ppol: Persistent Pollution
Variable
Persistent pollution sector
Persistent Pollution [Pollution units]
ppol = ppol + dt*( ppapr - ppasr )

ppol in the updating graph: ( how to read )
In nodes
ppapr : persistent pollution appearance rate
ppoli : initial persistent pollution
ppasr : persistent pollution assimilation rate
ppol : Persistent Pollution
Out nodes
ppolx : persistent pollution index
ppasr : persistent pollution assimilation rate
ppol : Persistent Pollution