The visibility and length of chimney plumes

Abstract

N onhebel's (1960) theoretical analysis of chimney smoke visibility is extended to incorporate the scattering of sunlight and skylight by plume particles. Relations are derived theoretically which show how both the visible length and the smoke contrast depend on the particles in the plume and the weather conditions. The theory treats inert particles which do not change by chemical reactions in the atmosphere or coagulation, etc. It is also limited to low particle concentrations where multiple scattering effects may be ignored. The accuracy of the calculation of the visibility and length of plumes from commercial power stations is limited because the optical properties of the smoke particles are not known accurately. However, using approximations for the optical properties of the complex smoke particles, given by the Mie theory for uniform spheres, it was found that there was reasonable agreement between the observed and calculated plume lengths and contrasts. It is known that in stable atmospheric conditions some plumes occasionally remain visible for some miles downwind of the chimney. These persistent plumes are discussed briefly and simple criteria for their occurrence are derived.