Physical and Chemical Properties of Atmospheric Aerosols

Abstract

Particles in the atmosphere are distinguished by their size, shape and composition. They can be directly emitted from sources and can be formed in the atmosphere by chemical reactions and physical processes. Once particles are formed, their properties can be modulated in space and time by atmospheric physical and chemical processes, such as condensation, evaporation and coagulation. Eventually, the particles are removed from the atmosphere by wet or dry deposition, with such removal occurring minutes to weeks after their release or formation, and after travelling metres to thousands of kilometres. This chapter summarises key physical and chemical properties of atmospheric aerosols. These properties can be described at various levels of aggregation (bottom-up, from the single-particle perspective) or discretisation (topdown, from the aerosol perspective). An accounting of each particle by size and composition would provide the finest resolution. In contrast, the bulk composition over all particles would be the coarsest resolution. Furthermore, given the dynamic nature of atmospheric aerosols, the degree of time integration is important. Particles in the atmosphere are directly emitted by sources (primary emissions) or formed in the atmosphere by physicochemical processes (secondary formation) [1]. Combustion and other high-temperature processes are largely responsible for primary emissions of fine-mode particles, while mechanical processes such as grinding, entrainment of dust and soil and droplet formation by waves generate coarse-mode particles. Condensable gases can homogeneously nucleate to form new particles in the atmosphere if there is insufficient surface area for their uptake. Heterogeneous condensation is the uptake of vapour by pre-existing particles, with the reverse process of evaporation also important. A summary of the atmospheric aerosol properties must necessarily reflect upon the methods used to arrive at the current state of knowledge. As tools to probe the atmosphere continue to advance, refinements will be made to the understanding of atmospheric aerosols. Starting with the dimensions of size, composition, space and time, observational data are collected from measurements that integrate across these dimensions. Most observational data have been collected at fixed locations, although more data are being obtained from satellite and other profiling instruments that provide spatial resolution [2, 3]. The spatial zone represented by monitoring at a fixed site will depend on the property of interest. It may be possible to infer