Deducing Ground-to-Air Emissions from Observed Trace Gas Concentrations: A Field Trial with Wind Disturbance

Abstract

Inverse-dispersion techniques allow inference of a gas emission rate Q from measured air concentration. In "ideal surface layer problems," where Monin-Obukhov similarity theory (MOST) describes the winds transporting the gas, the application of the technique can be straightforward. This study examines the accuracy of an ideal MOST-based inference, but in a nonideal setting. From a 6 m ϫ 6 m synthetic area source surrounded by a 20 m ϫ 20 m square border of a windbreak fence (1.25 m tall), Q is estimated. Open-path lasers gave line-averaged concentration C L at positions downwind of the source, and an idealized backward Lagrangian stochastic (bLS) dispersion model was used to infer Q bLS . Despite the disturbance of the mean wind and turbulence caused by the fence, the Q bLS estimates were accurate when ambient winds (measured upwind of the plot) were assumed in the bLS model. In the worst cases, with C L measured adjacent to a plot fence, Q bLS overestimated Q by an average of 50%. However, if these near-fence locations are eliminated, Q bLS averaged within 2% of the true Q over 61 fifteen-minute observations (with a standard deviation Q/Q ϭ 0.20). Poorer accuracy occurred when in-plot wind measurements were used in the bLS model. The results show that when an inverse-dispersion technique is applied to disturbed flows without accounting for the disturbance, the outcome may still be of acceptable accuracy if judgment is applied in the placement of the concentration detector.

Quantification of methane emissions from full-scale open windrow composting of biowaste using an inverse dispersion technique

“…It could be shown that the inverse dispersion technique agreed within 10-30% with the tracer release method. This is a reasonable result and corresponds with other literature studies (Flesch et al, 2005b;Andersen et al, 2010;Goldsmith et al, 2012). It should be mentioned that the current setup of the C 2 H 2 laser showed interferences with water vapour at measurement paths longer than 60 m (data not shown) and higher air humidity.…”

“…open path tunable diode laser spectroscopy -OP-TDLS) combined with meteorological data. This method has been used to characterise fugitive emissions from large area sources including landfills (Zhu et al, 2013), agricultural operations (Flesch et al, 2005b;McGinn et al, 2006McGinn et al, , 2011 and biodigesters (Flesch et al, 2011). Emission rates could be quantified with an uncertainty less than 10-20% (Flesch et al, 2004(Flesch et al, , 2005a.…”

Ground-to-Air Gas Emission Rate Inferred from Measured Concentration Rise within a Disturbed Atmospheric Surface Layer

“…Flesch et al (1995) introduced a particularly flexible variant based on the computation of an ensemble of backwardin-time trajectories from detector to source to establish the needed theoretical C-Q relationship (the backward Lagrangian stochastic, or bLS, method). Flesch et al (1995) introduced a particularly flexible variant based on the computation of an ensemble of backwardin-time trajectories from detector to source to establish the needed theoretical C-Q relationship (the backward Lagrangian stochastic, or bLS, method).…”

“…Flesch et al (1995) introduced a particularly flexible variant based on the computation of an ensemble of backwardin-time trajectories from detector to source to establish the needed theoretical C-Q relationship (the backward Lagrangian stochastic, or bLS, method). The emergence of line-averaging gas detectors having proven opportune for the application of this technique (largely because modeling crosswind spread is more difficult than vertical spread), bLS has now been applied by the present authors and colleagues to estimate gas emissions from confined animal operations, such as hog farms (Flesch et al 2005a), feedlots , and dairies ). The emergence of line-averaging gas detectors having proven opportune for the application of this technique (largely because modeling crosswind spread is more difficult than vertical spread), bLS has now been applied by the present authors and colleagues to estimate gas emissions from confined animal operations, such as hog farms (Flesch et al 2005a), feedlots , and dairies ).…”

“…An earlier investigation (Flesch et al 2005b) of MO-bLS in intentionally disturbed winds suggested that, provided the concentration detector was sufficiently remote from the source to ensure that a good fraction of the path from source to detector lay in undisturbed flow, the technique could be applied with adequate accuracy. For this reason we here relabel the bLS technique used by previous authors as MO-bLS.…”