The variable ASTMX defines the spectral dependence of the aerosol extinction coefficient through the Ångström approximation (Ångström, 1908):
$$ε_k(λ)=ε_k(λ_0){(λ_0/λ)^α}$$where εk is the extinction coefficient, λ0 if a reference wavelength (typically 550 nm) and α is the Ångström exponent.
Thus, this variable has an impact on the absorption and scattering effects caused by aerosols.
For typical aerosols, $α$ values range from 1 to 2 (Holben et al., 1998, Hess et al., 1998, Dubovik et al., 2002). If ASTMX=0 (default value) , MODTRAN will use the spectral dependency of the aerosol extinction coefficient from the aerosol model selected in IHAZE.
Angström, A., (1929), "On the Atmospheric Transmission of Sun Radiation and on Dust in the Air." Geogr. Ann., Vol. 11, pp. 156-166.
Holben, B., Eck, T., Slutsker, I., Tanré, D., Buis, J., Setzer, A., Vermote, E., Reagan, J., Kaufman, Y., Nakajima, T., et al., (1998), "AERONET—A Federated Instrument Network and Data Archive for Aerosol Characterization." Remote Sensing of Environment, Vol. 66, No. 1, pp. 1-16.
Hess, M., Koepke, P., & Schult, I. (1998), "Optical properties of aerosols and clous: the software package OPAC." Bulletin of the Americal Meteorologic Soctiety. Vol. 79, No. 5, pp. 831‒ 844.
Dubovik, O.; Holben, B., Eck, T., Smirnov, A., Kaufman, Y., King, M., Tanré, D., Slutsker, I., (2002), "Variability of absorption and optical properties of key aerosol types observed in worldwide locations." Journal of the Atmospheric Sciences, Vol. 59, pp. 590-608.