Aerosols
Kenneth Hunu and partner
Aerosols are a mixture of tiny particles or liquids that are suspended in air and can range
in size from 0.01 microns to several tens of microns. Volcanoes, dust storms, biomass
burning, and sea spray from the ocean, naturally produce aerosols. Human activities,
such as fossil fuel burning also generate aerosols, which tend to be smaller in diameter
than naturally formed aerosols by a factor of ten. Averaged over the globe,
anthropogenic aerosols account for about 10 percent of the total amount of aerosols in the
atmosphere. Aerosols have a short lifespan and tend to be found close to their source.
Most of the aerosols on the Earth are concentrated in the Northern Hemisphere,
especially downwind of industrial sites, slash-and-burn agricultural regions, and
overgrazed grasslands (NASA).
Figure 1: The natural and man made sources of aerosols are depicted in this diagram. Windblown dust and
sea salt from bursting ocean bubbles produce aerosols that are about 1 micron in size. Smaller aerosols are
produced from volcanic eruptions, fossil fuel combustion, and biomass burning. (NASA)
The Direct Effect of Aerosols
The direct effect of aerosols describes how they directly influence the Earth's radiation
budget. Aerosols tend to cool the Earth by reducing the amount of solar radiation that
reaches the surface, but different types of aerosols do so in different ways. Sulfate
aerosols cool the surface because they reflect sunlight back into space while aerosols
composed of soot particles absorb incoming radiation. Measurements show that black
carbon has reduced the amount of sunlight that reaches the surface by 15%. It is thought
that aerosol cooling partially offsets the global warming that is attributed to increases in
human produced carbon dioxide concentrations (Kaufman et al. 2002).
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The First Indirect Effect
By changing the properties of clouds, aerosols indirectly change the albedo of the Earth.
Clouds need cloud condensation nuclei (CCN), which mostly consist of aerosol particles
in order to form because cloud droplets need a surface onto which they can coalesce.
Increases in aerosol concentration from man-made sources produce clouds with more, but
smaller droplets because if the water vapor is held constant, the water vapor is spread
across a larger surface area. Droplet size has seen to be reduced by 20-30% in polluted
regions. This results in an increase in a cloud's optical thickness, at about the cubed root
of the difference in cloud droplet number. This increases the Earth's albedo and reduces
the amount of radiation that reaches the surface, thereby cooling it (Andreae et al. 2005,
Kaufman et al. 2002, Penner et al. 2006).
The Second Indirect Effect
As seen in the first indirect effect, increases in aerosol concentration lead to more, but
smaller droplets. Smaller cloud droplets tend not to fall out as raindrops, resulting in
lowered precipitation efficiency. This results in an increase in the lifespan of a cloud and
increases the amount of time that the cloud can reflect sunlight. Quantifying the second
indirect effect is difficult because of the already existing complexity and uncertainty of
cloud dynamics and processes (Penner et al. 2006).
Figure 2: The image on the left shows a cloud with low aerosol concentration. This results in the
formation of a few large droplets and allows a significant portion of solar radiation to pass through the
cloud. The image on the right shows a cloud with a higher concentration of aerosols in it. This results in
the formation of many small droplets. This increases the albedo of the cloud and the cloud reflects more
sunlight than the cloud on the left.(NASA)
The Semi-Direct Effect
Aerosols, such as black carbon and soot that absorb sunlight will warm the atmosphere.
This reduces the sunlight that reaches the surface, but also increases the temperature of
the mid-to-upper troposphere. This causes the lapse rate to decrease and stabilizes the
boundary layer. This reduces the convection that occurs in the atmosphere and hinders
cloud formation. In addition, soot particles that exist within a cloud can warm the
surrounding area and lead to the evaporation of a part of the cloud. This reduces the
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lifespan of the cloud and causes a warming effect on the planet because the amount of
cloud cover on the planet is reduced (Andreae et al. 2005, Ackerman et al. 2000).
Aerosols and Ozone depletion
Aerosols may also act as sites for chemical reactions to take place (heterogeneous
chemistry). During winter in the stratosphere in the Polar Regions, aerosols grow to form
polar stratospheric clouds. The large surface areas of these cloud particles provide sites
for chemical reactions to take place. These reactions produce large amounts of reactive
chlorine that lead to the destruction of stratospheric ozone (NASA).
Figure 3: After a volcanic eruption, large amounts of sulfur dioxide (SO 2), hydrochloric acid (HCl) and ash
are spewed into the Earth's stratosphere. Hydrochloric acid, in most cases, condenses with water vapor and
is rained out of the volcanic cloud formation. Sulfur dioxide from the cloud is transformed into sulfuric
acid (H2SO4). The sulfuric acid quickly condenses, producing aerosol particles which linger in the
atmosphere for long periods of time. The interaction of chemicals on the surface of aerosols, known as
heterogeneous chemistry and the tendency of aerosols to increase levels of chlorine, are prime contributors
to stratospheric ozone destruction. (NASA)
Evidence for anthropogenis aerosols and the effects of aerosols on climate forcing:
1. Sulfur dioxide forms sulfate aerosol particles in the atmosphere, which causes the
clouds to be more reflective, carry more water and possibly stop precipitation. This has
been ascertained by the study of “ship tracks”.
This is proof that humans have been creating and modifying clouds for generations
through the burning of fossil fuels and explains why global warming is affecting the
Southern Hemisphere much more quickly than the Northern Hemisphere (King et al.
1993).
