponded with the highest sedimentary Fe(III) and SO42−. We suggest that wetland remediation strategies should consider geochemical profiles to help to mitigate excessive and unwanted methane emissions, especially during early system recovery periods.

scholarly journals Evaluation of CH4MOD<sub>wetland</sub> and TEM models used to estimate global CH<sub>4</sub> emissions from natural wetlands

2020 ◽  
Author(s):  
Tingting Li ◽  
Yanyu Lu ◽  
Lingfei Yu ◽  
Wenjuan Sun ◽  
Qing Zhang ◽  
...  
Keyword(s):  
Coastal Wetlands ◽  
Coastal Wetland ◽  
Poor Performance ◽  
Model Accuracy ◽  
Ch4 Flux ◽  
Ch4 Emissions ◽  
Natural Wetlands ◽  
Flux Measurements ◽  
Model Efficiency ◽  
Process Based Models

Abstract. Reliable models are required to estimate global wetland CH4 emissions. This study aimed to test two process-based models, CH4MODwetland and TEM, against the CH4 flux measurements of marsh, swamps, peatland and coastal wetland sites across the world; specifically, model accuracy and generality were evaluated for different wetland types and in different continents, and then the global CH4 emissions from 2000 to 2010 were estimated. Both models showed similar high correlations with the observed seasonal CH4 emissions, and the regression of the observed versus computed total seasonal CH4 emissions resulted in R2 values of 0.78 and 0.72 by CH4MODwetland and TEM, respectively. The CH4MODwetland predicted more accurately in marsh, peatland and coastal wetlands, with model efficiency (EF) values of 0.22, 0.55 and 0.72, respectively; however, the model showed poor performance in swamps (EF 

scholarly journals Rhizosphere to the atmosphere: contrasting methane pathways, fluxes, and geochemical drivers across the terrestrial–aquatic wetland boundary

2019 ◽  
Vol 16 (8) ◽  
pp. 1799-1815 ◽  
Author(s):  
Luke C. Jeffrey ◽  
Damien T. Maher ◽  
Scott G. Johnston ◽  
Kylie Maguire ◽  
Andrew D. L. Steven ◽  
...  
Keyword(s):  
Climatic Conditions ◽  
Freshwater Wetlands ◽  
Sediment Geochemistry ◽  
Emission Rates ◽  
Redox Potentials ◽  
Soil Redox Potential ◽  
Ch4 Flux ◽  
Soil Redox ◽  
Ch4 Emissions ◽  
Seasonal Wetland

Abstract. Although wetlands represent the largest natural source of atmospheric CH4, large uncertainties remain regarding the global wetland CH4 flux. Wetland hydrological oscillations contribute to this uncertainty, dramatically altering wetland area, water table height, soil redox potentials, and CH4 emissions. This study compares both terrestrial and aquatic CH4 fluxes in permanent and seasonal remediated freshwater wetlands in subtropical Australia over two field campaigns, representing differing hydrological and climatic conditions. We account for aquatic CH4 diffusion and ebullition rates and plant-mediated CH4 fluxes from three distinct vegetation communities, thereby examining diel and intra-habitat variability. CH4 emission rates were related to underlying sediment geochemistry. For example, distinct negative relationships between CH4 fluxes and both Fe(III) and SO42- were observed. Where sediment Fe(III) and SO42- were depleted, distinct positive trends occurred between CH4 emissions and Fe(II) ∕ acid volatile sulfur (AVS). Significantly higher CH4 emissions (p < 0.01) in the seasonal wetland were measured during flooded conditions and always during daylight hours, which is consistent with soil redox potential and temperature being important co-drivers of CH4 flux. The highest CH4 fluxes were consistently emitted from the permanent wetland (1.5 to 10.5 mmol m−2 d−1), followed by the Phragmites australis community within the seasonal wetland (0.8 to 2.3 mmol m−2 d−1), whilst the lowest CH4 fluxes came from a region of forested Juncus spp. (−0.01 to 0.1 mmol m−2 d−1), which also corresponded to the highest sedimentary Fe(III) and SO42-. We suggest that wetland remediation strategies should consider geochemical profiles to help to mitigate excessive and unwanted methane emissions, especially during early system remediation periods.

