Understanding the chemical processing experienced by aerosol is particularly challenging given the vast diversity in chemical composition encountered in atmospheric particulate matter. Furthermore, the physical properties discussed in the previous page play a role in regulating how chemical processing proceeds, which ultimately leads to changes in those properties. Thus, chemical processing and physical properties are closely coupled, further complicating our understanding.
Aerosol may experience chemistry in a number of different ways. The most simple is bulk phase chemistry, where the reaction constituents are already present in the particle phase. This may occur when liquid droplets coalesce and become mixed, or if chemistry becomes initiated due to illumination with appropriate wavelengths (UV light for examples). Gas-to-particle partitioning of reactive gas phase species can lead to heterogeneous chemical processing involving gas uptake of molecules such as ozone, or radical species such as hydroxyl (OH).
The rate at which chemical processing occurs depends on the particular reactions, the concentrations of the substituents, and the physical state of the aerosol. For example, due to slow molecular movement in viscous and solid phases, reactions are generally much slower. This is particularly important in aerosol which can enter supersaturated states, leading to a large increase in viscosity, and is a key area of current research in the field. The role of interfacial absorption of species and intermediates may lead to enhanced rates for some reactions relative to those which are measured in the bulk. This highlights the potential use of aerosol in chemical synthesis, with droplets acting as micro-compartments to enhance the rate of formation of products.
Figures © 2014 James Davies