Internal and external mixing in atmospheric aerosols by coagulation: Impact on the optical and hygroscopic properties of the sulphate-soot system

A sectional aerosol model is used to study the impact of internal/external aerosol mixing on the optical and hygroscopic properties of two-component aerosol populations. Time evolution of the aerosol spectrum due to mixing by coagulation of two initially different particle populations is simulated. The impact of the state of mixing is determined through comparison of model results assuming either internal mixture (IM) only or both internal and external mixture (IEM). The model is first validated using the analytic solution for an idealized IEM problem and secondly against experimental data in an urban plume. Then, a preliminary application is made which consists in a scenario of mixing between a plume containing soot particles and its environment mainly loaded with accumulation-mode sulphate particles. Emphasis is put on the evolving state of mixing of the aerosols thus formed. Evolution of the optical properties is computed using Mie theory for both homogeneous and concentric spheres (coated aerosols). For this particular scenario, the IEM model is more light-diffusive and less light-absorbant than the IM model. The extinction coefficient is practically insensitive to the way of modelling of the state of mixing. Hygroscopic properties are also derived, based on empirical growth laws, showing more activation in the IEM than in the IM model. However, further such studies are necessary to determine more fully the variability range in the optical and hygroscopic properties of aerosols at different degrees of mixing.