Areal-averaged trace gas emission rates from long-range open-path measurements in stable boundary layer conditions

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 more readily done using spatial-integrating micro-meteorological methods than the widely-utilized small chamber measurements. Several micro-meteorological flux-gradient methods utilizing a non-intrusive path-averaging measurement method were evaluated for determining land-surface emission rates of trace gases under stable boundary layers. 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. 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 involved 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 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 fluxes in the stable boundary layer.