Size distributions for the three dust populations in the Jones et al. (2013) dust model. Small and large carbonaceous grain size distributions (dot-dashed and dotted lines respectively). a-C coated silicate grains (triple-dot-dashed line). The grey line represents the overall carbonaceous grain size distribution, while the blue line is the power-law graphitic-type and silicate grain size distribution assumed by Jones et al. (1994, 1996). The assumed dust-to-gas mass ratio, M/M_H, is the same for the two dust models.