Treatment of (ammonium (bi))sulfate in optics
The refractive index for 'sulfate' is taken from the OPAC database (Hess, et al., 1998; Koepke et al., MPI report no. 243, 1997). The value used is that of 'sulfate solution', i.e. particles consisting for 75% of sulfuric acid (H2SO4), based on Fenn et al. (chapter 18 in Handbook of Geophysics and the Space Enviroment, 1985). This is in line with Kinne et al. (JGR, 2003) who write that refractive indices for sulfate are "usually based on 75% sulfuric acid solution".
The OPAC refractive index agrees very well with the expression given by Boucher and Anderson (JGR, 1995) for H2SO4 at visible wavelengths (see figure below). Thus, the OPAC value can be considered to apply to pure H2SO4, as is done when applying the mixing rules in TM5.
Ammonium sulfate, ammonium bisulfate and MSA are not described by M7. The question is how to account for the presence of these components in the optics calculations.
The degree of ammonia neutralization can be characterized by the ammonium-to-sulfate ratio, which varies from 0 (sulfuric acid, H2SO4) to 1 (ammonium bisulfate, NH4HSO4) and 2 (ammonium sulfate, (NH4)2SO4). Sulfuric acid particles are extremely hygroscopic and will draw significant water mass into the aerosol phase at any relative humidity (RH). If these particles are partially or completely neutralized by drawing ammonia from the gas phase, there will be an increase in particle mass due to the added ammonium but a decrease in particle hygroscopicity at low to moderate RH, and thus a decrease in particulate water mass. On the other hand, ammonium bisulfate and ammonium sulfate both have a higher refractive index than sulfuric acid (see e.g. Boucher and Anderson, 1995). The net result of these competing factors is that one mole of sulfuric acid scatters about 25% more sunlight than one mole of ammonium bisulfate at 80% relative humidity. Ammonium sulfate is intermediate between the two. For more information see Boucher and Anderson (1995) and Boucher et al. (JGR, 1998). An excellent discussion of the optical properties of ammonium (bi)sulfate versus sulfuric acid, is also given by Adams et al. (JGR, 2001). They use optical properties based on Tang and Munkelwitz (1994) and Tang (1996; 1997).
Given that ammonia neutralization introduces competing effects, and the change in water uptake and particle size by the presence of ammonium (bi)sulfate is not described by M7, the best option seems not to try to account for the presence of ammonium (bi)sulfate in the optics calculations.