Microplastics travelled thousands of kilometres across oceans and continents in a fast-moving layer of the atmosphere before being captured on a mountain in the French Pyrenees
21 December 2021
Microplastics found at a mountain top in the French Pyrenees may have crossed continents and oceans, travelling around 4500 kilometres in a fast-moving region of the troposphere, which is the lowest layer of the atmosphere. The finding suggests the particles can circulate the world and reach even the most remote regions.
Microplastics are tiny pieces of plastic, each less than 5 millimetres in diameter. They have been previously discovered in a lower region of the troposphere called the boundary layer, where friction between the air and Earth’s surface occurs and wind speeds are relatively low.
Now, for the first time, we have evidence that microplastics can travel at a higher altitude in the troposphere in a layer that doesn’t feel the effects of friction with Earth’s surface. In this layer – called the free troposphere – higher wind speeds give microplastics a greater potential for long-distance travel than was previously known.
“Once microplastics hit the free troposphere it’s the super highway for pollution movement. There’s high wind speed and very little rain up there, so the pollution doesn’t get rained out and it just travels much faster [than in the planetary boundary layer below],” says Steve Allen at the University of Strathclyde in the UK, a member of the research team.
“We’re not surprised that it’s up there but we’re sad that it is. These tiny particles are excellent transporters of pollution, they act as little balls of Velcro, collecting viruses and other pollutants on the outside of the particle as it moves,” says team member Deonie Allen, also at the University of Strathclyde.
The researchers captured 15 samples of microplastic particles over several months at the Pic du Midi Observatory in the Pyrenees in south-west France, which sits at nearly 3000 metres above sea level and provides access to the free troposphere.
The team used computational models to map the likely routes taken by the microplastics in the week leading up to their capture. The models were fed with data on the movement of airflow around the globe and took account of the sizes and densities of the microplastics to find that particles travelled around 4500 kilometres on average in the free troposphere. Potential sources included the US, Canada, North Africa, the UK, and the Mediterranean Sea and Atlantic Ocean.
“Some of the samples we got showed a marine source, coming out of the ocean and managing to get up into the free troposphere,” says Steve Allen. “That basically completes the cycle of what we think plastics are doing – it doesn’t stop anywhere, there’s never a sink, but a way station on to somewhere else.”
Most of the particles were between 5 and 20 micrometres in diameter. These are particles that can be inhaled and potentially cause breathing problems.
“This is the size of particle you breathe that causes respiratory disease – the stuff that makes you cough and gives you asthma,” says Deonie Allen.
Using a laser, the team determined that the most abundant type of plastic was polyethylene, which is commonly used in plastic packaging.
“Wealthy countries think that they’re getting rid of plastic waste when they ship it off to be burned or landfilled in other parts of the world – they’re not, it’s just coming back in a few weeks’ time. There’s no borders in nature,” says Steve Allen.
Journal reference: Nature Communications, DOI: 10.1038/s41467-021-27454-7
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