A recent study carried out by MIT has characterized the cleansing effect that raindrops have on our atmosphere in removing aerosol and other pollutants from the air. The results of the research could be instrumental in creating reliable forecasts for air quality, and creating more accurate models of climate change impact due to clouds.
Atmospheric aerosols come from three main sources: as by-products of volcanic eruptions; desert dust; and increasingly as a product of human actions, often as a result of burning coal and oil as part of the industrial process. These aerosols have a number of effects on Earth's protective atmospheric shell, including acting as a catalyst for chemical reactions that seriously undermine our planet's stratospheric ozone.
Raindrops are able to remove these detrimental particles as they fall through the air via a process known as coagulation, which attracts aerosol pollutants, trapping them in the raindrop and returning them to the surface.
We know that the key to attracting an aerosol lies with the water droplet's electric charge, however this charge is so low that the phenomenon is difficult to observe in a laboratory setting. Previous attempts to study it have seen experiments conducted with charges 10 to 100 times that of the normal level, but such unrealistic levels of charge would only be expected during intense lightening storms, and therefore fail to translate to any practical application.
The MIT experiment avoided this pitfall through the use of a 3 ft (0.9 m) tall glass chamber that allowed the researchers to manipulate the size and current of the test drops using a strip of radioactive material. Aerosols were pumped into chamber, and a series of experiments were carried out with varying sizes of drops and environments.
Once the drops had passed through the aerosol gas, they were allowed to evaporate, and the residual particles were then passed through a mass spectrometer that allowed the scientists to quantify the amount of aerosol particles captured by the drop.
The team discovered that smaller water drops falling in a high humidity were the most successful in attracting the aerosol particles, with the tests representing the most accurate values for precipitation coagulation to date.
In the future, data from tests such as these will allow researchers to create more detailed air quality forecasts – a boon in cities such as Bejing, where millions of citizens are routinely blighted by smog. On a larger scale, the values produced in the test could be used to incorporate precipitation-based cleansing in global climate models.