When we talk about aerosols, we mean
solid particles and liquid droplets that can be found in the air over oceans,
deserts, mountains, forests, ice and every ecosystem in between. There are many
different ways of classifying aerosols, but climatologists use the chemical
composition, so we can find the following groups: sulphates, organic carbon,
black carbon, nitrates, mineral dust, and sea salt. In practice, many of these
terms are imperfect, as aerosols often form complex mixtures.
The main part
of aerosols, about 90 percent by mass, has natural origins. For instance,
volcanoes eject columns of ash into the air, as well as sulfur dioxide and
other gases. Another example is the
ocean, some microalgae produce dimethylsulfide which converses into sulfates in
the atmosphere. Sea salt and dust are two of the most abundant aerosols.
The
remaining 10 percent are considered anthropogenic and they have many sources including, fossil
fuel combustion and biomass burning, among others.
Aerosols
and incoming sunlight – direct effects
Aerosols
reflect a quarter of the Sun’s energy back to space. Different aerosols scatter
or absorb sunlight in different degrees, depending on their physical
properties; and this is a direct effect of aerosols on the Earth’s radiation
field.
On one
hand, pure sulphates and nitrates reflect nearly all radiation they encounter,
cooling the atmosphere. On the other hand, Black carbon absorbs radiation,
warming the atmosphere and the surface.
Volcanoes eruptions eject large amount of sulphur dioxide to the atmosphere. As a result of
the global sulfate infusion, global temperatures drop.
In addition
to scattering or absorbing radiation, they can alter the albedo of the planet.
Bright surfaces reflect radiation and cool the climate, whereas darker surfaces
absorb radiation and produce a warming effect.
Aerosols
and clouds – indirect effects
On a global
scale, the aerosols can form clouds that cause cooling. Those clouds are
bright, it means that they will scatter more light and become more reflective;
blocking the sunlight from reaching Earth’s surface and producing a net
cooling. The “cloud albedo effect”, as we know this cloud brightening effect,
make a significant impact on the climate.
Current
estimates suggest the cooling driven by aerosol indirect effects is less than
half as much as the warming caused by greenhouse gases when average over the
globe. But if we consider a smaller space and time scale, the climate effects
of aerosols can be significant.
For further
information on aerosols, visit the following link:
NASA Earth
observatory – Aerosols http://earthobservatory.nasa.gov/Features/Aerosols/
This
explanation about the aerosols and its interaction with the climate, including
the direct and the indirect effects are part of the exercises on a Future Learn
course on Climate Change by the University of Exeter.
