Atmospheric chemists at MIT claims to have obtained the most accurate estimates of rates of coagulation between rain droplets and aerosol particles that in turn could be used to determine the extent to which natural rain may contribute to keeping the atmosphere cleaner.
Before hitting the ground, raindrops attract a number of aerosol particles in the atmosphere. This attraction called coagulation helps to keep the atmosphere cleaner by removing pollutants like soot, sulfates, and organic particles from air. Though not the first of its kind, an experiment on coagulation between droplets and aerosol particles was run by atmospheric chemists at MIT in August 2015, in which they determined the likelihood of raindrops attracting aerosol particles. The results, published in the journal Atmospheric Chemistry and Physics, have been claimed by Dan Cziczo, a professor of atmospheric chemistry at MIT, to be the most accurate estimates of coagulation between rain droplets and aerosol particles obtained so far.
The researchers used a 0.91 m (3 feet) tall glass chamber that was used to generate rain droplets in controlled rates and sizes, with aerosol particles being pumped in at the same time. The rate at which the droplets and aerosol particles attracted each other in various conditions of humidity was measured. Conditions of low relative humidity seemed to encourage coagulation. As electric charges in rain droplets played a big role in attracting particles, the researchers also altered the charge in the droplets and particles for coagulation to occur.
In this illustration, a falling droplet of water is surrounded by the airstream. Aerosol particles (yellow) are attracted to the droplet. The process by which droplets and aerosols attract is coagulation, a natural phenomenon that can act to clear the air of pollutants like soot, sulfates, and organic particles. Credit: Jose-Luis Olivares/MIT.
From the measurements obtained, the researchers calculated coagulation efficiency of rain droplets, the ability of a droplet to attract aerosol particles. It was found that the smaller the size of a droplet, the likelier it was to attract more aerosol particles. The measurements, as Cziczo things, can be used to predict rain’s potential to clear a range of aerosol particles from the atmosphere.
To get more accurate results, the researchers also used a different chamber with a single-droplet generator that generated single droplets of specific sizes, charges and also at fixed frequencies. As charges carried by droplets in nature were actually lower than that imparted on droplets in the chambers by droplet generators, the researchers used a radioactive source to strip away a small amount of charge from each droplet. Various conditions of relative humidity were used during multiple similar experiments.
During the experiments, aerosol particles of known sizes were pumped in the lower portion of the chamber. As droplets hit the ground after attracting aerosol particles, they would leave salt on the surface. Otherwise, only aerosol particles were to be found. From observation of how much salt was found, the scientists were able to determine the extent of coagulation.
As clouds turn into rainfalls, and heat released by surface is trapped by clouds thereby causing greenhouse effect, Cziczo thinks it is very essential to understand the relationship between rain droplets and aerosol particles in the atmosphere. The research was funded partially by the National Oceanic and Atmospheric Administration.
- "Laboratory studies of collection efficiency of sub-micrometer aerosol particles by cloud droplets on a single-droplet basis" - K. Ardon-Dryer, Y.-W. Huang, and D. J. Cziczo - Atmospheric Chemistry and Physics (2015) - doi:10.5194/acp-15-9159-2015, 2015
Featured image: During some very heavy rainfall, it almost looks like fog - Daniel Ramirez (CC- Flickr)