Atmospheric particles are produced both by natural sources (e.g., desert dust, biomass burning, sea salt, volcanoes) and anthropogenic sources (e.g., industrial processes, fuel combustion, agriculture, transportation, heating). Particles can be emitted directly into the atmosphere as primary emissions (e.g., dust, sea salt, soot), or can be formed by gas-to-particle conversion of precursor gases as secondary emissions (e.g., sulfates, ammonium nitrate, secondary organic aerosol).

Once in the atmosphere, particles can change their size and composition by coagulation (i.e. collision and coalescence), condensational growth (gas molecules condense on existing particles) or shrinkage (volatile compounds evaporate from existing particles), by chemical reactions or by activation in the presence of water super-saturation to become fog or cloud droplets. Atmospheric particles are transported by advection and turbulence, and they are finally removed from the atmosphere by dry or wet deposition.

The size ranges of atmospheric particles cover several orders of magnitude and may be subdivided into the ‘nucleation mode’ (diameter < 20 nm), the ‘Aitken mode’ (diameter 20 – 100 nm), the ‘accumulation mode’ (diameter 100 – 1000 nm), and the ‘coarse mode’ (> 1 µm). In principle, there is no upper size limit, but for atmospheric science an upper limit between 20 to 100 µm is commonly applied. The atmospheric residence time is longest in the accumulation size range from 0.1 – 1.0 µm because smaller particles quickly grow to larger sizes by condensation and coagulation, and larger particles are efficiently removed by gravitational settling. Usually, the highest number concentrations are found in the size range smaller than some hundreds of nanometers (e.g., soot, sulfates, nitrates). The highest mass and volume concentrations are found in the coarse mode (e.g., mineral dust, sea salt, pollen, bacteria). The particle size fraction with an aerodynamic diameter below 10 µm is readily inhalable (in particular particles < 1 µm) and might contain toxicologically relevant substances, which can lead to adverse human health effects. Viruses like the SARS-CoV-2 have sizes between 0.06 to 0.14 µm. However, in the atmosphere they adhere mostly to other, larger atmospheric particles.