The recognition that particle phase, liquid to glassy, can both influence and be influenced by the organic aerosol ageing process in the atmosphere has led to intense experimental and modeling work to understand the underlying chemistry. The primary manifestation of phase is in the overall uptake coefficient and reaction rate between oxidants and the aerosol particle, with glassy particles reacting more slowly than liquid ones. In this work, we develop a detailed reaction-Fickian diffusion model that describes the early stages of oxidation of a model liquid system: reaction of squalane by OH radical. The simulations are performed using stochastic methods that enable an absolute time base to be calculated and validated by comparison to experiment. The results show that although the measured uptake coefficient is about 0.3, the uptake is not equivalent to a probability for surface sticking even though reaction rates are high. In order to reproduce experimental observations, the microscopic sticking probability for OH on the aliphatic organic surface in the model is 1x10<sup>-4</sup>. We show that the uptake coefficient is an intrinsically emergent quantity that depends upon particle size, viscosity, and OH concentration. The OH collision frequency determines the influence of intrinsic glassiness of the aerosol particle on overall reactivity, with very small and viscous particles at atmospheric OH densities having liquid-like characteristics, and larger liquid particles appearing to be glassy under laboratory conditions with high OH densities.

On the connection of organic aerosol ageing to viscosity

249th National Meeting (2015)

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