A parameterisation of the soot aging for global climate models

Abstract

The representation of soot in global climate models is desirable since it contributes to both the direct and indirect climate effect. While freshly emitted soot is initially hy-drophobic and externally mixed, it can be transferred into an internal mixture by coag-ulation, condensation or photochemical processes. These aging processes affect the 5 hygroscopic qualities and hence the growth behaviour, the optical properties and eventually the lifetime of the soot particles. However, due to computational limits the aging of soot in global climate models is often only parameterised by an estimated turnover rate resulting in a lifetime of soot of several days. Based on the results of our simulations with a comprehensive mesoscale model, we derive the timescale on which diesel 10 soot is transferred from an external to internal mixture, and propose a parameterisation for the use in global climate models. This parameterisation is applicable to continental conditions in industrialised areas as can be found in Central Europe and North Amer-ica. For daytime conditions, away from the sources, condensation is dominant and the aging process occurs very fast with a timescale of τ=2 h. During night time conden-15 sation is not effective. Then coagulation is the most important aging process and our parameterisation leads to a timescale between 10 h and 40 h.