Primary biogenic aerosols

Primary biogenic aerosols are particles that are produced by the disintegration and dispersion of plant and animal material, and the dispersion of microbes from a variety of surfaces into the atmosphere. These primary particles can be categorized as follows: 1. living/viable particles:
bacteria viruses protozoa fungi (including yeast) algae pollen spores
2. nonliving particles:
dead microbes dead skin (e.g. dander) leaf abrasions (traceable by chemical signatures of waxes)
How biogenic aerosols enter the atmosphere
Animals (including humans) are continually shedding dead skin cells, and bacteria and fungus that can be associated with them. For example, it is estimated that it's estimated that just the mere act of changing clothes could propel 10,000 bacteria/minute into the air. Bacteria, fungi, viruses, pollen, and other primary particles enter the atmosphere during agricultural activities, such as plowing, mowing, and harvesting of crops and timber. Pollen, spores, and leaf abrasions are lofted into the air by wind. The ocean serves as an important source of primary biogenic aerosols; microbes collect in microlayers around air bubbles rising to the surface of the ocean. Subsequent bubble bursting can eject bacteria, algae and protozoa into the air.
Sizes of biogenic aerosols
Viruses and bacteria rank as the smallest of the primary biogenic particles, having diameters typically between 0.5 and 2.0 µm. These particles are known to serve as cloud condensation nuclei (CCN). The cytoplasm of some pollen is hygroscopic (can attact water), and thus are able to serve as CCN also. Most of the other primary particles are much larger, and it is estimated that 20-30% of all aerosol particles >4 µm in diameter have a biological origin.

Secondary biogenic aerosols

Secondary biogenic aerosols are formed in the atmosphere by the condensation (gas to particle conversion) of volatile organic compounds (VOC) that are released from terrestrial or marine environment. Only higher hydrocarbons (C10-C28 n-alkanes) are candidates for being forming aerosol particles. Isoprene (C5H8) and terpenes (such as a-pinene) are the most common hydrocarbons released from vegetation. Terpenes and isoprenoid compounds evolved as qualitative herbivore defense compounds; being volatile, they escape from vegetation via stomata. Higher hydrocarbons are also released from the ocean. These include organic coatings that get aerosolized by bubble bursting (these would have sodium in them), and marine organohalogens, such as chlorine containing terpenes. It is possible to some extent to differentiate between hydrocarbons originating from the marine and the terrestrial sources. Hydrocarbons released from the terrestrial biosphere have more odd-number n-alkanes than even-number n-alkanes. Although it is not an organic compound, COS (carbonyl sulfide) is another biogenically produced compound that can result in the formation of secondary aerosol particles. COS gas is the most abundant sulfur compound in the atmosphere; its S-mass is 15 times that of sulfate aerosol. COS results from biomass burning, and emission from soil and oceans. If COS is oxidized in the atmosphere to sulfate, it can contribute to sulfate aerosol via homgeneous or heterogeneous condensation. COS is less readily oxidized than other sulfur containing precursor gases; much of it ends up getting transported all the way to the stratosphere. Background concentrations of isoprene are ca. 1 ppb. Over the canopy of a tropical forest, however, isoprene concentrations can be as high as 10 ppb. Atmospheric oxidation of biogenic hydrocarbons yields compounds of low volatility that readily form aerosols. One of the first steps in the oxidation is the attack by OH radicals or ozone molecules of one of the double bonds of the compounds. Photo-oxidation is another pathway. In general, oxidation by O3 or NO3 individually yields more aerosol than oxidation by OH. However, because of the low concentrations of NO3 and O3 in the troposphere outside of polluted areas, on a global scale most VOC oxidation occurs through reaction with OH. Although terpenes and other biogenic VOC are natural, human activities may be increasing the extent to which terpenes produce aerosols. This is because O3 and NO3 are increasing via pollution, and these oxidants may have caused a 3-4 fold increase in the oxidation of terpenes since pre-industrial times. Recent studies in Amazonia confirm low aerosol yields and little production of new particles from VOC oxidation under unpolluted conditions. Given the vast amount of VOC emitted in the humid tropics, a large increase of organic aerosol production could be expected as anthropogenic emissions increase in this region.