and plant pollen), and various fragments or excretions (e.g., plant debris and brochosomes). Bioaerosol particle diameters range from nanometers up to about a tenth of a millimeter. The upper limit of the aerosol particle size range is determined by rapid sedimentation, i.e., larger particles are too heavy to remain airborne for extended periods of time. Bioaerosols include living and dead organisms as well as their fragments and excrements emitted from the biosphere into the atmosphere. Included are archaea, fungi, microalgae, cyanobacteria, bacteria, viruses, plant cell debris, and pollen. Historically, the first investigations of the occurrence and dispersion of microorganisms and spores in the air can be traced back to the early 19th century. Since then, the study of bioaerosols has come a long way, and air samples collected with aircraft, balloons, and rockets have shown that bioaerosols released from land and ocean surfaces can be transported over long distances and up to very high altitudes, i.e., between continents and beyond the troposphere. Bioaerosols play a key role in the dispersal of reproductive units from plants and microbes (pollen, spores, etc.), for which the atmosphere enables transport over geographic barriers and long distances. Bioaerosols are thus highly relevant for the spread of organisms, allowing genetic exchange between habitats and geographic shifts of biomes. They are central elements in the development, evolution, and dynamics of ecosystems. Transport and distribution Once aerosolized, microbial cells enter the planetary boundary layer, defined as the air layer near the ground directly influenced by the planetary surface. The concentration and taxonomic diversity of airborne microbial communities in the planetary boundary layer has been recently described, though the functional potential of airborne microbial communities remains unknown. From the planetary boundary layer, the microbial community might eventually be transported upwards by air currents into the free troposphere (air layer above the planetary boundary layer) or even higher into the stratosphere. Microorganisms might undergo a selection process during their way up into the troposphere and the stratosphere. Subject to gravity, aerosols (or particulate matter) as well as bioaerosols become concentrated in the lower part of the troposphere that is called the planetary boundary layer. Microbial concentrations thus usually show a vertical stratification from the bottom to the top of the troposphere with average estimated bacterial concentrations of 900 to 2 × 107 cells per cubic metre in the planetary boundary layer and 40 to 8 × 104 cells per cubic