In the ever-complex theatre of Earth’s climate system, clouds play a pivotal role, regulating weather patterns and precipitation, which in turn govern ecosystems and human activities alike. Recent groundbreaking research emerging from the Laboratory of Atmospheric Processes and Their Impacts at EPFL (École Polytechnique Fédérale de Lausanne) reveals a sophisticated connection between biological particles suspended in the atmosphere and cloud ice formation—a link that promises to revolutionize weather prediction and climate modeling. This new study, published in the esteemed journal Climate and Atmospheric Sciences, unpacks the crucial role of organic aerosols such as pollen, bacteria, fungal spores, and plant matter, unraveling how their abundance fluctuates diurnally and how these cycles influence the microphysics of clouds.
Clouds largely form around particles in the atmosphere known as cloud condensation nuclei (CCN) and ice nucleating particles (INPs). While mineral dust and sea salt have long been recognized as significant contributors to this process, biological aerosols have recently taken center stage. It turns out that biological particles are remarkably efficient catalysts for ice formation, a process critical to precipitation worldwide. Since ice crystals precipitate more rapidly than liquid droplets, the heterogeneous nucleation of ice accelerates the progression of storms and can intensify extreme weather phenomena such as flash floods and blizzards. Understanding the dynamic behavior of these ice-active biological particles is therefore essential for better forecasting models.
The EPFL team, led by Professor Athanasios (Thanos) Nenes along with researcher Kunfeng Gao, conducted an in-depth analysis of atmospheric samples taken from Mount Helmos, a mountain situated at 2350 meters in Greece. This alpine location experiences frequent cloud cover and lies beneath a vast forest whose biological emissions contribute significantly to local aerosol populations. By meticulously monitoring the diurnal (daily) variation in the concentration of biological particles, the researchers observed a striking correlation between biological particle abundance and ice nucleation events. As daytime temperatures rise, pollen grains, bacteria, and spores are lofted into the atmosphere from the forest canopy, reaching peak numbers at midday before declining through the night.
These cyclical fluctuations in bioaerosol levels have profound implications for cloud microphysics. The daytime surge in ice nucleating particles facilitates the formation of ice crystals within clouds, which can hasten precipitation processes and potentially influence the development of convective storms. This tight coupling between biology and atmospheric physics has historically been overlooked in climate models, which typically do not account for the temporal dynamics and biological origin of aerosols in their computations. As such, current forecasts may underrepresent the variability of precipitation intensity, especially under a warming climate where biological emissions are expected to increase.
The interdisciplinary approach adopted by the EPFL team is particularly notable. Beyond simple aerosol sampling, their ongoing campaign named CHOPIN (Cloud Hosting On-Peak Ice Nucleation) integrates advanced instrumentation including cloud radars, aerosol lidars, unmanned aerial vehicles (UAVs), tethered balloons, and direct in situ sampling. This comprehensive suite enables an unprecedented characterization of the types and efficiencies of biological particles in nucleating cloud droplets and ice crystals. These efforts aim to tease apart which bioaerosols dominate ice nucleation under varying atmospheric conditions and to quantify their relative impact on cloud lifetime and precipitation.
From a technical perspective, biological aerosols often possess complex surface chemistries and morphologies that make them particularly adept at catalyzing ice nucleation at relatively warm subzero temperatures, compared to mineral or anthropogenic particles. For instance, proteinaceous compounds on bacterial cell walls or the robust structures of pollen exine can serve as efficient nucleation sites. Quantifying such mechanisms involves laboratory ice nucleation assays coupled with field observations, enabling modelers to parameterize ice formation rates as a function of aerosol composition and environmental variables like temperature and humidity.
Professor Nenes emphasizes the urgent need to incorporate these biological processes into global weather and climate models. Not only do biological particles exhibit concentration cycles tied to environmental variables, but their source strengths are also likely to evolve with climate change. Increased global temperatures may lead to longer growing seasons for vegetation and elevated microbial activity, thus changing atmospheric aerosol loads. Ignoring these factors renders climate models less accurate in predicting future precipitation patterns, particularly extreme events that can have devastating social and economic consequences.
In addition to enhancing numerical models, the data from CHOPIN and related campaigns holds promise for remote sensing advancements. Satellite instruments such as the recently launched EarthCare mission, developed by the European Space Agency in collaboration with consortia including CERTAINTY and AIRSENSE, rely on algorithms derived from ground truth measurements to interpret aerosol and cloud properties. Improved characterization of biological aerosols will refine these retrieval algorithms, thereby enhancing global monitoring of aerosol-cloud interactions and their climate effects in real time.
The Mount Helmos study also offers valuable insight into alpine cloud dynamics. Mountainous regions often serve as natural laboratories due to their unique meteorological regimes and the interplay between terrestrial biospheres and atmospheric processes. The discovery that biological particles track daily temperature cycles so closely suggests a delicate biosphere-atmosphere feedback loop, where forest emissions modulate precipitation that in turn impacts forest health and growth patterns—a complex system with broad ecological ramifications.
This research arrives at a pivotal moment in climate science. As humanity grapples with the challenges of a post-fossil-fuel era, understanding the fundamental processes driving weather extremes gains critical importance. The integration of biological ice nucleation into climate frameworks stands to enhance predictive capabilities, informing disaster preparedness, agricultural planning, water resource management, and policy decisions worldwide. EPFL’s work underscores how interdisciplinary collaborations, blending atmospheric physics, biology, and remote sensing technology, open new frontiers in comprehending and mitigating the impacts of climate change.
Looking forward, the CleanCloud European project spearheaded by Professor Nenes seeks to expand these findings by conducting broader observational campaigns across various ecosystems and incorporating artificial intelligence models for predictive analytics. Together with partners across Europe, this initiative aims to translate complex microphysical insights into actionable climate intelligence. The nuanced recognition of the biosphere’s role snuggly nested within cloud physics heralds a paradigm shift—a call to treat Earth’s atmosphere as a living system where life’s microscopic agents orchestrate grand-scale climatic phenomena.
In summary, this compelling study from EPFL advances the scientific understanding that biological aerosols are not passive bystanders but active architects of cloud ice formation, precipitation, and ultimately, weather extremes. The dynamics of these particles are intricately tied to environmental cycles and are set to evolve under global warming. This new knowledge challenges existing meteorological conventions and equips researchers with the tools to enhance forecasting accuracy, better anticipate extreme weather, and develop informed climate strategies. As science continues peeling back the layers of Earth’s climate system, integrating the microscopic world of bioaerosols into the grand scale of atmospheric modeling emerges as not just necessary, but transformative.
Subject of Research: The role of biological aerosols in cloud ice formation and their impact on weather and climate models.
Article Title: Biological Ice Nucleating Particles Drive Diurnal Cloud and Precipitation Dynamics at Mount Helmos
News Publication Date: 5-May-2025
Web References:
- EPFL CleanCloud Project: https://projects.au.dk/cleancloud
- CHOPIN Campaign: http://go.epfl.ch/chopin-campaign
- DOI Link to Article: http://dx.doi.org/10.1038/s41612-024-00817-9
References:
Contained within the published article in Climate and Atmospheric Sciences, DOI: 10.1038/s41612-024-00817-9
Keywords: Cloud microphysics, biological aerosols, ice nucleation, climate modeling, weather extremes, atmospheric particles, bioaerosols, diurnal cycles, precipitation, alpine clouds, remote sensing, EarthCare satellite
