Aerosols
 

According to Wallace and Hobbs (page 143), an aerosol is defined as:
"An aerosol is a suspension of solid or liquid matter (with small settling velocity) in a gaseous medium (air, in our case).

When scientists think about aerosols, they want to know a number of things about them:

• composition
• sizes
• concentrations
• location
• origin
• loss

What are some examples of aerosols?

Aerosols can be either solid or liquid or a combination of both.

• wind-blown dust
• particles of sulfuric acid and water (sulfate particles)
• sea salt
• soot
• other combustion products

Thus the chemical composition of aerosols can vary widely.  Some common components in the troposphere include:

• sulfate (SO₄₎
• ammonia (NH₄)
• nitrate (NO₃)
• elemental carbon (C)
• organic carbon (contains CH's)
• minerals and metals

Aerosol sizes vary over such a wide range that we must use logarithms to describe them.  They vary from a few tens of nanometers up to a few tens of micormeters in diameter.  Figure 4.7 (Wallace and Hobbs, page 154) shows different aerosols and their sizes.

Not all aerosols are the same size.  Aerosols have a size distribution.  This distribution results from processes that create, combine, and remove aerosols.
Not all aerosols are spherical.

Aerosols are most abundant near their sources at the ground.  Generally their source is at the surface with surface processes.

There is a layer of sulfuric acid aerosol (H₂SO₄) in the lower stratosphere just above the tropopause.  This layer is called the Junge Layer for the scientist who discovered it.  The Junge Layer is very important for the formation of the Antarctic Ozone Hole.

What are typical aerosol concentrations?

Aerosol concentrations vary widely depending on the place, time, and meteorological circumstances.  Near Earth's surface, typical concentrations are 10³ cm^-3 over the oceans, 10⁴ cm^-3 over rural land areas, and 10⁵ cm^-3 or greater over urban areas.

If we add up all the sources of atmospheric aerosols, about 1200 megatons of anthropogenic and about 1400-3600 megatons of natural aerosols are put into the atmosphere each year.

• Windblown dust is created by abrasion.
• Soot and other combustion products are created by combustion.
• Most of the sulfate and nitrate aerosols are created by a process called gas-to-particle conversion.  Gases emitted into the atmosphere react, producing nitric acid (HNO₃) and surfuric acid (H₂SO₄), which under the right conditions, can actually form new particles.

• Aerosol size often change as aerosol particles collide with each other, a process called coagulation.  This creates a few larger aerosols out of many smaller aerosols.
• Aerosols are scavenged by raindrops and deposited on Earth's surface.
• Aerosols have mass and therefore are affected by gravity.  They settle out in a process called dry deposition.

What roles do aerosols play in the atmosphere?

• act as cloud condensation nuclei (mostly aerosols with diameters between 0.01 and 100 microns)
• act as ice condensation nuclei, mainly in the upper troposphere (same range)
• carry charge for the Earth's electric field (0.0001 to 0.1 microns)
• reduce visibility (0.01 to a few microns)
• influence global temperatures (0.01 to a few microns)
• affect human health (less than a few microns)
• participate in atmospheric chemistry (smog formation, acid rain, stratospheric ozone reduction)

• concentration and distribution with size
• surface area and distribution with size
• volume and distribution with size
• composition