Urban – Wetland contrast in turbulent exchange of methane

Publication date: November 2016
Source:Atmospheric Environment, Volume 145
Author(s): Włodzimierz Pawlak, Krzysztof Fortuniak, Mariusz Siedlecki, Mariusz Zieliński
Continuous eddy-covariance measurements of turbulent methane exchange between the ground and the atmosphere were carried out during the period from July 2013 to September 2015. The measurement sites were located in areas characterized by different types of use: the city (Łódź, central Poland) and the wetlands of Biebrza National Park (northeastern Poland). Regardless of the type of surface, such long-term, continuous measurements of the turbulent fluxes of methane are rare. Our aim was to investigate the temporal variability of the turbulent methane exchange in the surface–atmosphere system, while considering the impact of land use. Because cities are a huge source of atmospheric carbon dioxide, we compared the intensity of methane emissions from the city as compared to the wetland, which is considered to be the most intense natural source atmospheric methane. In both cases, the results show a clear prevalence of positive as compared with negative fluxes. This shows that both areas are net sources of methane to the troposphere. The measurements also demonstrated the existence of a clear annual cycle of turbulent methane flux in the centre of the city (average values in winter ranged from ∼40 to 60 nmol m⁻²·s⁻¹ and were significantly greater than values measured in summer) as well as in the wetlands, where maximum values were observed during the warm part of the year (approximately 80 nmol m⁻²·s⁻¹ or more). The different times of maximum CH4 flux (FCH4) occurrence resulted from the differentiation of processes that determine methane emissions: in the city these are anthropogenic emissions (strongest in winter); in the wetland, natural processes dominate. The diurnal variability of FCH4 was faintly detectable, except for the cold half of the year in the city and the warm half of the year in the wetland. The studied area of the centre of Łódź, in turn, is also characterized by a weekly cycle of methane exchange: the values measured on working days were higher by 6.6% (winter) to 5.6% (summer) than those observed on weekends. Differences in the annual exchange of methane in 2014 were insignificant: the centre of Łódź emitted 17.4 g m⁻² net, while the wetland emitted about 18.7 g m⁻² net. The results of the measurements performed during the period from January to September 2015, however, suggest that a very dry summer, drying of the soil and, consequently, a reduced water level in the fen significantly reduced the ability of the wetland to emit methane. This pattern was not observed in the city.

Graphical abstract

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