Tiny Biological Particles in Mountain Air Drive Ice Formation in Clouds

A team of international researchers has uncovered a surprising daily rhythm in the air above mountainous regions that could reshape our understanding of extreme precipitation. The study, published Sunday in the journal npj Climate and Atmospheric Science, reveals how microscopic biological particles from forests create a 24-hour cycle that influences cloud formation and potentially triggers intense rain and snow events.

Scientists from EPFL (École Polytechnique Fédérale de Lausanne) and collaborating institutions discovered that the concentration of ice nucleating particles (INPs) – tiny airborne materials that help create ice in clouds – follows a distinct pattern throughout the day. These particles, which include pollen, bacteria, spores, and plant matter, peak around midday and reach their lowest levels during the night.

“Biological particles are very effective at forming ice in clouds, and the formation of ice is responsible for most of the precipitation the planet receives worldwide, because ice falls very quickly from the sky,” explains Athanasios Nenes of EPFL’s Laboratory of Atmospheric Processes and Their Impacts, who led the study alongside researcher Kunfeng Gao.

The research team conducted their measurements at Mount Helmos in Greece, a site rising 2,314 meters above sea level. When morning temperatures rise, forests below the mountain release biological particles that float upward, reaching maximum levels in the early afternoon before declining as evening approaches.

The importance of these findings extends beyond academic interest. Current weather and climate models largely ignore these biological particles and their daily cycles – a significant oversight that could affect the accuracy of both current forecasts and future climate predictions.

The study examined multiple atmospheric conditions, comparing days influenced by desert dust with days dominated by local biological particles. While both contribute to ice formation, the researchers found that biological particles have a particularly strong influence during daytime hours and in the absence of dust events.

According to the study, different types of biological particles appear to have varying impacts. Certain bacteria-sized particles were strongly linked to ice formation when local forest sources dominated, while larger fungal spores and pollen fragments became more important during dust events.

By contrast, the researchers found that particles containing black carbon – typically from fossil fuel combustion – had negligible effects on ice formation, despite their known influence on other cloud processes.

Nenes, who recently participated in the IPCC scoping meeting for the organization’s 7th Assessment Report, is now leading a second campaign at Mount Helmos called CHOPIN. This follow-up research deploys an expanded arsenal of tools including cloud radars, aerosol lidars, UAVs, and tethered balloons to further characterize how each type of biological particle influences cloud formation.

“Given our findings, weather and climate models absolutely need to take biological particles into account, especially since biological particles are expected to be present in larger amounts in the atmosphere as the climate warms up,” Nenes emphasized.

The findings add complexity to our understanding of precipitation in mountainous regions worldwide. When combined with subsequent processes like ice multiplication, these biological particles can rapidly glaciate clouds and potentially generate heavy snowfall and extreme precipitation.

The research team is now collaborating with the European Space Agency and other consortia to apply their findings to satellite data, improving our ability to understand and predict how airborne particles influence clouds and precipitation patterns in what they describe as “a post-fossil world.”