MIT study examines the cleansing effect of rain drops on our atmosphere
A recent study carriedout by MIT has characterized the cleansing effect that raindrops haveon our atmosphere in removing aerosol and other pollutants from theair. The results of the research could be instrumental in creatingreliable forecasts for air quality, and creating more accurate modelsof climate change impact due to clouds.
Atmospheric aerosolscome from three main sources: as by-products of volcaniceruptions; desert dust; and increasingly as a product of human actions,often as a result of burning coal and oil as part of the industrialprocess. These aerosols have a number of effects on Earth'sprotective atmospheric shell, including acting as a catalyst forchemical reactions that seriously undermine our planet'sstratospheric ozone.
Raindrops are able toremove these detrimental particles as they fall through the air via aprocess known as coagulation, which attracts aerosol pollutants,trapping them in the raindrop and returning them to the surface.
We know that the key toattracting an aerosol lies with the water droplet's electric charge,however this charge is so low that the phenomenon is difficult toobserve in a laboratory setting. Previous attempts to study it have seen experiments conducted with charges 10 to 100 timesthat of the normal level, but such unrealistic levels of chargewould only be expected during intense lightening storms, andtherefore fail to translate to any practical application.
The MIT experimentavoided this pitfall through the use of a 3 ft (0.9 m) tallglass chamber that allowed the researchers to manipulate the size andcurrent of the test drops using a strip of radioactive material. Aerosolswere pumped into chamber, and a series of experiments were carriedout with varying sizes of drops and environments.
Oncethe drops had passed through the aerosol gas, they were allowed toevaporate, and the residual particles were then passed through a massspectrometer that allowed the scientists to quantify the amount ofaerosol particles captured by the drop.
The team discoveredthat smaller water drops falling in a high humidity were the mostsuccessful in attracting the aerosol particles, with the testsrepresenting the most accurate values for precipitation coagulationto date.
In the future, datafrom tests such as these will allow researchers to create moredetailed air quality forecasts – a boon in cities such as Bejing,where millions of citizens are routinely blighted by smog. On alarger scale, the values produced in the test could be used toincorporate precipitation-based cleansing in global climate models.