Suspended particulate material in the atmosphere gives rise to a number of poorly determined problems of major current concern. Direct and indirect radiative forcing (the ability of aerosol particles to affect the Earth's radiation budget) each carries larger uncertainties than all other agents of climate change. Furthermore, there are significant challenges associated with the uncertainties of the impacts of particulate material on air quality and human health. A significant contributor to these uncertainties is the vast heterogeneity in the distribution of aerosols by virtue of their disparate sources (both primary and secondary) and transformations in the moist oxidising atmosphere. The complexity of aerosol precursors and variability in the oxidising environment leads to a highly variable loading of particles of widely ranging size, age and property. There are a number of emerging frameworks aiming to integrate our theoretical or empirical understanding of the atmospheric aerosol, able to capture behaviour of physico-chemical aspects to various degrees and for a variety of applications at a range of complexity. We look forward to exploring these aspects during this meeting. The Scientific Committee warmly invites you to take part in the Discussion and looks forward to welcoming you to Leeds.