Refining an Inverse Dispersion method to Quantify Gas Sources on Rolling Terrain

  • Author / Creator
    Hu, Nan
  • In this thesis, an indirect methodology for estimating gas emission rate to the atmosphere from small surface point or area sources is investigated. More specifically, the thesis is a study of an“inverse dispersion” method which has become widely used for estimating ground-air gas fluxes (Q) from agricultural sources. Like all inverse dispersion methods, the “bLS” (for “backward Lagrangian stochastic”) method hinges on the placement of concentration detectors upwind and downwind from the source, allowing to determine the rise (Δc) in mean gas concentration for which it is responsible. Also in common with the norm for “inverse dispersion”, a mathematical model of atmospheric dispersion is invoked so that the wanted source strength Q can be inferred from the measured concentration rise Δc, which however must be supplemented with relevant meteorological information (such as mean wind direction and speed, and thermal stratification). In the case of “‘bLS,” the dispersion model is a backward Lagrangian stochastic model, which computes fluid element trajectories backwards in time and space from their arrival at the concentration detectors to their earlier point or points of contact with the source or sources. An assumption inherent to most implementations of inverse dispersion method is that wind statistics in the atmospheric surface layer are horizontally-homogeneous (i.e. wind statistics vary only with height). It is of interest to establish how robust the inverse dispersion approach may be, when applied to quantify sources on land surfaces that are patently not flat and uniform – complications that result in horizontal variability of the wind statistics. In that context, this thesis analyses a trace gas dispersion experiment with multiple fixed point sources arrayed on gently rolling terrain, to investigate the performance of inverse dispersion using a well known bLS dispersion model (WindTrax) that, strictly speaking, is appropriate only for the case of horizontally-homogeneous winds. Despite the fact that measured mean wind speeds revealed spatial variation of order ±10% over the site, results of the inversion to estimate source strength indicate that the unwanted impact of this moderate terrain can be compensated by assigning every concentration detector its true height above local ground (for line-averaging optical detectors, this means the straight line light path transforms to a curved line). This strategy permits easy extension of a well proven method to conditions that, `a priori, had been considered unsuitable; and the thesis culminates with application of the bLS approach to deduce an aggregate methane emission rate from a herd of (twenty) cattle confined within a long, narrow field of pasture.

  • Subjects / Keywords
  • Graduation date
    Spring 2016
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.