Aerosols

Alaskan sunset: The intense red and orange hues of the sky at sunset and sunrise are mainly caused by scattering of sunlight off dust particles, soot particles, other solid aerosols and liquid aerosols floating in the earth’s atmosphere.

What are Aerosols?

Aerosols are solid or liquid particles suspended in air ranging in size from 0.01 microns to several tens of microns. One of the activities in the GEOmon project is to monitor atmospheric aerosols and to assess their impact on climate and health.

Types of Aerosols

Most aerosols occur naturally in the form of windblown dust and sand, soot and smoke from forest fires, SO2 from volcanic eruptions, salt from sea spray. Other anthropogenic aerosols originate with human activities, the main causes of which are the burning of fossil fuels and changes made to the earth’s natural surface cover.

Lifetime of Aerosols

The majority of aerosols form a thin haze in the lower atmosphere (troposphere), where they are washed out of the air by rain within about a week. However some aerosols such as the volcanic ash from large eruptions can be blown into the stratosphere where it does not rain and may remain there for up to a year.

Aerosols affect human health and play an important role in weather and climate change processes, both regionally and globally. However, it remains an area of research where we still have a lot to learn about the complex interactions involved.

Aerosols and cooling effects on the climate

Overall, it is believed that aerosols have a ‘direct’ cooling effect on the planet because they reflect sunlight back into space, thus reducing the amount of solar radiation that reaches the surface. The magnitude of this cooling effect depends on the size and composition of the aerosol particles, as well as the reflective properties of the underlying surface. This may partially offset the warming effect of increased levels of carbon dioxide in the atmosphere.

Aerosols are also believed to have an 'indirect' cooling effect on climate by changing properties of clouds. Indeed, if there were no aerosols in the atmosphere, there would be no clouds. It is very difficult to form clouds without small aerosol particles acting as "seeds" to start the formation of cloud droplets. As aerosol concentration increases within a cloud, the water in the cloud gets spread over many more particles, each of which is correspondingly smaller. Smaller particles fall more slowly in the atmosphere and decrease the amount of rainfall. In this way, changing aerosols in the atmosphere can change the frequency of cloud occurrence, cloud thickness, and rainfall amounts.

Aerosols and Cloud Formation

Left: Clouds with low aerosol concentration and a few large droplets do not scatter light well, and allow much of the Sun's light to pass through and reach the surface.Right: The high aerosol concentrations in these clouds provide the nucleation points necessary for the formation of many small liquid water droplets. Up to 90% of visible radiation (light) is reflected back to space by such clouds without reaching Earth's surface. Source: NASA

If there are more aerosols, scientists expect more cloud drops to form. Since the total amount of condensed water in the cloud is not expected to change much, the average drop must become smaller. This has two consequences -- clouds with smaller drops reflect more sunlight (as explained in Figure 2a & b), and such clouds last longer, because it takes more time for small drops to coalesce into drops that are large enough to fall to the ground. Both effects increase the amount of sunlight that is reflected to space without reaching the surface.

Understanding More

Still, key details about the amount and properties of aerosols are needed to calculate their current effect on surface temperatures; so far, it has not been possible to make these measurements on a global scale.

What GEOmon is contributing to our understanding of aerosols?

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