An updated microphysical model for particle activation in contrails: the role of volatile plume particles

Abstract. Global simulations suggest the mean annual contrail cirrus net radiative forcing is comparable to that of aviation's accumulated CO2 emissions. Currently, 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 model to include vPM and benchmark this against a more advanced parcel model (pyrcel) modified to treat contrail formation. We explore how the apparent emission index (EI) of contrail ice crystals (AEIice) scales with EInvPM, vPM properties, ambient temperature, and aircraft/fuel characteristics. We find model agreement within 10 %–30 % 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, AEIice approaches 1016 kg−1 for low ambient temperatures (< 210 K) and sulfur-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 that ranges from 1013–1016 kg−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 AEIice, especially for flights powered by lean-burn engines. Furthermore, our results imply that, under these conditions, AEIice might be reduced by (i) reducing fuel sulfur content, (ii) minimising organic emissions, and/or (iii) avoiding cooler regions of the atmosphere.