Abstract
Abstract. Measurements of land-surface emission rates of greenhouse and other gases at large spatial scales (10 000 m2) are needed to assess the spatial distribution of emissions. This can be readily done using spatial-integrating micro-meteorological methods like flux-gradient methods which were evaluated for determining land-surface emission rates of trace gases under stable boundary layers. Non-intrusive path-integrating measurements are utilized. Successful application of a flux-gradient method requires confidence in the gradients of trace gas concentration and wind, and in the applicability of boundary-layer turbulence theory; consequently the procedures to qualify measurements that can be used to determine the flux is critical. While there is relatively high confidence in flux measurements made under unstable atmospheres with mean winds greater than 1 m s−1, there is greater uncertainty in flux measurements made under free convective or stable conditions. The study of N2O emissions of flat grassland and NH3 emissions from a cattle lagoon involves quality-assured determinations of fluxes under low wind, stable or night-time atmospheric conditions when the continuous "steady-state" turbulence of the surface boundary layer breaks down and the layer has intermittent turbulence. Results indicate that following the Monin-Obukhov similarity theory (MOST) flux-gradient methods that assume a log-linear profile of the wind speed and concentration gradient incorrectly determine vertical profiles and thus flux in the stable boundary layer. An alternative approach is considered on the basis of turbulent diffusivity, i.e. the measured friction velocity as well as height gradients of horizontal wind speeds and concentrations without MOST correction for stability. It is shown that this is the most accurate of the flux-gradient methods under stable conditions.
Highlights
The determination of trace gas and especially greenhouse gas (GHG) emission rates from open land is important to understand our climate and current and future changes in climate (Forster et al, 2007)
While intermittent turbulence may be representable in path-integrated measurements and contribute to the flux under stable conditions, the non-transporting wave structure must be removed from the computation of fluxes, It is the objective of this paper to present a method for areal-averaged emission rates made during periods of low wind speed and stable boundary layers using open-path gas measurement techniques
There was no distinct relationship between the vertical NH3 and N2O concentration gradients and the vertical gradient in horizontal wind speed, there was a tendency for greater concentration gradients corresponding to low horizontal wind speed gradients less than 0.2 m s−1 m−1
Summary
The determination of trace gas and especially greenhouse gas (GHG) emission rates from open land is important to understand our climate and current and future changes in climate (Forster et al, 2007). K. Schafer et al.: Areal-averaged trace gas emission rates from long-range open-path measurements (10 000 m2) require the application of micro-meteorological methods (Smith et al, 1994). Such path-integrated measures have been developed using laser-scintillometers (Nakaya et al, 2007) and acoustic travel-time tomography (Barth et al, 2007) These measurement techniques possess wind speed detection limits of about 0.2 m s−1, and are not sensitive enough to describe the turbulence during stable and very stable conditions (Andreas, 2000). While intermittent turbulence may be representable in path-integrated measurements and contribute to the flux under stable conditions, the non-transporting wave structure must be removed from the computation of fluxes, It is the objective of this paper to present a method for areal-averaged emission rates made during periods of low wind speed and stable boundary layers using open-path gas measurement techniques. Note that the methods evaluated here substitute point measurements in the usual flux-gradient methods by integrated open-path concentration measurements (Wichink Kruit et al, 2007, 2010; Luhar et al, 2009) in order to extend the spatial representativeness of flux measurements
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