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 m 2 ) are needed to assess the spatial distribution of emissions. This can be readily done using spatial-integrating micrometeorological 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 N 2 O emissions of flat grassland and NH 3 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 loglinear 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.

Assessment of Open-path Spectrometer Accuracy at Low Path-integrated Methane Concentrations

“…It has an integrated transmitter/receiver unit including a detector, a micro-computer, and a distributed feedback diode laser that operates in continuous wave output. The laser is thermally stabilized and is tuned to the 1654 nm absorption line for CH 4 , which avoids interference from other gases including water vapour. The laser emits a beam through the air that is reflected and returned to the photodiode detector.…”

“…Tubing (6.35 mm inner diameter) at the reflective end of the calibration chamber allowed it to be filled with standard concentrations of CH 4 . An open "window" (diameter = 2.5 cm) at one end was a passive outlet for CH 4 and kept the chamber at atmospheric pressure. The GF3 laser was positioned at the "window" end of the chamber, aimed at the reflector within.…”

“…Open-path sensors are also advantageous because they enable the use of path-switching, in which a single sensor is used to measure a series of paths in succession or based on wind direction. Path lengths vary depending on the size of the emission source, ranging from 10 to 300 m in the literature [3,4]. The primary unit of measurement for open-path sensors is a path-integrated concentration (in units of ppm·m, or parts per million × metre), which is greatly affected by path length.…”

Line-averaging measurement methods to estimate the gap in the CO<sub>2</sub> balance closure – possibilities, challenges, and uncertainties

“…Ziemann et al: Line-averaging measurement methods their potential for special applications (Fischer et al, 2012). First joint investigations using A-TOM and optical spectrometers confirmed the suitability of combined line-integrating measurements of GHG exchange between surface and atmosphere Schäfer et al, 2012).…”

“…Furthermore, ground-based optical remote sensing methods like OP-FTIR are well suited to study dynamic atmospheric processes due to their avoidance of any disturbances upon emission and/or sampling processes (Reiche et al, 2014;Schütze and Sauer, 2016). Several successful applications of active and passive OP-FTIR are reported in terms of air quality monitoring, dynamic atmospheric processes observations, and emission rate estimations in boundary layer (e.g., Griffith et al, 2002;Allard et al, 2005;Schäfer et al, 2012;Chen, 2015). The technique is often combined with other micrometeorological investigations and provides information on several GHG target gases, such as CO 2 , CH 4 , NH 3 , and N 2 O (Griffith et al, 2012;Wilson and Flesch, 2016).…”