Global simulations suggest the mean annual contrail-cirrus net radiative forcing is comparable to that of aviation’s accumulated CO₂ emissions. However, these simulations assume non-volatile particulate matter (nvPM) and ambient particles are the only source of condensation nuclei, omitting activation of volatile particulate matter (vPM) formed in the nascent plume. Here, we extend a microphysical framework to include vPM and benchmark this against a parcel model (pyrcel) modified to treat contrail formation. We explore how the apparent emission index (EI) of contrail ice crystals (AEI_ice) scales with EI_nvPM, vPM properties, ambient temperature and aircraft/fuel characteristics. We find model agreement within 20 % in the previously defined “soot-poor” regime. However, discrepancies increase non-linearly (up to 60 %) in the “soot-rich” regime, due to differing treatment of droplet growth. Both models predict that in the “soot-poor” regime, AEI_ice approaches 10¹⁶kg^-1 for low ambient temperatures (< 210 K) and sulphur-rich vPM, which is comparable to estimates in the “soot-rich” regime. Moreover, our sensitivity analyses suggest that the point of transition between the “soot-poor” and “soot-rich” regimes is a dynamic threshold on EI_nvPM that ranges from 10¹³kg^-1 – 10¹⁶kg^-1 and depends sensitively on ambient temperature and vPM properties, underlining the need for vPM emission characterisation measurements. We suggest that existing contrail simulations omitting vPM activation may underestimate AEI_ice, especially for flights powered by engines with very low EI_nvPM (<10¹³kg^-1). Under these conditions, AEI_ice might be reduced by reducing fuel sulphur content, minimising organic emissions and/or avoiding cooler regions of the atmosphere.