Aerosol particle density typically follows a decreasing vertical distribution with, in normal meteorological conditions, higher densities in the boundary layer (0 km to H km) and troposphere (H km to 10 km). Among these two layers, variability and mixture of aerosol types is higher in the boundary region due to interactions between land surface and atmosphere (e.g., biomass burning, dust storms or urban pollution). The vertical distribution in each layer largely depends on particle density and meteorological conditions, particularly wind, turbulence and vertical profiles of pressure and temperature.
In ALG, an exponential decay function is used to model the vertical distribution of the extinction coefficient in
the boundary layer following the approach in (Hess et al., 1998).
This exponential decay function is characterized by the scale-height variable Z (km), which has typical values
between 0.5 km and 99 km (for a homogeneous layer). If the user introduces a value for Z, ALG will use by
default a value of H=2 km for the boundary layer top height.
If this value is not introduced, ALG will use instead (default option)
the predefined MODTRAN aerosol vertical distribution.
This variable controls the distribution of aerosols within the boundary layer, which influences the absorption and scattering effects caused by aerosols.
Hess, M., Koepke, P., & Schult, I. (1998), "Optical properties of aerosols and clous: the software package OPAC." Bulletin of the Americal Meteorologic Soctiety. Vol. 79, No. 5, pp. 831‒ 844.