atmosphere continue to rise. The coupling
between air pollution, air quality, and climate
change has led to the synergetic development of
air pollution and climate change policy to achieve
optimal effect in attempting to mitigate and abate
climate change while simultaneously striving for
clean air.
The challenge, however, is great in terms of
producing accurate assessments of aerosol
impacts, particularly those relating to climate
change because the atmospheric aerosol system
is incredibly complex and this complex system
needs to be incorporated into large-scale and
global climate assessment and prediction models.
From the emission and formation stage of the
aerosol life cycle, through its evolution, interaction with clouds and radiation, to its removal from
the atmosphere is a highly complex non-linear
system spanning spatial and temporal scales
from nanometers and seconds (in terms of nucleation of new particles, to global and decadal
scales, respectively, in terms of climatic impacts.
The complexity is further compounded by the
diverse and even incomplete quantification of
the chemical speciation of atmospheric aerosol
particles. This chapter summarizes the nature, formation, transformation, distribution and impacts
of aerosols in the global atmosphere and documents the recent advances in the knowledge relating to atmospheric aerosols and their impacts.
Aerosol Formation, Dimensions, and
Removal
Aerosols are produced via two generic mechanisms: (1) primary aerosol formation and (2) secondary aerosol formation. Primary aerosols are
formed from the mechanical breakup of a parent
material, with the resultant aerosol inheriting the
chemical properties of the parent bulk material. In
addition, primary aerosol production can result
from incomplete combustion processes. These
processes result in the direct emission of aerosols
into the atmosphere. Secondary aerosols, on the
other hand, are formed via gas-to-particle conversion processes such as oxidation or other means of
chemical reduction of volatile precursor gases,
condensation of low-volatility gases onto existing
aerosol, homogeneous nucleation (phasetransition) producing new aerosol particles, aqueous phase reactions of dissolved gases producing
stable aerosol phase species, or chemical reactions
on aerosol surfaces leading to an increase in the
condensed aerosol mass. In contrast to direct ejection of primary aerosols into the atmosphere,
which necessitates a surface source for primary
aerosol (excluding aviation emissions), secondary
aerosols are formed in situ in the atmosphere,
although many of their precursors are likely to
originate at the surface. The formation of secondary aerosols can occur at all levels in the atmosphere, from the boundary layer in the lower
troposphere to above the stratosphere.
Once formed, an aerosol particle may undergo
physical and chemical transformation, but generally there are only two main sinks – that is through
dry deposition at the surface, or wet deposition
involving washout by precipitation. Dry deposition is the transport of aerosol particles from the
atmosphere to the surface in the absence of precipitation. The level of turbulence in the atmosphere, particularly that close to the ground,
determines the rate at which particles are deposited [1]. The deposition rate depends on surface
type (i.e., roughness) and particle size since there
also is a gravitational settling component to dry
deposition. Combining both wet and dry deposition removal processes, on average, submicron
particles reside in the boundary layer for up to
~4–8 days [2] depending on precipitation, while
supermicron particles, depending on the size,
have lifetimes from an hour to 1–2 days. For
example, a 100-nm particle takes approximately
36 years to fall 1 km whereas a 10-mm particle
takes 3.6 days. Clearly, if particles are vertically
transported to the free troposphere or the stratosphere, their lifetime in the atmosphere is greatly
increased even to timescales of a year or more in
the stratosphere. Removal in the atmospheric
boundary layer is increased due to turbulent
effects. In terms of aerosol-related air pollution,
effects are more so on a local-to-regional scale
rather than on a global scale due to removal timescales; however, as it will be seen later, in a
climate context, a small amount of aerosol
Aerosol in Global Atmosphere
241
between air pollution, air quality, and climate
change has led to the synergetic development of
air pollution and climate change policy to achieve
optimal effect in attempting to mitigate and abate
climate change while simultaneously striving for
clean air.
The challenge, however, is great in terms of
producing accurate assessments of aerosol
impacts, particularly those relating to climate
change because the atmospheric aerosol system
is incredibly complex and this complex system
needs to be incorporated into large-scale and
global climate assessment and prediction models.
From the emission and formation stage of the
aerosol life cycle, through its evolution, interaction with clouds and radiation, to its removal from
the atmosphere is a highly complex non-linear
system spanning spatial and temporal scales
from nanometers and seconds (in terms of nucleation of new particles, to global and decadal
scales, respectively, in terms of climatic impacts.
The complexity is further compounded by the
diverse and even incomplete quantification of
the chemical speciation of atmospheric aerosol
particles. This chapter summarizes the nature, formation, transformation, distribution and impacts
of aerosols in the global atmosphere and documents the recent advances in the knowledge relating to atmospheric aerosols and their impacts.
Aerosol Formation, Dimensions, and
Removal
Aerosols are produced via two generic mechanisms: (1) primary aerosol formation and (2) secondary aerosol formation. Primary aerosols are
formed from the mechanical breakup of a parent
material, with the resultant aerosol inheriting the
chemical properties of the parent bulk material. In
addition, primary aerosol production can result
from incomplete combustion processes. These
processes result in the direct emission of aerosols
into the atmosphere. Secondary aerosols, on the
other hand, are formed via gas-to-particle conversion processes such as oxidation or other means of
chemical reduction of volatile precursor gases,
condensation of low-volatility gases onto existing
aerosol, homogeneous nucleation (phasetransition) producing new aerosol particles, aqueous phase reactions of dissolved gases producing
stable aerosol phase species, or chemical reactions
on aerosol surfaces leading to an increase in the
condensed aerosol mass. In contrast to direct ejection of primary aerosols into the atmosphere,
which necessitates a surface source for primary
aerosol (excluding aviation emissions), secondary
aerosols are formed in situ in the atmosphere,
although many of their precursors are likely to
originate at the surface. The formation of secondary aerosols can occur at all levels in the atmosphere, from the boundary layer in the lower
troposphere to above the stratosphere.
Once formed, an aerosol particle may undergo
physical and chemical transformation, but generally there are only two main sinks – that is through
dry deposition at the surface, or wet deposition
involving washout by precipitation. Dry deposition is the transport of aerosol particles from the
atmosphere to the surface in the absence of precipitation. The level of turbulence in the atmosphere, particularly that close to the ground,
determines the rate at which particles are deposited [1]. The deposition rate depends on surface
type (i.e., roughness) and particle size since there
also is a gravitational settling component to dry
deposition. Combining both wet and dry deposition removal processes, on average, submicron
particles reside in the boundary layer for up to
~4–8 days [2] depending on precipitation, while
supermicron particles, depending on the size,
have lifetimes from an hour to 1–2 days. For
example, a 100-nm particle takes approximately
36 years to fall 1 km whereas a 10-mm particle
takes 3.6 days. Clearly, if particles are vertically
transported to the free troposphere or the stratosphere, their lifetime in the atmosphere is greatly
increased even to timescales of a year or more in
the stratosphere. Removal in the atmospheric
boundary layer is increased due to turbulent
effects. In terms of aerosol-related air pollution,
effects are more so on a local-to-regional scale
rather than on a global scale due to removal timescales; however, as it will be seen later, in a
climate context, a small amount of aerosol
Aerosol in Global Atmosphere
241