scholarly journals Geochemical and sedimentary constraints on the formation of the Venta Micena early Pleistocene site (Guadix-Baza Basin, Spain)

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Alejandro Granados ◽  
Oriol Oms ◽  
Pere Anadón ◽  
Jordi Ibáñez-Insa ◽  
Anu Kaakinen ◽  
...  
Keyword(s):  
Critical Role ◽  
Freshwater Wetlands ◽  
Freshwater Wetland ◽  
Early Pleistocene ◽  
Bulk Rock ◽  
X Ray Diffraction ◽  
Marginal Areas ◽  
Invertebrate Fossils ◽  
Venta Micena ◽  
Lake Dynamics

AbstractDespite the paleontological relevance of the terrestrial Early Pleistocene Venta Micena bonebed (Baza Basin, Spain), it lacks a comprehensive geochemical/sedimentological study. Here, we demonstrate that the 1.5-m-thick Venta Micena limestone formed in a relatively small freshwater wetland/pond located at the periphery of the large saline Baza paleolake. Two microfacies are observed, with high and low contents of invertebrate fossils, and which originated in the centre and margin of the wetland, respectively. X-ray diffraction (XRD) mineralogy and paleohydrological characterization based on ostracod and bulk-rock geochemistry (δ13C and δ18O) indicate that the limestone reflects a general lowstand of the Baza lake, permitting the differentiation of freshwater wetlands that were fed by adjacent sources. Conversely, during highstands, the Baza lake flooded the Venta Micena area and the freshwater fauna was replaced by a saline one. Bulk-rock isotopic data indicate that the lower interval C1 of the limestone (bone-rich in marginal settings) displays general negative values, while the upper interval C2 (bone free) displays less negative values. The bones of predated mammals accumulated in the marginal areas, which were flooded and buried by recurring water-table fluctuations. Lake dynamics played a critical role in bone accumulation, which was previously considered as representing a hyena den.

scholarly journals The effect of atmospheric turbulence and chamber deployment period on autochamber CO<sub>2</sub> and CH<sub>4</sub> flux measurements in an ombrotrophic peatland

2012 ◽  
Vol 9 (2) ◽  
pp. 1439-1482 ◽  
Author(s):  
D. Y. F. Lai ◽  
N. T. Roulet ◽  
E. R. Humphreys ◽  
T. R. Moore ◽  
M. Dalva
Keyword(s):  
Atmospheric Turbulence ◽  
Concentration Gradient ◽  
Co2 Concentration ◽  
Time Of Day ◽  
Reliable Estimate ◽  
Organic Soils ◽  
Atmospheric Conditions ◽  
Near Surface ◽  
Ch4 Flux ◽  
Flux Measurements

Abstract. Accurate quantification of soil-atmosphere gas exchange is essential for understanding the magnitude and controls of greenhouse gas emissions. We used an automatic closed dynamic chamber system to measure the fluxes of CO2 and CH4 for several years at the ombrotrophic Mer Bleue peatland near Ottawa, Canada and found that atmospheric turbulence and chamber deployment period had a considerable influence on the observed flux rates. With a short deployment period of 2.5 min, CH4 flux exhibited strong diel patterns and both CH4 and nighttime CO2 effluxes were highly and negatively correlated with friction velocity as were the CO2 concentration gradients in the top 20 cm of peat. This suggests winds were flushing the very porous and relatively dry near surface peat layers, altering the concentration gradient and resulting in a 9 to 57% underestimate of CH4 flux at any time of day and a 13 to 21% underestimate of nighttime CO2 fluxes in highly turbulent conditions. Conversely, there was evidence of an overestimation of ~100% of CH4 and nighttime CO2 effluxes in calm atmospheric conditions possibly due to enhanced near-surface gas concentration gradient by mixing of chamber headspace air by fans. These problems were resolved by extending the deployment period to 30 min. After 13 min of chamber closure, the flux rate of CH4 and nighttime CO2 became constant and were not affected by turbulence thereafter, yielding a reliable estimate of the net biological fluxes. The measurement biases we observed likely exist to some extent in all chamber flux measurements made on porous and aerated substrate, such as peatlands, organic soils in tundra and forests, and snow-covered surfaces, but would be difficult to detect unless high frequency, semi-continuous observations are made.

