10 September 2008

Climatologists get to grips with aerosols

by Kate Melville

The airborne spicks and specks known as aerosol particles are created by both natural and human processes and can consist of sea salt, sand grains, soot particles, sulfates and other materials of organic and inorganic origin. We blow vast quantities of aerosols into the air with our cars, power plants and heating systems, and while there is no doubt that aerosols have a large impact on our climate, no one has been able to agree on exactly what those effects are. Some climatologists believe they lead to more clouds and more precipitation, while others say they mean fewer clouds and less precipitation.

Now, a new study from the Max Planck Institute for Chemistry has revealed that both sides of the argument are right. "It depends on the number of particles. This is what determines how the energy needed to evaporate water and transport air is distributed," explained Max Planck climatologist Meinrat Andreae. "These results finally allow us to predict the effects of aerosols in climate models more accurately."

The new study, published in Science, explains that aerosols act in two ways. Firstly, like a sun umbrella they reduce the amount of solar energy reaching the ground and less water evaporates. Furthermore, the ground does not heat up as much and less of the warm, wet air necessary for cloud formation rises. Dark particles of soot from forest fires or coal burning have a similar effect, in that they absorb solar energy. They heat up the air around them, so that the cloud droplets evaporate instead of falling as rain.

Secondly, rain drops cannot form without aerosols. They start the clouds off by providing the moisture with points around which to collect, called condensation nuclei. Every single raindrop needs one of these tiny particles, which have diameters of less than one thousandth of a millimeter as a starting point. The moisture from the rising air condenses on the aerosol particles. This releases the heat that was originally needed to evaporate the water. If there are only few particles in the air, the drops grow so quickly that they fall before all the water can condense. If there are many collection points, more drops form; however, they are smaller and remain suspended in the air for longer. The thermal energy that the extra water gives off when it condenses is sufficient to make the clouds rise again, and the process continues. It then rains heavily.

However, in strongly polluted air, there is an excess of collection points: the drops remain small and do not reach the weight they need to fall. Apart from that, the many small droplets, with their larger overall surface, scatter more sunlight, which cools the Earth's surface, just like the sun umbrella effect. Clouds and therefore rain are only created when wet, warm air rises from the ground and the water condenses or freezes around the aerosols at altitude. "The number of aerosols controls how the energy, which ultimately originates from the sun, is distributed in the atmosphere," says the main author of the study, Daniel Rosenfeld.

"The effects of the aerosols on the energy on the ground and on droplet formation at altitude have been considered separately up to now. Consequently, the results were so contradictory that the subject was often sidelined," added Andreae.

The common thread that the team has now followed through the labyrinth of conflicting effects is the flow of energy. With this approach, the team has linked the two processes. "Now, for the first time, we can estimate by how many watts the energy available for the circulation in the atmosphere changes when the number of aerosols changes," explains Andreae. The link between the quantity of aerosols and the energy in the atmosphere available for forming precipitation can be described with a curve. Initially, the amount of energy released rises as the number of aerosols increases; it reaches a peak and then falls considerably. Before the curve peaks, more aerosols provide more precipitation; after the peak, additional aerosols reduce the precipitation.

The curve reaches maximum concentration at 1,200 condensation nuclei per cubic centimeter of air. At this concentration, natural and man-made aerosols screen off around a fifth of the sun's energy; however, the additional energy from condensation and freezing compensates for this.

The link between energy flow and precipitation explains, for example, why rain in the Amazonian rainforest is frequent, short-lived and occurs where the water has also evaporated. In such an environment, the air is very clean and given these low aerosol concentrations, a lot of water evaporates to begin with as a lot of solar energy reaches the ground. Secondly, only a few, albeit large drops form, which fall to the ground quickly. While there is a lot of energy available on the ground, it never reaches the altitudes at which long-lived clouds form.

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Source: Max Planck Society for the Advancement of Science