Intermittent Surface Renewals and Methane Hotspots in Natural Peatlands

Peatlands account for a large fraction of global methane ( CH 4 ) emissions. These environments exchange CH 4 with the atmosphere via three main mechanisms: diffusion through the peat and water, plant-mediated diffusion, and sporadic release of CH 4 bubbles. While rapid advances have been made in measuring CH 4 fluxes above peatlands on sub-daily time scales, partitioning CH 4 fluxes into ebullition and background diffusion remains a formidable challenge. Such partitioning is becoming necessary for future projection of methane concentration as atmospheric, hydrologic, and edaphic drivers of these two types of methane releases may differ significantly. Using surface renewal theory, a framework for partitioning measured methane fluxes based on the mass transfer mechanism is introduced with the overall objective of characterizing the intermittency of CH 4 source and its strength at the ground. This approach is tested using a large dataset of measured turbulent air velocity and multiple scalar concentrations (including heat, water vapour, and carbon dioxide) for flow above a boreal peatland in Finland. The transport efficiencies of different gas transfer mechanisms are then evaluated for scalars characterized by background diffusion (e.g., water vapour) or by intermittent sources (e.g., methane). Whether environmental variables such as water-table levels and atmospheric conditions have a signature on the occurrence of CH 4 hotspots is then investigated. Building upon the classical surface renewal theory, this work introduces a novel approach for inferring the intermittent nature of scalar sources at the ground and for exploring how non-homogeneity affects the efficiency of gas turbulent transport in the atmospheric surface layer.