This paper investigates which parameters can be used to model local soot mass growth rates in laminar diffusion flames. Measurements of soot concentrations and number densities are made in a range of ethylene diffusion flames. The data are combined with modelled flame field velocities and mixture fraction to extract local soot formation rates which are evaluated and presented in two ways: #x2014;as surface specific mass growth rates in terms of local temperature and mixture fraction; #x2014;as particle reactivity per active site in terms of local temperature and mixture fraction. The conclusion is that the surface specific growth rate function shows better consistency than the active sites model. However, the latter model is probably handicapped at present by inadequate knowledge of particle aging in diffusion flames and it has better potential to unify diffusion and premixed flame soot growth rates.
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