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
- 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
- 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.
"Global dimming": Is a gradual reduction in the amount of
global direct irradiance at the Earth's surface.
- 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
- 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
- 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
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
- 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,
- 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,
- 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.
Seinfeld, H. J., Spyros N. Pandis, (2006) Atmospheric
Chemistry and Physics: From Air Pollution to Climate
Change, John Wiley and Son, Hoboken, NJ.