Approaches to measuring fluxes of methane and nitrous oxide between landscapes and the atmosphere

Abstract

The theory, applications, strengths and weaknesses of approaches commonly used for measuring trace gas fluxes are reviewed. Chambers, representing the smallest scale (∼1 m2), are the most common tools. Their operating principle is simple, they can be highly sensitive, the cost can be low and field requirements small. Problems include leaks, stickiness of some gases, inhibition of fluxes through concentration build-up, pressure effects and spatial and temporal variability in gas fluxes. Mass balance techniques are suitable for small, defined source areas, typically tens to thousands of square metres in extent. Emissions are calculated from the difference in the rates at which the gas is carried into a control volume above the source area by the wind and carried out. The required primary data are profiles of gas concentration on the downwind boundaries as well as the wind speed profile, the wind direction and the upwind background gas concentration. They have been used to measure gas emissions from landfills, treated fields and small animal herds. Circular test areas make the method independent of wind direction. A newly developed technique based on a backward Lagrangian stochastic dispersion model is also applicable to small, well-defined source areas of any shape. The surface flux is calculated form measurements of atmospheric turbulence and stability and the gas concentration at any height downwind. Implementation of the method is aided greatly by a software package WindTrax. The combination provides a powerful new tool for measuring gas emissions from treated areas and intensive animal production systems. Finally, techniques suitable for measuring gas emissions on large landscape scales (ha) are discussed. Eddy covariance is the micrometeorologist’s preferred technique for this scale. The method uses fast response anemometers and gas sensors to make direct measurements of the vertical gas flux at a point, several times a second. However, it is not feasible for many trace gases for a variety of reasons. These are discussed. Relaxed eddy accumulation is an alternative technique that retains the attraction of eddy covariance by providing a direct point measurement. It removes the need for a fast response gas sensor by substituting for it a fast solenoid valve sampling system. Flux–gradient methods are in more common use. Fluxes are calculated as the product of an eddy diffusivity and the vertical concentration gradient of the gas or the product of a transfer coefficient and the difference in gas concentration between two heights. Assumptions of the method and precautions in its application are discussed.