A parameterisation for the co-condensation of semi-volatile organics into multiple aerosol particle modes

A new parameterisation for the cloud droplet activation of multiple aerosol modes is presented that includes the effects of the co-condensation of semi-volatile organic compounds (SVOCs). The novel work comes from the dynamic condensation parameterisation that approximates the partitioning of the SVOCs into the condensed phase at cloud base. The dynamic condensation parameterisation differs from equilibrium absorptive partitioning theory by calculating time-dependent condensed masses that depend on the updraft velocity. Additionally, more mass is placed on smaller particles than at equilibrium, which is in better agreement with parcel model simulations. All of the SVOCs with saturation concentrations below 1×10−3µgm−3 are assumed to partition into the condensed phase at cloud base, defined as 100% relative humidity, and the dynamic condensation parameterisation is used to distribute this mass between the different aerosol modes. An existing cloud droplet activation scheme is then applied to the aerosol particles at cloud base with modified size distributions and chemical composition to account for the additional mass of the SVOCs. Parcel model simulations have been performed to test the parameterisation with a range of aerosol size distributions, composition, and updrafts. The results show excellent agreement between the parameterisation and the parcel model and the inclusion of the SVOCs does not degrade the performance of the underlying cloud droplet activation scheme.