An advective hypothesis for the formation of the stratospheric aerosol layer

In earlier work it was concluded that the large (0.3μ rad) particles of the stratospheric aerosol layer are formed by a heterogeneous nucleation process, and that a primary factor controlling the particle size distribution of the aerosol is the number of particles available to serve as nucleation sites for the subsequent heterogeneous formation process. In this paper two possible sources of these sites are considered; transport from the troposphere, and homogeneous formation in the stratosphere. Using a simple numerical model, it is shown that if homogeneous nucleation occurred in the stratosphere it would, in general, result in the formation of relatively large (10cm-3) numbers of new particles, which previous work has shown to be incompatible with the formation of the observed large particles. It is also pointed out that vertical diffusive transport from the troposphere would also introduce unacceptably large numbers of particles, but that vertical advection, in which air enters the stratosphere in the updrafts of intense tropical thunderstorms, would have the requisite low particle concentrations and also be compatible with other observed properties of the stratospheric aerosol layer