Factors Affecting the Uptake and Reactivity of OH with Organic Aerosol

Abstract:

The uptake of gas phase species onto an aerosol surface is the primary process that governs oxidative aging of aerosols. The complexity of the resulting multiphase chemical and physical transformations has been challenging to describe at the level of accuracy required to predict aerosol lifetimes, chemical composition, phase and other key properties. Stochastic simulations of a modelcoupling free radical reactions and Fickian diffusion that quantitatively reproduces experimental observations areused to examine the early stages of a well-mixed liquid model system, the oxidation of squalane by hydroxyl radical (OH). The results elucidate the physical meaning of the uptake coefficient and reveal internal details of the particle as it undergoes oxidation. The uptake coefficient is not equivalent to an accommodation coefficient: is an intrinsically emergent process that depends upon particle size, viscosity, and OH concentration. Well-mixed, liquid behavior is also found to depend on these systems characteristics. The small particle size creates large instantaneous concentration gradients, leading to dispersal of OH within the top few nanometers and rapid mixing of long-lived peroxy radicals throughout. The implications of these results for connecting laboratory and natural oxidative processes will be discussed.