Micrometeorological measurements of CH 4 and CO 2 exchange between the atmosphere and subarctic tundra

Abstract

Exchanges of methane and carbon dioxide between the atmosphere and the Arctic tundra were measured continuously near Bethel, Alaska (61°05.41'N 162°00.92'W), for 5 weeks during July and August 1988. Fluxes were obtained directly using eddy correlation at 12-m altitude, and concentrations were measured sequentially at eight altitudes between 0 and 12 m. A prototype differential infrared absorption methane instrument based on a Zeeman-split HeNe laser was used for determination of methane and a flame ionization detector for total hydrocarbons (THC). Methane was found to account for nearly all the THC concentrations and fluxes. Methane fluxes at the tower site were apportioned to various methane-producing habitats, using a satellite image to classify surface vegetation at 20 × 20 m resolution. The “footprint” of the tower was computed using a Gaussian plume model for dispersion in the surface layer. Grid cells classified as dry tundra (water level 5 cm below surface) emitted methane at an average rate of 11±3 (standard error) mgCH4/m2/d, and wet meadow tundra (water level near surface) emitted 29±3 mgCH4/m2/d. Fluxes from lakes depended on wind speed, averaging 57±6 mgCH4/m2/d at the site, where the mean wind speed was 5ms−1. The mean emission rate for tundra (including lakes) around the tower was 25±1 mgCH4/m2/d, notably smaller than adopted for boreal wetlands in recent inventories of global methane sources. Emissions from major habitats derived from the tower measurements were in reasonable agreement with data from chamber studies. Errors of a factor of ∼2 accrued in scaling up the chamber data, representing 1 m2 plots, to the footprint of the tower measurements (103 m), because the satellite could not fully resolve heterogeneous methane-producing habitats. Fluxes obtained at the tower site were in harmony with fluxes from aircraft overflights. The aircraft data represent mainly afternoon periods with good flying weather, conditions associated with maximum CH4 fluxes in the tower time series. Mean fluxes from the aircraft are consequently ∼2 × higher than seasonal means from the region. Solar irradiance provided the primary control on the net ecosystem exchange (NEE) of carbon dioxide. The mean maximum uptake near the local solar noon was 1.4±0.2 gC/m2/d, and nocturnal respiration averaged 0.73±0.18 gC/m2/d. Net uptake of carbon dioxide averaged 0.30 gC/m2/d (0.1 tons C/hectare) during the period of the Arctic Boundary Layer Experiment (ABLE 3A). About 6% of the seasonal net uptake was returned to the atmosphere as methane.