[RegCNET] Background Aerosols

[Jonathan Winter]

I was always under the impression that even without using the aerosol
model, there were background aerosols scattering and absorbing light in
RegCM3.  For a sensitivity study, I wanted to reduce SWI, so I changed
"tauvis", which is the visible aerosol optical depth from 0.04 to 0.4.
Nothing happened.  SWI, SWN, and TA were exactly the same.  Looking in
the code and putting in some print statements, I found that in aermix.F,
"tauvis" is used to calculate "aermmb" (Background aerosol mmr).
"aermmr" (Radiation level aerosol mass mixing ratio) is then set equal
to "aermmb".  But then in aeroppt.F, "aermmr" is never used because
"chtrname(itr)" doesn't seem to be defined if ichem=0.  So I don't see
how "aermmr" ever impacts the radiation.

So my questions are: Does RegCM3 include background aerosols?  If so,
where in the code do they affect radiation?  If not, has this always
been the case?

Thanks in advance for the information,

jonathan