Rainfall causes microplastic transport into the atmosphere

Oceans, lakes and rivers often con­tain a large num­ber of microplas­tic par­ti­cles on their sur­face. Hit­ting rain­drops cause many droplets with an almost equal­ly high microplas­tic con­cen­tra­tion to be hurled into the air. If they evap­o­rate in the air, the par­ti­cles enter the atmos­phere. Researchers* at the Uni­ver­si­ty of Bayreuth describe these process­es in a new study pub­lished in “Microplas­tics and Nanoplas­tics.” In an ini­tial esti­mate, which is still sub­ject to uncer­tain­ties in sev­er­al respects, they come to the con­clu­sion: World­wide, up to 100 tril­lion microplas­tic par­ti­cles could enter the atmos­phere each year as a result of rainfall.

The stud­ies show that when a rain­drop hits a water sur­face, droplets from a small ring-shaped area around the impact site are eject­ed into the air. They orig­i­nate from a depth of a few mil­lime­ters below the water sur­face. The microplas­tic par­ti­cles con­tained in the droplets have almost the same con­cen­tra­tion as in this nar­row lay­er of water. The Bayreuth sci­en­tists have also cal­cu­lat­ed their tra­jec­to­ries in the air and their flight duration.

This gives a clear pic­ture: The water from rain­drops that is free of microplas­tics ends up in the oceans, while water con­tain­ing plas­tic from the oceans ends up in the air. As the droplets fly in the air until they evap­o­rate, they release the microplas­tic par­ti­cles into the atmos­phere. This is par­tic­u­lar­ly com­mon above the water sur­faces of oceans, where wind con­di­tions and tem­per­a­tures favor com­par­a­tive­ly long flight times and rapid evap­o­ra­tion. How­ev­er, most of the microplas­tic par­ti­cles eject­ed into the air fall back into the water due to a short flight duration

“It was a huge chal­lenge to deter­mine how many droplets are flung up by a sin­gle impact­ing rain­drop, how large and how fast those droplets are, and how many microplas­tic par­ti­cles they may con­tain. Exper­i­ments alone would have pro­vid­ed too lit­tle infor­ma­tion. There­fore, we worked out a com­plete­ly new code for sim­u­la­tions of these process­es and devel­oped a com­put­er mod­el that allows us to answer these ques­tions with high accu­ra­cy and in unprece­dent­ed detail.”

- Prof. Dr. Stephan Gek­le, coor­di­na­tor of the study

“How real­is­tic our sim­u­la­tions are becomes appar­ent when com­par­ing them with tech­ni­cal­ly demand­ing exper­i­ments: High-speed images of impact­ing rain­drops con­firm the cal­cu­la­tions based on our mod­el,” says first author Moritz Lehmann, a physics doc­tor­al stu­dent at the Uni­ver­si­ty of Bayreuth.

To find out how many microplas­tic par­ti­cles ulti­mate­ly end up in the atmos­phere as a result of these process­es, the Bayreuth researchers* brought togeth­er a vari­ety of empir­i­cal­ly avail­able data and includ­ed them in their cal­cu­la­tions. These data include microplas­tic con­cen­tra­tions at sea sur­faces, annu­al pre­cip­i­ta­tion amounts, the size of rain­drops, which depends on rain inten­si­ty, and the tem­po­ral dis­tri­b­u­tion of rain inten­si­ty. A first esti­mate leads to the con­clu­sion that the impact of rain­drops on water sur­faces world­wide could release up to 100 tril­lion microplas­tic par­ti­cles per year into the atmosphere.

The authors empha­size that this esti­mate is still sub­ject to numer­ous uncer­tain­ties and inac­cu­ra­cies: tur­bu­lence in the wind, which can influ­ence the impact force of rain­drops, has not yet been includ­ed in the cal­cu­la­tions. In addi­tion, the ocean sur­faces on Earth do not have the same high con­cen­tra­tion of microplas­tic par­ti­cles every­where — on the con­trary, the dif­fer­ences are very large. How­ev­er, satel­lite mea­sure­ments in con­junc­tion with weath­er mod­els could soon pro­vide more pre­cise infor­ma­tion about the “hotspots” where par­tic­u­lar­ly large num­bers of microplas­tic par­ti­cles are trans­port­ed from the ocean into the atmosphere.

Research funding and cooperation

The study, pub­lished in the jour­nal “Microplas­tics and Nanoplas­tics,” emerged from a research project of the DFG-fund­ed Col­lab­o­ra­tive Research Cen­ter “Microplas­tics” at the Uni­ver­si­ty of Bayreuth. The Bayreuth research team, led by Prof. Dr. Stephan Gek­le, col­lab­o­rat­ed with Prof. Dr. Andreas Held, Pro­fes­sor of Envi­ron­men­tal Chem­istry and Air Pol­lu­tion Con­trol at TU Berlin.