Bioaerosols include fungi, bacteria, viruses, and pollen. Their concentrations are greatest in the planetary boundary layer (PBL) and decrease with altitude. Survival rate of bioaerosols depends on a number of biotic and abiotic factors which include climatic conditions, ultraviolet (UV) light, temperature and humidity, as well as resources present within dust or clouds.
Bioaerosols found over marine environments primarily consist of bacteria, while those found over terrestrial environments are rich in bacteria, fungi and pollen. The dominance of particular bacteria and their nutrient sources are subject to change according to time and location.
Bioaerosols can range in size from 10 nanometer virus particles to 100 micrometers pollen grains. Pollen grains are the largest bioaerosols and are less likely to remain suspended in the air over a long period of time due to their weight.
Consequently, pollen particle concentration decreases more rapidly with height than smaller bioaerosols such as bacteria, fungi and possibly viruses, which may be able to survive in the upper troposphere. At present, there is little research on the specific altitude tolerance of different bioaerosols. However, scientists believe that atmospheric turbulence impacts where different bioaerosols may be found.