Fumigation of pollutants in and above the entrainment zone into a growing convective boundary layer: a large-eddy simulation

Fumigation of a passive plume located in or above the entrainment zone (EZ) into a growing convective boundary layer (CBL) has been simulated by large-eddy simulation (LES). Three non-dimensional parameters, α(=we0/w*0), z0/zi0, and σz0/zi0, are used to classify different cases, where w*0 is the convective velocity scale, we0 the initial entrainment velocity, zi0 the initial CBL height, z0 the initial plume height, and σz0 is the initial plume half-depth. Forty cases have been run and analysed. The crosswind-integrated concentrations have been compared with existing laboratory data from a saline convection tank. The results show that LES is a promising tool to reproduce fumigation processes. With a relatively coarse grid mesh near the EZ, LES derives reliable results that are in a good agreement with the laboratory data. The first parameter, α, containing the effects due to inversion strength, plays an important role in determining C0(T), the ground-level concentration (GLC) as a function of dimensionless time, T. For large α (say >0.03, corresponding to fast entrainment), variation of α gives significant change in C0(T); whereas for a wide range of α between 0.01 and 0.02 (corresponding to slow entrainment), C0(T) is almost independent of α. The starting time of fumigation does not vary significantly with the second parameter, z0/zi0 (relative height of plume), although C0(T) is, in general, smaller for a higher plume. This confirms laboratory findings that the traditional notion of zero fumigation for a high plume (say above 1.10zi) is not correct. The effect of the third parameter, σz0/zi0, is on the magnitude of C0(T); thinner initial plumes have higher GLCs.