Moderate quantities of dirt make more rain
An international team of scientists sums up the current effects of aerosols on precipitation
Drought or deluge - scientists working with Meinrat O. Andreae, Director at the Max Planck Institute for Chemistry in Mainz, have now discovered how aerosols affect the when, where and how much of rainfall. Until now, the answers to these questions have been as varied as they have been inconsistent. Andreae and his co-authors are now tracing a common theme through the sometimes contradictory effects that these tiny particles have on precipitation. Their new approach: they are observing how aerosols change the flow of energy in the atmosphere and thus air circulation, the way drops form and the way they fall. Because the role of aerosols has to date been very much a subject of dispute and a source of great uncertainty in climate predictions made by researchers, this work removes one of the largest obstacles to the development of more accurate climate forecasts.(Science, September 5, 2008)
Human beings blow vast quantities of aerosols into the air with their cars, power plants and heating systems. Fires set to clear forests also release these floating particles which, in some cases, measure just a few thousandths of a millimetre, or even less. Before humans had any impact, the aerosol load in air over land was only double that in the air over the sea. However, today, the former can amount to one hundred times more than the latter. There is no doubt that natural and human-made aerosols have an impact on our climate. But what effect do they have, exactly? Some say they lead to more clouds and more precipitation. Others say they mean fewer clouds and less precipitation. "Both sides are right," says Meinrat Andreae, Director at the Max Planck Institute for Chemistry in Mainz, "but it depends on the number of particles. This is what determines how the energy needed to evaporate water and transport air is distributed."
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, from the Hebrew University in Jerusalem.
Aerosols act in two ways: firstly, like a sun umbrella, they reduce the amount of solar energy reaching the ground: 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 millimetre, 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.
"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," says Meinrat 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 Meinrat 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 centimetre of air - the equivalent to the volume of a sugar cube. 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. There the air is very clean. 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. Although there is a lot of energy available on the ground, it never reaches the altitudes at which long-lived clouds form.
Aerosols are created in natural and human processes. Aerosol particles can consist of sea salt, sand grains, soot particles, sulfates and other materials of organic and inorganic origin. The natural processes include volcanic eruptions, the occasional forest fire, sandstorms, and breaking ocean waves, while traffic, forest clearance by fire, changes in land use and industrial emissions are the major human sources. Not all aerosol particles can act as condensation nuclei; among other things, this depends on whether they are water-soluble and how large they are. Clean air over land contains typically around 2,000 particles per cubic centimetre. Polluted air over land contains around 10,000; air over a city can contain up to 100,000 particles per cubic centimetre. In pure oceanic air, the value is around 500. The cleanest air is over the Antarctic plateau, with readings of only 43 particles per cubic centimetre.
By contrast, moderate concentrations of aerosols delay the fall of rain, as initially less water evaporates, and more and therefore lighter drops form. These rise again and reach altitudes where the atmosphere is so cold that they freeze. This releases heat, just as condensing does. The air becomes warmer and can continue to rise. The moisture bound to the aerosols transports energy to where larger clouds form. This stimulates the circulation of the atmosphere and more rain can fall, possibly even as hail. This precipitation can also be transported over quite large distances as clouds do not rain immediately, but first mature. Moderate concentrations of aerosols cause the highest rainfall, as well as extreme events and storms, since the energy for forming clouds and circulation is at a maximum.
Where concentrations of aerosols are very high, the sun umbrella effect and the cloud processes weaken the atmospheric circulation. On the one hand, less water evaporates. On the other, there are so many aerosol particles that the small amount of moisture is very thinly spread: only tiny rain drops and powdery ice crystals are created. As the microdrops and the powdery crystals are too light to fall, they evaporate after a while. In the process, they extract the heat from the air that they released when they condensed and froze. This brings the circulation to a standstill as the air masses can no longer further rise. The result: rain fails to materialize - and droughts become more frequent.
"These results finally allow us to predict the effects of aerosols in climate models more accurately. Currently, it is the conflicting effects of the aerosols that are preventing us from making more precise predictions about the future of the climate," says Meinrat Andreae with reference to the study’s significance.