An Airborne Study of Methane Point Source Dispersion And Mixed Layer Scaling

The dispersion of natural gas in the atmospheric boundary layer is investigated using light aircraft equipped with temperature, humidity, methane and horizontal wind sensors flying in the lower portion of the mixed layer under a variety of micrometerological conditions. Natural gas leaking from a high pressure (~500 PSI) transmission line is initially denser than the surrounding air, inhibiting vertical mixing. On the other hand, flight safety requires that most aerial patrol operations be conducted at a minimum altitude of 150 meters. Research funded by the Pipeline Research Council International to investigate the viability of transmission line leak detection from fixed wing aircraft using cavity ring down spectroscopy included 10 days of low altitude, low speed flights over controlled natural gas releases to map the distribution of methane plumes above the surface layer. Data from these flights is presented and compared with existing dispersion parameterizations. In addition to the boundary layer structure obtained from airplane and rawinsonde profiles, the convective velocity scale (w_*), heat fluxes, and winds are measured at 2 m height using a 3D sonic anemometer. Of particular interest here is how well we can determine the convective velocity scale armed only with the boundary-layer height (Venkatram, 1978) using the relation w_*=1.12x10-3 zi. This parameterization is tested against winter and spring time boundary layers in northern California, southern California and near Las Vegas, Nevada. Finally, the theoretical plume dispersion coefficients (σ_y, σ_z ) are compared with observed plume distributions.