Lift-off of ground-based buoyant plumes

Abstract

If a ground-based plume has enough buoyancy to overcome the effects of ambient turbulence and other physical processes, it will rise or lift-off the ground, thus reducing the health and environmental impacts of chemicals released accidentally to the atmosphere. The approach described below was developed using wind tunnel observations of plumes for which buoyancy was conserved, but we also propose it for use for plumes whose buoyancy flux varies with distance; this can occur due to the presence of aerosols, depolymerization, reactions with water vapor or other chemicals to form new products, and evaporation and condensation processes. It is assumed that the lift-off phenomenon can be parameterized by defining a dimensionless buoyancy flux, F∗∗= F/ u 3 W, where F is the local plume buoyancy flux, u is the local effective wind speed advecting the ground-based plume, and W is the local lateral plume width. All variables can vary with plume travel time or downwind distance. It is suggested that the effects of plume lift-off can be accounted for by multiplying the calculated ground-level concentration in the absence of lift-off by the term exp(−6 F∗∗ 0.4). Special emphasis is given to the development of simple empirical lift-off equations for buoyant plumes which are trapped in building wakes. In this case, the empirical formula that is proposed combines the exp(−6 F∗∗ 0.4) term with four additional terms related to the spread of plumes in building wakes, and has been demonstrated to agree with wind tunnel observations.