Oceans, lakes and rivers often contain a large number of microplastic particles on their surface and impacting raindrops cause many droplets with an almost equally high concentration of microplastics to be thrown up into the air. When they evaporate in the air, the particles enter the atmosphere.
Researchers from the University of Bayreuth have investigated the phenomenon and found that when a raindrop hits a water surface, droplets from a small ring-shaped area around the impact site are thrown into the air.
They originate from a depth of a few millimetres below the water surface. The microplastic particles contained in the droplets have almost the same concentration as that in this narrow layer of water.
The trajectories and flight duration of the particles showed that water from raindrops, which is free of microplastics, ends up in the oceans, while water containing plastics from the oceans ends up in the air.
The droplets are suspended in the air until they evaporate, upon which they release microplastic particles into the atmosphere.
This happens particularly often above the surfaces of oceans, where wind conditions and temperatures favour a comparatively long flight duration and rapid evaporation. Most of the microplastic particles return to the water due to their short flight duration.
“It was a huge challenge to determine how many droplets are thrown up by a single impacting raindrop, how large and how fast these droplets are, and how many microplastic particles they might contain,” said study coordinator Dr Stephan Gekle.
“Experiments alone would have provided too little information. That is why we have devised some completely new coding for simulations of these processes and developed a computer model that allows us to answer these questions with high accuracy and in an unprecedented level of detail.”
The researchers found that high-speed recordings of impacting raindrops confirmed their model’s calculations.
In order to find out how many microplastic particles ultimately end up in the atmosphere as a result of these processes, other, pre-existing data was used in their calculations.
This included the concentrations of microplastics at sea surfaces, annual precipitation amounts, the size of the raindrops, which depends on rain intensity, and the temporal distribution of rain intensity.
An initial estimate leads to the conclusion that the impact of raindrops on water surfaces worldwide could release up to 100 trillion microplastic particles into the atmosphere per year.
While the researchers admit that their estimate does not take all factors into account, such as wind turbulence, which can influence the impact force of raindrops and consideration of the fact that the Earth’s sea surfaces do not have the same high concentration of microplastic particles everywhere.
However, satellite measurements in conjunction with weather models could soon provide more precise information on “hotspots” from which particularly large numbers of microplastic particles are transported from the ocean into the atmosphere, the researchers said.
Earlier this month, another study aimed to track the movement of single-use plastic pollution created by the Covid-19 pandemic, such as face masks, gloves, and face shields, around the world’s oceans.