Atmospheric aerosols are suspensions of liquid, solid, or mixed particles with highly variable chemical composition and size distribution (Putaud et al. 2010). Their variability is due to the numerous sources and varying formation mechanisms (Figure 1). Aerosol particles are either emitted directly to the atmosphere (primary aerosols) or produced in the atmosphere from precursor gases (secondary aerosols). Primary aerosols consist of both inorganic and organic components. Inorganic primary aerosols are relatively large (often larger than 1 μm) and originate from sea spray, mineral dust, and volcanoes. These coarse aerosols have short atmospheric lifetimes, typically only a few days. Combustion processes, biomass burning, and plant/microbial materials are sources of carbonaceous aerosols, including both organic carbon (OC) and solid black carbon (BC). BC is the main anthropogenic light-absorbing constituent present in aerosols. Its main sources are the combustion of fossil fuels (such as gasoline, oil, and coal), wood, and other biomass. Primary BC and OC containing aerosols are generally smaller than 1 µm. Secondary aerosol particles are produced in the atmosphere from precursor gases by condensation of vapours on pre-existing particles or by nucleation of new particles. A considerable fraction of the mass of secondary aerosols is formed through cloud processing (Ervens et al. 2011). Secondary aerosols are small; they range in size from a few nanometres up to 1 µm and have lifetimes of days to weeks. Secondary aerosols consist of mixtures of compounds; the main components are sulphate, nitrate, and OC. The main precursor gases are emitted from fossil fuel combustion, but fires and biogenic emissions of volatile organic compounds (VOCs) are also important. Occasionally volcanic eruptions result in huge amounts of primary and secondary aerosols both at the ground and in the stratosphere (Boulon et al. 2011). The size and chemical composition of the particles evolve with time through coagulation, condensation, and chemical reactions. Particles may grow by uptake of water, a process that depends on chemical composition, particle size, and ambient relative humidity. The different particles have varying impacts in the atmosphere depending on composition, and the numerous sources and large range in size distributions further complicate a quantification of their effects. Both particle growth and the mixing of different particle types influence the climate effect of aerosols.
