Aerosols

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.