LIFT-OFF OF GROUND-BASED BUOYANT PLUMES

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 descr ibed below was developed using wind tunnel observations of plumes for which buoyancy was conserved, but we also propose it for use for plume s 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 paramete rized by defining a dimensionless buoyancy flux, F * = F/u(3)W, wher e F is the local plume buoyancy flux, u is the local effective wind sp eed advecting the ground-based plume, and W is the local lateral plume width. All variables can vary with plume travel time or downwind dist ance. It is suggested that the effects of plume lift-off can be accoun ted for by multiplying the calculated ground-level concentration in th e absence of lift-off by the term exp(-6F * (0.4)). Special emphasis is given to the development of simple empirical lift-off equations fo r buoyant plumes which are trapped in building wakes. In this case, th e empirical formula that is proposed combines the exp(-6F * (0.4)) t erm with four additional terms related to the spread of plumes in buil ding wakes, and has been demonstrated to agree with wind tunnel observ ations. (C) 1998 Elsevier Science B.V.