Removal of tracer variable so4_ac

[oyvindseland]

What is the feature/what would you like to discuss?

The process tracer so4_ac should be removed and merged into so4_a1. so4_ac is created from coagulation of small so4 particles with other components. However when included in the mixtures the coagulation process is approximated by the condensation equation. The concentration of coagulate is typically a factor 10 lower than condensate. Secondary organic aerosols should be treated exactly the same as sulphate. Given the argument above any soa_ac was never included. Even if only for consistency also so4_ac shold be removed. This will save both some cpu cost and reduce the complexity. The results will not be bit-identical given that the sum of tracers will give different transport of two individual tracers, but the climate should be the same.

Is there anyone in particular you want to be part of this conversation?

@gold2718 @DirkOlivie @m

Will this change (regression test) answers?

Yes

Will you be implementing this enhancement yourself?

Yes, but I will need some help