Ultrafine aerosol particles found in polluted urban air can contribute to more intense storms in the Amazon rainforest, with potential knock-on effects for weather and climate patterns in the region and beyond, researchers have warned.

Particles smaller than 50 nanometers in diameter have a substantial influence on cloud formation in the Amazon. In turn, the rainforest has a strong influence on climate regulation worldwide, and aerosol effects observed in this region could also trigger climate shifts globally, according to a study published in Science.

Until this study, ultrafine particles were thought to be too small to affect the formation of clouds. This has now been overturned, at least in the specific circumstances of the Amazon. "These tiny particles were thought to be too small to aid droplet formation," says meteorologist Luiz Augusto Machado, from the Brazilian Institute for Space Research's Center for Weather Forecasting and Climate Studies, and one of the study's authors. "Now we verified that their presence is one reason why some storms become so strong and produce so much rain in the Amazon region."

Pérola de Castro Vasconcellos, a chemist at the University of Sao Paulo's Laboratory of Atmospheric Chemistry Studies, added: "This study changes the way clouds are to be represented in models used for weather forecasting, as well as the way researchers understand the workings of tropical rain."

The research team analyzed the influence of ultrafine particles on cloud cycles in the rainforest during the 2014 rainy season, when there were no forest fires and the only source of pollution was Manaus, the largest city in the Amazon.