John:

The model calculates the major species in mass mixing ratio. The post-processor,
when requested, will convert MMR to "number density mixing ratio" as follows:

f(cm3-mr) = f(mmr) * barm / w

where:
  barm is mean molecular weight: 1/(o2/32+o/16+n2/28) (o2,o,n2 in mmr), and
  w is molecular weight of the species being converted.

Keep in mind that figure 2 shows difference fields (large impact minus base
case, I think). Maybe it would make things clearer if I plot out full fields
of both perturbed and control cases -- I could also plot the densities in
mass mixing ratio and number density as well, would that help?

--Ben