Atmospheric pollution is a serious problem, having irreversible effects on human health and the global climate. The increasing number of diesel cars on the streets, coupled with coal-burning power plants and industries, release enormous amounts of pollutants and tiny particles (aerosols) into the atmosphere, putting in direct danger not only millions of people around the world, but also reducing the ozone layer, and causing changes that no one yet knows how to prevent.
It has been hypothesized for a while that the electrical charge in raindrops should be able to attract fine particles from the atmosphere. In theory, because raindrops fall through a strict process, known as coagulation, they should be able to trap the aerosols, and carry them to the Earth’s surface.
I say hypothesized, because no one to date has been able to prove it. The reason behind this is the super small size of the electric charge carried by raindrops, which does not show in laboratory experiments. Scientists have tried to test what would happen if the charge increases, however, even if the theory holds, such huge charge could almost never be observed in reality.
Now, a team from MIT, led by postdoc Karin Ardon-Dryer and former postdoc Yi-Wen Huang, decided to give it another shot. Taking into account previous failures, they decided to try and control the size of raindrops, rather than the charge.
To achieve this, the team used a tall glass chamber, and a strip made of radioactive material. Then, they introduced aerosols into the chamber, and they simulated various sizes of raindrops and environmental conditions. The runoff was then dried, and measured with a mass spectrometer to establish the amount of aerosols being taken out.
The results were very promising, showing that smaller raindrops, in highly humid environments, were the most efficient in removing aerosols. This is the most accurate estimation of coagulation of precipitation that has been reported to date. This results were very promising, and as the team points out, they could contribute to more accurate forecast predictions and improvements of climate models.
Technical details and more results from the experiment can be found in the latest issue of the journal Atmospheric Chemistry and Physics.
Image (c) MIT