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What Aerosols Are

  • Aerosols are defined as collodial systems of liquid or solid particles suspended in a gas.
  • Can be natural or anthropogenic (man-made) in origin. Most occur naturally from dust, volcanoes, sea salt spray, etc.
  • Colloidal suspension that may be stable for <1 second or > 1 year.
  • Particles can range anywhere in diameter from 1nm to 1mm.

Sources, Formation, & Deposition:

  • Anthropogenic sources of aerosols include burning of fossil fuels and the alteration of the natural Earth surface.
  • Natural sources of aerosols include sea salts from the ocean's sea spray and bubble bursting, to phytoplankton emissions, windstorms, viruses & bacteria.
  • Understanding the source of aerosols is an important consideration when comparing continental vs. oceanic environments or in-town vs. rural locations.

  • Primary emissions are aerosols that result from direct emission sources whether they be natural or anthropogenic in origin.
  • In Situ formation or "Secondary" organic aerosols (SOA) are photochemically produced particles from gaseous pollutants.
  • Deposition: Most particles are washed out of the atmosphere by rain, while some can fall out or accumulate with other particles to eventually fall out. The smaller the particle is the longer it can stay suspended before falling out, and therefore we say it has a "longer lifetime" or is susceptable to chemical and physical transformation (aging). Below is a diagram illustrating these general processes.

Aerosol Lifetimes:

  • tbc


  • The solar irradiance received from Earth's surface back into space is referred to as reflectivity or Earth's albedo. It's estimated that approximately 31% of all the incoming solar light of every wavelength is reflected, while the rest is absorbed and re-emmited as infrared radiation (IR).
  • Aerosols have one of the least understood effects on climate, they can interact both directly and indirectly with the Earth's radiative budget, and can also serve as sites for chemical reactions to take place (heterogeneous chemistry).
  • Aerosols are commonly known to have a cooling effect on the atmosphere since they scatter light, but unlike the warming effect of of the greenhouse gases taking place across the globe, the cooling effect of anthropogenic aerosols is expected to be regionally dependent on industrial areas.
  • The "Indirect effect" of aerosols is believed to effect climate by changing properties of clouds. This can be explained by increased concentrations of aerosols within clouds that act as "seeds" (or cloud condensation nuclei (CCN)) to start the formation of cloud droplets. The water in this case gets spread over more and more particles, each of which is correspondingly smaller. Smaller particles fall more slowly in the atmosphere and therefore decrease the amount of rainfall. This means that fluctuating levels of aerosols in the atmosphere can change also the frequency of cloud occurence, cloud thickness, and rainfall amounts that could hypothetically lead to drought.
  • "Twomey effect":
  • "Global dimming": Is a gradual reduction in the amount of global direct irradiance at the Earth's surface.


    • Aerosols can range in size from 0.01 microns to a few tens of microns, and can therefore be a challenge to measure. Ideally atmospheric measurements would have high sensitivity, high time resolution, no sample collection required, be selective, small and durable, have low power consumption and be inexpensive.
    • Basic particulate matter measurements involve PM1.0 and PM2.5 which stand for particulate matter of diameter 1.0 micrometers or less, and particulate matter of diameter 2.5 micrometers or less respectively. PM1.0 has become a particular concern since the small size of the particle has a higher chance of affecting the respiratory system.
    • Difficulties with Impacters and collecting samples on filters: The principle of impacters is that as air travels through enclosed areas at a high velocity more and more particles become caught on sequential filters (ideally the larger to smaller ones depending on the number of stages set). An illustration is available to the lower right.
    • Other Measurement Systems Include:
      FTIR - Using infrared spectrometry an IR photon can cause vibrational or rotational excitation that can be read if there is no net dipole moment, to produce a spectra that can be analyzed.
    • Microscopy: NEXAFS-STXM - Using non-destructive soft X-ray beams organic functional groups can be distinguished from the different bonding energies absorbed by carbon-containing molecules, then a a two-dimensional map of particle composition and morphology can be obtained.
    • Online Mass Spectrometer - Similiar to the AMS (Aerosol Mass Spectrometer) insofar as filters are analyzed immediately by the instrument instead of filters collected and analyzed in a lab.


    • How Aerosols Grow: There are three general size modes for aerosols, and the middle, accumulation mode, is the one most related to aerosol growth and increased atmospheric lifetimes.
    • Coagulation - Is the process when aerosol particles suspended in a fluid come into contact and adhere as a result of hydrodynamic, electrical, gravitational, or other forces (Seinfield, 2006).
    • Deposition - The process by which the species (gas or aerosol) is in the presence of condensed water and scavenged by the hydrometeors to finally be delivered to the Earth's surface (Seinfield, 2006).
    • Nucleation scavenging - When an aerosol acting like a CCN (cloud condensation nuclei) will prompt the formation of a cloud droplet and therefore become integrated into cloud water as either an insoluable or soluble component.
    • Condensation - The process in which a vapor compound condenses on a particle (Sienfield, 2006).
    • Condensing - Is a common term when speaking about aerosols as particles often grow from gaseous molecules condensing on them.

    Cited Material

    Seinfeld, H. J., Spyros N. Pandis, (2006) Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, John Wiley and Son, Hoboken, NJ.