Introduction: The Nature and Impacts of
Atmospheric Aerosols
Atmospheric aerosols are tiny airborne liquid or
solid droplets or particles, ranging from nanometers to tens or even hundreds of microns in size.
They are diverse in shape and chemical composition and can posses varying optical properties and
hygroscopic (water uptake) properties. Particle
shape ranges from the most simple to characterize,
that is, spherical, to complex fractal agglomerates
with similar diversity in aerosol density. Chemical
composition ranges from relatively simple inorganic composition to very complex organic matter
composition and mixtures thereof. The vast
majority of organic chemical species have not, to
date, even been identified. The aerosol population
is a dynamic physical and chemical evolving system with fresh aerosol plumes often being externally mixed in terms of chemical composition,
evolving with time into more complex internal
mixtures. Primary aerosol production is the
breakup of a parent material or incomplete combustion and results in the direct injection of aerosol particles into the atmosphere. Secondary
aerosol production is the formation of aerosol
phase products from nucleation, condensation,
and multiphase chemical reactions resulting in
aerosol production in the atmosphere itself as
opposed to injection into the atmosphere. Aerosol
particles in the size range from 100 nm to 1 mm are
particularly efficient at scattering, and depending
on the chemical composition, absorption of solar
radiation. Sea-salt, sulfate, and nitrate are all efficient scatters, while soot or black carbon and dust
are efficient absorbers. These important optical
properties lead to a reduction of visibility from
aerosol haze layers under conditions of high aerosol concentrations and such haze layers are
responsible for a number of atmospheric optical
phenomena such as beautiful sunsets often seen
under clear sky conditions. Aerosol particles also
act as condensation nuclei for cloud droplets to
form on. Without condensation nuclei, it would be
virtually impossible for clouds to form, therefore
aerosols also play an important role in the hydrological cycle. Cloud drops are also efficient scatters of solar radiation and consequently, both
aerosol haze and cloud layers play an important
role in the global radiative budget through contributing to 50% or more of the Earth’s albedo.
The significant contribution to global albedo
necessitates an important role in climate, and in
terms of a changing global aerosol population, a
role in climate change also exists. Aerosols are
both natural and anthropogenic, the former produced by the biosphere (sulfate and organics) and
wind interaction at the Earth’s surface (sea spray
and dust production), the latter produced by
industrial, transport, and combustion emissions
(sulfate, nitrate, soot, and organics). Biosphere
emissions are connected to climate change feedbacks and virtually impossible to control while
anthropogenic emissions can be controlled to a
certain extent. Although some aerosol species
absorb solar radiation, partially contributing to
global warming, the predominant climate effect
is one of net global cooling. Since the start of the
industrial era, aerosol-related air pollution has, to
a significant degree, been masking the global
warming impact of greenhouse gases and has
been slowing the rate of global warming. The
effect on climate can be considered as a welcome
impact of atmospheric aerosols; however, micronsized and submicron-sized aerosol also have a
high penetration efficiency through the human
respiratory system leading to increased respiratory and cardiovascular health impacts. The
adverse health impacts lead to increased mortality
rates and reduced life span. For example, the
London black smoke pollution events in the
1950s resulted in thousands of excess deaths
over periods lasting the order of several days. To
counteract the adverse health impacts of atmospheric aerosols, various clean air acts have been
introduced in various continents, particularly
developed industrial continents. Such legislation,
while having the required successful impact on air
quality and public health, has had a negative
impact on global warming in that the reflective
haze and cloud layers have become less reflective
with the effect of reducing the masking effect on
global warming. Particularly over the last two to
three decades, cleaner air has resulted in a remarkable increase in the rate of global temperature
increase as greenhouse gas concentrations in the
240
Aerosol in Global Atmosphere
Atmospheric Aerosols
Atmospheric aerosols are tiny airborne liquid or
solid droplets or particles, ranging from nanometers to tens or even hundreds of microns in size.
They are diverse in shape and chemical composition and can posses varying optical properties and
hygroscopic (water uptake) properties. Particle
shape ranges from the most simple to characterize,
that is, spherical, to complex fractal agglomerates
with similar diversity in aerosol density. Chemical
composition ranges from relatively simple inorganic composition to very complex organic matter
composition and mixtures thereof. The vast
majority of organic chemical species have not, to
date, even been identified. The aerosol population
is a dynamic physical and chemical evolving system with fresh aerosol plumes often being externally mixed in terms of chemical composition,
evolving with time into more complex internal
mixtures. Primary aerosol production is the
breakup of a parent material or incomplete combustion and results in the direct injection of aerosol particles into the atmosphere. Secondary
aerosol production is the formation of aerosol
phase products from nucleation, condensation,
and multiphase chemical reactions resulting in
aerosol production in the atmosphere itself as
opposed to injection into the atmosphere. Aerosol
particles in the size range from 100 nm to 1 mm are
particularly efficient at scattering, and depending
on the chemical composition, absorption of solar
radiation. Sea-salt, sulfate, and nitrate are all efficient scatters, while soot or black carbon and dust
are efficient absorbers. These important optical
properties lead to a reduction of visibility from
aerosol haze layers under conditions of high aerosol concentrations and such haze layers are
responsible for a number of atmospheric optical
phenomena such as beautiful sunsets often seen
under clear sky conditions. Aerosol particles also
act as condensation nuclei for cloud droplets to
form on. Without condensation nuclei, it would be
virtually impossible for clouds to form, therefore
aerosols also play an important role in the hydrological cycle. Cloud drops are also efficient scatters of solar radiation and consequently, both
aerosol haze and cloud layers play an important
role in the global radiative budget through contributing to 50% or more of the Earth’s albedo.
The significant contribution to global albedo
necessitates an important role in climate, and in
terms of a changing global aerosol population, a
role in climate change also exists. Aerosols are
both natural and anthropogenic, the former produced by the biosphere (sulfate and organics) and
wind interaction at the Earth’s surface (sea spray
and dust production), the latter produced by
industrial, transport, and combustion emissions
(sulfate, nitrate, soot, and organics). Biosphere
emissions are connected to climate change feedbacks and virtually impossible to control while
anthropogenic emissions can be controlled to a
certain extent. Although some aerosol species
absorb solar radiation, partially contributing to
global warming, the predominant climate effect
is one of net global cooling. Since the start of the
industrial era, aerosol-related air pollution has, to
a significant degree, been masking the global
warming impact of greenhouse gases and has
been slowing the rate of global warming. The
effect on climate can be considered as a welcome
impact of atmospheric aerosols; however, micronsized and submicron-sized aerosol also have a
high penetration efficiency through the human
respiratory system leading to increased respiratory and cardiovascular health impacts. The
adverse health impacts lead to increased mortality
rates and reduced life span. For example, the
London black smoke pollution events in the
1950s resulted in thousands of excess deaths
over periods lasting the order of several days. To
counteract the adverse health impacts of atmospheric aerosols, various clean air acts have been
introduced in various continents, particularly
developed industrial continents. Such legislation,
while having the required successful impact on air
quality and public health, has had a negative
impact on global warming in that the reflective
haze and cloud layers have become less reflective
with the effect of reducing the masking effect on
global warming. Particularly over the last two to
three decades, cleaner air has resulted in a remarkable increase in the rate of global temperature
increase as greenhouse gas concentrations in the
240
Aerosol in Global Atmosphere