Characterizing methane flux hotspots over boreal wetlands using surface renewal theory

Abstract:

Boreal wetlands emissions largely contribute to the global methane (CH4) budget. In these environments, CH4 is emitted through three main mechanisms: diffusion, plant-mediated transport, and the localized release of bubbles. The interplay of these different pathways determine CH4 fluxes which are highly variable in space and time when compared to other gases such as carbon dioxide and water vapor. For this reason, characterizing the variability of CH4 fluxes and the relative contributions of different gas transfer mechanisms in these environments remains a challenging area of research. Here we introduce a framework to partition the CH4 fluxes based on the spatial homogeneity of the gas sources at the surface when compared to a reference gas such as water vapor. Combining a wavelet partition scheme with the theory of surface renewals, we then characterize strength and intermittency of localized CH4 “hotspot” sources relative to a background steady emission. We apply and test this approach using a large dataset of turbulent air velocity and multiple scalar concentrations measured over a boreal peatland in southern Finland. This methodology applied to an extensive dataset allows us to I) characterize the effects of CH4 localized hotspots on the gas transport efficiency, and II) analyze the role of possible environmental controls such as water table levels and atmospheric pressure on the measured methane fluxes.