Microbial activity and diversity in the rhizosphere soil of the invasive species Zizania latifolia in the wetland of Wuchang Lake, China

2020 ◽  
Vol 71 (12) ◽  
pp. 1702
Author(s):  
Baohua Zhou ◽  
Zhaowen Liu ◽  
Guo Yang ◽  
Hui He ◽  
Haijun Liu
Keyword(s):  
Invasive Species ◽  
Rhizosphere Soil ◽  
Remote Sensing Data ◽  
Wetland Loss ◽  
Freshwater Wetlands ◽  
Total Carbon ◽  
Freshwater Wetland ◽  
Available Nitrogen ◽  
Zizania Latifolia ◽  
Rhizosphere Soils

Information about the consequences of invasive species overgrowing freshwater wetlands is limited. According to remote sensing data, the invasive species Zizania latifolia spreads at an annual rate of 1.78km2 in the freshwater wetland of Wuchang Lake, China, resulting in wetland loss and degradation due to the overgrowth. This species not only increases soil organic matter, total carbon, total nitrogen, total sulfate, available nitrogen and the C/N ratio in the rhizosphere soil, but also results in increased urease, sucrose and catalase activity, as well as fluorescein diacetate hydrolysis. In this study, we have analysed microbial diversity in rhizosphere soils among different habitat types of Z. latifolia. Microbial communities in different habitats invaded by Z. latifolia differed considerably at the genus level, although all soil samples were predominated by the phyla Proteobacteria, Acidobacteria and Chloroflexi. The dominant bacterial taxa in the rhizosphere soil from the floating blanket included Acidimicrobiales, Thiomonas, Alicyclobacillus, Acetobacteraceae and Acidocella, whereas those in rhizosphere soils from the lake sludge were Acidobacteria, Anaerolineaceae, Bacteroidetes and Nitrospirae. The bacterial community in the rhizosphere soil differed significantly from that in the non-rhizosphere soil. Z. latifolia potentially creates suitable habitats and provides substrate for a unique set of microbes, further facilitating the succession of this species.

scholarly journals Methane emissions associated with the conversion of marshland to cropland and climate change on the Sanjiang Plain of northeast China from 1950 to 2100

2012 ◽  
Vol 9 (12) ◽  
pp. 5199-5215 ◽  
Author(s):  
T. Li ◽  
Y. Huang ◽  
W. Zhang ◽  
Y.-Q. Yu
Keyword(s):  
Climate Change ◽  
Northeast China ◽  
Model Simulation ◽  
Linear Trend ◽  
Sanjiang Plain ◽  
Lower Amount ◽  
The Sanjiang Plain ◽  
Ch4 Flux ◽  
Ch4 Emissions ◽  
The 1950S

Abstract. Wetland loss and climate change are known to alter regional and global methane (CH4) budgets. Over the last six decades, an extensive area of marshland has been converted to cropland on the Sanjiang Plain in northeast China, and a significant increase in air temperature has also been observed there, while the impacts on regional CH4 budgets remain uncertain. Through model simulation, we estimated the changes in CH4 emissions associated with the conversion of marshland to cropland and climate change in this area. Model simulations indicated a significant reduction of 1.1 Tg yr−1 (0.7–1.8 Tg yr−1) from the 1950s to the 2000s in regional CH4 emissions. The cumulative reduction of CH4 from 1960 to 2009 was estimated to be ~36 Tg (24–57 Tg) relative to the 1950s, and marshland conversion and the climate contributed 86% and 14% of this change, respectively. Interannual variation in precipitation (linear trend with P > 0.2) contributed to yearly fluctuations in CH4 emissions, but the relatively lower amount of precipitation over the period 1960–2009 (47 mm yr−1 lower on average than in the 1950s) contributed ~91% of the reduction in the area-weighted CH4 flux. Global warming at a rate of 0.3 ° per decade (P < 0.001) has increased CH4 emissions significantly since the 1990s. Relative to the mean of the 1950s, the warming-induced increase in the CH4 flux has averaged 19 kg ha−1 yr−1 over the last two decades. In the RCP (Representative Concentration Pathway) 2.6, RCP 4.5, RCP 6.0 and RCP 8.5 scenarios of the fifth IPCC assessment report (AR5), the CH4 fluxes are predicted to increase by 36%, 52%, 78% and 95%, respectively, by the 2080s compared to 1961–1990 in response to climate warming and wetting.

