The layers are defined by their natural average temperature profiles. The troposphere is the layer closest to the earth. Within the troposphere there are several important layers. The thickness of the PBL is determined by surface driven convection. Atmospheric stability is defined qualitatively as a measure of the degree of vertical mixing in the atmosphere.
Stable atmospheres are poorly mixed in the vertical direction.
Unstable atmospheres are those that are significantly mixed in the vertical direction. The assumptions used to derive the GPM should be understood in order to understand the uncertainties associated with modeling results. The GPM is often used as a screening calculation method for exposure assessments. The GPM is derived from the advection-diffusion equation. The GPM is derived for continuous point sources.