Methane flux, vertical gradient and mixing ratio measurements in a tropical forest

C A S Querino,A Martinewski,L V Gatti,A C De Araújo,I Vigano,V Moura,R Holzinger,C J P P Smeets,T Röckmann,A O Manzi

doi:10.5194/acp-11-7943-2011

Abstract

Abstract. Measurements of CH4 mixing ratio, vertical gradients and turbulent fluxes were carried out in a tropical forest (Reserva Biológica Cuieiras), about 60 km north of Manaus, Brazil. The methane mixing ratio and flux measurements were performed at a height of 53 m (canopy height 35 m). In addition, vertical CH4 gradients were measured within the canopy using custom made air samplers at levels of 2, 16 and 36 m above ground. The methane gradients within the canopy reveal that there is a continuous methane source at the surface. No clear evidence for aerobic methane emission from the canopy was found. The methane fluxes above the canopy are small but consistently upwards with a maximum early in the morning. The measured fluxes are in agreement with the observed CH4 gradient in the canopy. In the morning hours, a strong canopy venting peak is observed for both CH4 and CO2, but for CO2 this peak is then superimposed by photosynthetic uptake, whereas the peak lasts longer for CH4. Monthly averaged diurnal cycles of the CH4 mixing ratio show a decrease during daytime and increase during nighttime. The magnitude of the difference in CH4 mixing ratio between day and night gradually increases throughout the wet season. The fluxes required to explain the nighttime increase are in agreement with the nighttime fluxes measured above the canopy, which implies that the CH4 increase in the nighttime boundary layer originates from local sources.

Highlights

Amazonia plays a prominent role in the global carbon and methane cycle and its land use change implies numerous modifications to the biogeochemical cycles
The site belongs to the Instituto Nacional de Pesquisas da Amazonia (INPA), and it is coordinated by the Large Scale Biosphere and Atmospheric Experiment in Amazonian – LBA project
The collective evidence from vertical CH4 gradient measurements in an upland tropical forest, CH4 fluxes above the canopy, and the diurnal cycle of CH4 mixing ratio above the canopy, suggests that this ecosystem is a source of CH4 to the atmosphere

Summary

Introduction

Amazonia plays a prominent role in the global carbon and methane cycle and its land use change implies numerous modifications to the biogeochemical cycles. Methane (CH4) is the second most important anthropogenic greenhouse gas and its mixing ratio (presently ∼1.8 ppm) has increased by about 150 % since pre-industrial times (Etheridge et al, 1998). It is the most predominant hydrocarbon and the most abundant organic trace gas in the Earth’s atmosphere, with 25 times higher greenhouse warming potential than CO2 resulting in a 20 % contribution to the current enhanced greenhouse effect (IPCC, 2007). The atmospheric CH4 budget has been intensively studied over the past two decades using numerous techniques, including flux measurements (Bartlett and Harriss, 1993; Singh et al, 1997; Hendriks et al, 2008; Smeets et al, 2009), mixing ratio measurements in global monitoring networks (Dlugokencky et al, 1998, 2009), isotope measurements (Quay et al, 1999; Miller et al, 2002), forward inverse modeling

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