AEROSOL INVARIANCE IN EXPANDING COAGULATING PLUMES

We demonstrate that the total number of particles in an evolving aeros ol plume is limited in a predictable way by the competing rates of coa gulation and atmospheric dispersion, and is generally independent of t he details of particle nucleation or growth. Using a simplified phenom enological model, expressions are derived from which the time variatio n in the total number of aerosols generated by localized sources can b e calculated, as can their size distribution and local concentrations. Consideration of various microphysical processes contributing to aero sol plume development leads to the conclusion that the potential nucle ation of enormous numbers of fine particles (for example, in high-alti tude aircraft wakes and volcanic eruption clouds) does not affect the total number eventually dispersed throughout the atmosphere. We show t hat, after a suitable period of time (which is quite short relative to the time scales of regional and global dispersion), the aerosol popul ation (total number, or concentration) is independent of the initial n umber, and instead depends in a simple way on the average coagulation kernel and plume dispersion rate. In terms of these basic physical par ameters, we define a unique dimensionless number that fully determines the time evolution of the aerosol population, and show how this invar iant number can be applied to estimate the properties of particulates emitted by high-altitude aircraft.