scholarly journals Impact of synthetic space-borne NO<sub>2</sub> observations from the Sentinel-4 and Sentinel-5P missions on tropospheric NO<sub>2</sub> analyses

2019 ◽  
Vol 19 (19) ◽  
pp. 12811-12833 ◽  
Author(s):  
Renske Timmermans ◽  
Arjo Segers ◽  
Lyana Curier ◽  
Rachid Abida ◽  
Jean-Luc Attié ◽  
...  
Keyword(s):  
Error Estimates ◽  
Temporal Resolution ◽  
Transport Model ◽  
Surface Ozone ◽  
Low Earth Orbit ◽  
Added Value ◽  
High Temporal Resolution ◽  
Chemistry Transport Model ◽  
European Continent ◽  
Global Coverage

Abstract. We present an Observing System Simulation Experiment (OSSE) dedicated to the evaluation of the added value of the Sentinel-4 and Sentinel-5P missions for tropospheric nitrogen dioxide (NO2). Sentinel-4 is a geostationary (GEO) mission covering the European continent, providing observations with high temporal resolution (hourly). Sentinel-5P is a low Earth orbit (LEO) mission providing daily observations with a global coverage. The OSSE experiment has been carefully designed, with separate models for the simulation of observations and for the assimilation experiments and with conservative estimates of the total observation uncertainties. In the experiment we simulate Sentinel-4 and Sentinel-5P tropospheric NO2 columns and surface ozone concentrations at 7 by 7 km resolution over Europe for two 3-month summer and winter periods. The synthetic observations are based on a nature run (NR) from a chemistry transport model (MOCAGE) and error estimates using instrument characteristics. We assimilate the simulated observations into a chemistry transport model (LOTOS-EUROS) independent of the NR to evaluate their impact on modelled NO2 tropospheric columns and surface concentrations. The results are compared to an operational system where only ground-based ozone observations are ingested. Both instruments have an added value to analysed NO2 columns and surface values, reflected in decreased biases and improved correlations. The Sentinel-4 NO2 observations with hourly temporal resolution benefit modelled NO2 analyses throughout the entire day where the daily Sentinel-5P NO2 observations have a slightly lower impact that lasts up to 3–6 h after overpass. The evaluated benefits may be even higher in reality as the applied error estimates were shown to be higher than actual errors in the now operational Sentinel-5P NO2 products. We show that an accurate representation of the NO2 profile is crucial for the benefit of the column observations on surface values. The results support the need for having a combination of GEO and LEO missions for NO2 analyses in view of the complementary benefits of hourly temporal resolution (GEO, Sentinel-4) and global coverage (LEO, Sentinel-5P).

scholarly journals Characterisation of short-term extreme methane fluxes related to non-turbulent mixing above an Arctic permafrost ecosystem

2019 ◽  
Vol 19 (6) ◽  
pp. 4041-4059 ◽  
Author(s):  
Carsten Schaller ◽  
Fanny Kittler ◽  
Thomas Foken ◽  
Mathias Göckede
Keyword(s):  
Steady State ◽  
Wavelet Analysis ◽  
Strongly Correlated ◽  
Atmospheric Conditions ◽  
Ch4 Flux ◽  
Ch4 Emissions ◽  
Stationary Signals ◽  
Methane Fluxes ◽  
Variable Flux ◽  
Mesoscale Processes

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.

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