2. Regional variability of fine aerosols: due to the relatively short lifetime of aerosols,
they tend to remain at their source areas. Fine hygroscopic particles (urban haze) are
found downwind of populated regions in polluted (from industrial and burning of fossil
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fuels) areas like China. Black carbon is also concentrated at regions where much burning
takes place. These indicate that they are anthropogenic.
Figure 4: (a) shows the global distribution of fine aerosols mainly as a result of pollution and vegetation
fires whereas figure (b) shows the global distribution of course aerosols which are mainly dust and
salt.(EESC W4400 lecture notes)
3. Cloud base: aircraft measurements show that in polluted air a six-fold increase in the
number of fine aerosols per unit volume of air produces a three-to fivefold increase in the
droplet concentration. Analysis of global satellite data shows that such a change in
aerosol concentration corresponds to 10-25% smaller cloud droplets because the
condensed water is divided into more numerous droplets. These clouds therefore have a
larger surface area and hence a larger reflectivity.
What do model simulations of past/present indicate?
Models show that the aerosol forcing ranges from 0 to - 4.4 W/m2(IPCC).
Furthermore, they seem to have presently offset greenhouse gases by 25-50% (Kaufman
et al. 2002).
What are the models suggesting for the future, and what are the assumptions in
those projections?
Using an aerosol forcing of -1.5 W/m2, models using several IPCC scenarios show a
difference in climate sensitivity of up to 6ºC for 2100 between a strong aerosol forcing
situation and one with no aerosol cooling effect (Andreae et al. 2005). One potential
weakness of the GCMs used is that their resolution may be too low to depict the
dynamics and processes of clouds well (Penner et al. 2006). Differences also occur
because modelers have to assume how aerosols interact with water vapor and how water
vapor will be distributed in the atmosphere. Since cloud processes are not known well,
the used assumptions affect the uncertainty of the calculated impact aerosols have on
climate. Each model also decides what cloud droplet number concentration to use and
how to predict future aerosol concentration changes (Penner et al. 2006). Another model,
which also takes into account future decreases in sulfate aerosol concentrations but uses
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one IPCC scenario (SRES-A2), shows differences in temperature increases of 3.5ºC
within the 21st century when both man-made reflective aerosol concentration changes are
included and when they are not. This insinuates that sulfate aerosols may not play a large
role in cooling the Earth in the 21st century, especially considering the large greenhouse
gas increase that is predicted to occur. Their models also show that during the first half
of the 21st century, greenhouse gas forcing increases much faster than in the 20th century
while aerosol forcing remains constant or becomes more positive. Forcing of sulfate
aerosols from 1945-1995 is about -.05, while it becomes almost zero from 1995-2045.
Their models also show that the aerosol forcing becomes positive (0.5) as the emissions
of sulfate decrease during 2045-2095 (Dufresne et al. 2005).
What are the principal uncertainties in the scientific evidence and in the forecasts?
Scientists disagree about the magnitude of aerosol forcing, but they all agree that aerosols
have a net cooling effect. This large uncertainty, which ranges from 0 to -4.4 W/m2, is
due to several factors. Scientists are still unclear about how clouds respond to aerosol
concentrations. In addition, they cannot exactly determine how clouds respond to
concentrations of aerosols. A substantial amount of information about cloud dynamics
and behavior is still unknown. This also makes it difficult to depict cloud processes in
models that predict future aerosol forcing and future temperature changes of the Earth
(Andreae et al. 2005, Kaufman et al. 2002). There are currently two main methods that
are used to calculate aerosol forcing, forward and inverse. Forward calculations are
based on physical and chemical properties of aerosols and use that information to
determine what effect aerosols have on the Earth's radiation budget. Inverse calculations
work backwards from model simulation results to determine what portion of the Earth's
total forcing is attributable to aerosols. Inverse calculations result in an uncertainty range
less than 2 W/m2, with an average forcing of -1 W/m2. Forward calculations have an
aerosol forcing that ranges from 0 to about -5 W/m2. There is no clear explanation as to
why such a disparity exists, as fairly well defined physics and observations are used in
both types of calculations (Anderson et al. 2003). Thus, more reliable observations about
aerosol emission rates, their lifetimes, and their concentrations are needed to improve
these predictions. In addition, there is a need to improve the understanding scientists
have about the impact aerosols have on the hydrologic cycle, specifically how they affect
clouds (Ramanathan et al. 2001). In addition, this uncertainty about the net radiative
forcing of aerosols makes it difficult to estimate the Earth's climate sensitivity. In order
to better determine how the Earth will respond to increases in greenhouse gases,
especially if aerosol concentrations change dramatically in the future, it is important to
understand how strong of an effect aerosols have on the Earth's radiation budget.
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Figure 5: This figure from the IPCC shows how low the understanding about aerosols is and how the
forcings from the first indirect effect and mineral dust are difficult to quantify (IPCC).
Main Points
Aerosols affect the Earth’s radiation budget and currently have a net cooling
effect by directly or indirectly changing the amount of solar radiation that reaches the
Earth’s surface. In the future, aerosol cooling is expected to decline relative to
greenhouse gas forcing.


High concentrations of small aerosols, mostly anthropogenic in source, change
cloud properties and thus, affect the hydrologic cycle by reducing the precipitation
efficiency of clouds.
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It is uncertain how large the aerosol forcing is even though aerosols have an
estimated radiative forcing of about 0 to -4.4W/m2. This is partly due to our
incomplete understanding of cloud dynamics.
References
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