Some recent developments in trace gas flux measurement techniques

Abstract

Meteorological techniques for measuring trace gas fluxes on three important scales not well catered for by conventional methods have been discussed: an inverse Lagrangian dispersion method appropriate for the canopy scale, mass balance methods for small and heterogeneous ecosystems and boundary layer budgeting schemes for the regional scale. The inverse Lagrangian analysis offers a relatively simple measurement scheme for inferring fluxes of trace gases and their source-sink distributions within plant canopies. Inputs are the profiles of mean gas concentration and turbulence within and above the canopy. The analysis provides a bridge between chamber and cuvette measurements on soil and foliage elements and flux measurements on a field scale. Mass balance methods are appropriate for flux measurements in small ecosystems, tens of meters in lateral extent. Fluxes from areas of known geometry are calculated from the rate at which the wind transports gas across the upwind and downwind boundaries of the designated area. The method can fill the gap between chambers of, say, 1 m 2 in area and conventional micrometeorological methods representing, say, 10 4 m 2 . It can suffer from errors arising from the large number of gas analyses required for a flux determination and may become unreliable when there are light winds and variable wind directions. On the other hand, it is non-disturbing, has a simple theoretical basis, smooths over surface heterogeneity and is independent of atmospheric stability or the shape of the wind profile. Convective and nocturnal boundary layer (CBL and NBL) budgeting techniques are discussed in the context of a recent experiment to estimate regional fluxes of carbon dioxide, methane and nitrous oxide in a rural area of southeast Australia. CBL techniques estimate the average surface flux over regions of order 100 km 2 through the buildup or drawdown of gas concentration in the atmospheric mixed layer and its depth. An integral form of CBL budgeting was used to estimate daily fluxes. Input data were gas concentrations at 22 m and CBL heights obtained with radiosondes. The atmospheric gas concentrations above the CBL were assumed to be the current clean-air baseline values. It was concluded that even with this simplified obsevation scheme, CBL budgeting can be a very useful survey tool and in regions that are homogeneous in the large, can provide better than order of magnitude estimates of trace gas fluxes. NBL budgeting techniques follow the change of gas storage in the surface layer at night when low-level radiative inversions inhibit vertical mixing. The footprint is difficult to estimate, but is of order 1 to 5 km. On one occasion during the experiment, balloon measurements were made up to a height of 100 m, but routinely, tower-based measurements were made to 22 m. It was concluded that for gases whose emissions do not exhibit marked diurnal cycles, NBL budgets may be simpler alternatives than either CBL budgets or conventional micrometeorological measurements made by day. When diurnal variation is large, both day and night measurements are needed to define the 24-hour flux.