pollution transported to very remote regions can
have a notable impact, despite relatively short
atmospheric residence timescales. Physical transformations include condensation growth and
coagulation (collisions of two particles resulting
in one larger particle), while chemical transformations include changes in chemical composition
from chemical reactions (e.g., oxidation), along
with condensation of different vapors to that
already in the aerosol phase. These transformations result in the aerosol population becoming
“internally mixed” at long distances from source
regions as opposed to “externally mixed” near
source regions.
Primary production results typically in largermicron- or supermicron-sized particles, while secondary production results in nanometer- to
submicron-sized particles, leading to size distributions spanning many orders of magnitudes. In
Fig. 1, a typical number size distribution is
displayed. The peak in the number frequency
distribution is in submicron sizes; however, for
the same distribution, the volume or mass distribution is more often than not in supermicron sizes,
even though the contribution of supermicron particles generally only comprises less than 1% of the
total number concentration. Similarly, the mass of
particles where the peak concentration occurs
contribute less than 1% of the total mass. The
peak in aerosol surface area (not shown) usually
lies between the number concentration and volume peaks. The size distribution comprises a
number of “log-normal” modes reflecting different formation and transformation processes. At
the smallest sizes is the nucleation mode where
new particles, nanometers in size, are formed from
nucleation of condensable vapors. As the nucleation mode grows through further condensation,
an Aitken mode (called after the Scottish Scientist
John Aitken) of a few tens of nanometers is
formed. Through further transformations and
growth processes, the Aitken mode grows into
an accumulation mode of between about 100 and
200 nm. Generally, the coarse or supermicron
mode arises from the direct injection of primary
particles. The accumulation mode is termed so as
removal processes are least efficient at these sizes
and particles tend to accumulate there. Both
smaller particles (in the nucleation and Aitken
modes) and larger (supermicron mode) particles
have notably shorter lifetimes than that associated
with accumulation mode particles but for different
reasons. Smaller particles are highly diffusive and
either self-coagulate to form a larger particle, or
they coagulate with larger particles (with a high
surface area) where they are effectively removed
from the size distribution. Their mass is so small
that gravitational settling is negligible. In contrast,
larger particles have a negligible diffusion coefficient, but significant mass so that gravitational
settling determines their lifetime. Also shown in
Fig. 1 are scanning electron micrographs of various aerosol types such as soot (black) carbon,
marine organic, sulfate, and bioaerosol particles.
These images demonstrate the complexity of
atmospheric aerosols in terms of shape, chemistry,
and resultant density, and this complexity is propagated through measurement and modeling challenges associated with quantification of aerosol
properties and effects.
Primary Aerosol Formation
In the atmosphere, the most important natural
primary aerosol types result from the interaction
of wind with terrestrial or oceanic surfaces. For
terrestrial primary aerosols, wind interaction with
agricultural and desert soils lifts loose fragments
of the parent material through saltation and suspension processes resulting in airborne dust particles [4]. These primary dust particles are typically
supermicron in size, comprising low number concentration, but high surface area and mass concentrations. The most dramatic example of this is
the vertical uplifting of large amounts of desert
dust into aerosol plumes that can extend for thousands of kilometers (Fig. 2). These plumes often
are transported and deposited into oceanic regions
where they can provide an important source of
nutrients for the marine biota [5].
Wind interaction with terrestrial vegetation can
also trigger the release and dispersion of
bioaerosol such as pollen and seeds as well as
waxlike and leaf-fragment debris. Humic matter,
plant decomposition products, fungi, and
microbes are also released from plants. Fungi, in
turn, release spores through a variety of ejection
242
Aerosol in Global Atmosphere
have a notable impact, despite relatively short
atmospheric residence timescales. Physical transformations include condensation growth and
coagulation (collisions of two particles resulting
in one larger particle), while chemical transformations include changes in chemical composition
from chemical reactions (e.g., oxidation), along
with condensation of different vapors to that
already in the aerosol phase. These transformations result in the aerosol population becoming
“internally mixed” at long distances from source
regions as opposed to “externally mixed” near
source regions.
Primary production results typically in largermicron- or supermicron-sized particles, while secondary production results in nanometer- to
submicron-sized particles, leading to size distributions spanning many orders of magnitudes. In
Fig. 1, a typical number size distribution is
displayed. The peak in the number frequency
distribution is in submicron sizes; however, for
the same distribution, the volume or mass distribution is more often than not in supermicron sizes,
even though the contribution of supermicron particles generally only comprises less than 1% of the
total number concentration. Similarly, the mass of
particles where the peak concentration occurs
contribute less than 1% of the total mass. The
peak in aerosol surface area (not shown) usually
lies between the number concentration and volume peaks. The size distribution comprises a
number of “log-normal” modes reflecting different formation and transformation processes. At
the smallest sizes is the nucleation mode where
new particles, nanometers in size, are formed from
nucleation of condensable vapors. As the nucleation mode grows through further condensation,
an Aitken mode (called after the Scottish Scientist
John Aitken) of a few tens of nanometers is
formed. Through further transformations and
growth processes, the Aitken mode grows into
an accumulation mode of between about 100 and
200 nm. Generally, the coarse or supermicron
mode arises from the direct injection of primary
particles. The accumulation mode is termed so as
removal processes are least efficient at these sizes
and particles tend to accumulate there. Both
smaller particles (in the nucleation and Aitken
modes) and larger (supermicron mode) particles
have notably shorter lifetimes than that associated
with accumulation mode particles but for different
reasons. Smaller particles are highly diffusive and
either self-coagulate to form a larger particle, or
they coagulate with larger particles (with a high
surface area) where they are effectively removed
from the size distribution. Their mass is so small
that gravitational settling is negligible. In contrast,
larger particles have a negligible diffusion coefficient, but significant mass so that gravitational
settling determines their lifetime. Also shown in
Fig. 1 are scanning electron micrographs of various aerosol types such as soot (black) carbon,
marine organic, sulfate, and bioaerosol particles.
These images demonstrate the complexity of
atmospheric aerosols in terms of shape, chemistry,
and resultant density, and this complexity is propagated through measurement and modeling challenges associated with quantification of aerosol
properties and effects.
Primary Aerosol Formation
In the atmosphere, the most important natural
primary aerosol types result from the interaction
of wind with terrestrial or oceanic surfaces. For
terrestrial primary aerosols, wind interaction with
agricultural and desert soils lifts loose fragments
of the parent material through saltation and suspension processes resulting in airborne dust particles [4]. These primary dust particles are typically
supermicron in size, comprising low number concentration, but high surface area and mass concentrations. The most dramatic example of this is
the vertical uplifting of large amounts of desert
dust into aerosol plumes that can extend for thousands of kilometers (Fig. 2). These plumes often
are transported and deposited into oceanic regions
where they can provide an important source of
nutrients for the marine biota [5].
Wind interaction with terrestrial vegetation can
also trigger the release and dispersion of
bioaerosol such as pollen and seeds as well as
waxlike and leaf-fragment debris. Humic matter,
plant decomposition products, fungi, and
microbes are also released from plants. Fungi, in
turn, release spores through a variety of ejection
242
Aerosol in Global Atmosphere