Gas dispersion trials: a surface area source enclosed by a windbreak

As a simple alternative to direct measurements, it is often useful to diagnose the strength (Q, kg m^-2 s^-1) of a finite surface area source of a trace gas, from nearby measurement(s) of concentration (C, kg m^-3); this is sometimes called an "inverse dispersion" exercise, and an example would be determining the volatilization rate of ammonia from a soil treatment plot.

In such a task, an atmospheric dispersion model plays a central role, in conjunction with appropriate measurements of C, and of the atmospheric state (wind strength and direction; stability). Modern Lagrangian stochastic (LS, particle trajectory) models have proven particularly well-suited to this task, but although they are capable of accommodating disturbed flows, they have so far only been applied (experimentally) to horizontally-uniform flows (ie. in which neither the source itself, nor any obstacle, disturbs the wind).

To study the vulnerability of the methodology to flow disturbance, and provide the experimental basis for an exercise in adaptation of an LS model to a known disturbed flow, we performed a series of trials in each of which tracer methane was released from a synthetic 6m x 6m area source on ground, and two nearby lasers measured line-average methane concentrations along paths of 50 - 100 m. We compare trials that took place on open, undisturbed land (thin stubble; roughness length z₀ about 1 – 2 cm) against others when the source was enclosed by a 20 m x 20 m windbreak fence, whose height was h=1.25 m, and which grossly disturbed the wind.