• Atmospheric particles have important impacts on air quality and human health (e.g., particulate matter (PM) related respiratory illnesses).
• Atmospheric particles are important in the Earth’s climate system because they scatter and absorb solar radiation, influencing directly the temperature at the surface and within the atmosphere, and exerting a positive or negative radiative forcing. Atmospheric particles influence the radiation budget in a direct, semi-direct and indirect manner (see graph below). The scattering and absorption of short and long wave radiation is called the direct effect. This can lead to either warming or cooling of the atmosphere depending on the proportion of light scattered to that absorbed. The semi-direct effect describes the warming of the boundary layer, through the absorption of radiation by aerosols, which can lead to evaporation of clouds. This will allow more solar radiation to reach the surface. The indirect effect concerns the ability of aerosols to act as cloud condensation nuclei which influences the micro-physical and optical properties of clouds, thus changing the radiative and precipitation properties and the lifetime of clouds.
• Atmospheric particles provide surfaces for and take part in (photo)chemical reactions in the atmosphere. Further, airborne particles contribute to the atmospheric transport of both nutrients and pollutants, to the advantage or disadvantage of ecosystems.
• Atmospheric particles can reduce visibility distinctly and create specific hazards, e.g. during desert dust storms or volcano eruptions.

Because of the high temporal and spatial variability of atmospheric particles and because of their influence on cloud formation and cloud properties, atmospheric particles form one of the major contributors to uncertainties in current climate models.