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

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.