In a groundbreaking advancement in atmospheric science, researchers at the Leibniz Institute for Tropospheric Research (TROPOS) in Leipzig have unveiled compelling findings that illuminate the presence of elusive, high-altitude aerosol layers previously invisible to conventional detection methods. Utilizing an innovative fluorescence lidar system integrated into the MARTHA (Multiwavelength Atmospheric Raman Lidar for Temperature, Humidity, and Aerosol Profiling) platform, scientists have been able to detect and characterize ultra-thin smoke layers originating from Canadian wildfires that drift across the Atlantic and settle above Europe’s upper troposphere. This new approach heralds a paradigm shift in our understanding of aerosol distribution and their climatic impacts.
The essence of this breakthrough lies in laser-induced fluorescence, a sophisticated technique that identifies aerosol particles by their unique glow when irradiated with specific wavelengths of laser light. Unlike standard lidar technologies, which rely on backscattered laser light and suffer from ambiguity when differentiating aerosol types, fluorescence lidar exploits the intrinsic spectroscopic fingerprint of organic compounds and biomass burning residues. This enables the unambiguous identification of volatile smoke aerosols, even when present in optically thin layers at altitudes as high as 10 kilometers.
The innovative fluorescence channel was appended to the MARTHA lidar system in August 2022. It utilizes an interference filter centered at 466 nanometers to isolate fluorescence emissions from atmospheric particles. Because fluorescence signals are intrinsically weak and can be easily drowned out by solar radiation, these measurements are constrained to nocturnal periods with minimal background noise. Despite the challenges, the researchers amassed over 250 hours of fluorescence observations across 50 measurement sessions from August 2022 to October 2023, yielding unprecedented insight into atmospheric aerosol dynamics.
One of the pivotal revelations from these observations is the frequent detection of thin, elevated smoke layers stemming from massive forest fires in Canada during the spring and summer of 2023. These fires, concentrated in the provinces of Alberta and British Columbia, emitted vast clouds of biomass smoke that were transported by prevailing westerlies to European skies. The fluorescence lidar technique enabled the precise detection of these smoke layers, some exceeding two kilometers in vertical extent, demonstrating pronounced fluorescence signals that betray their biomass burning origin. This discovery challenges prior understandings that largely underestimated the range and impact of transcontinental wildfire aerosols.
Conventional aerosol detection methods encountered difficulties resolving these tenuous layers. Prior to the fluorescence method, many such layers in the upper troposphere appeared transparent or clean, lacking significant backscatter signals. However, the fluorescence data revealed robust aerosol presence at altitudes of 5 to 10 kilometers, layers that would have otherwise gone unnoticed. These findings underscore the critical role of fluorescence lidar in enhancing atmospheric profiling resolution and aerosol characterization, especially in the upper atmospheric regions where direct sampling remains prohibitively challenging.
The climatic implications of these discoveries are profound. Aerosol particles act as cloud condensation nuclei (CCN) and ice nucleating particles (INPs), thereby influencing cloud formation, lifetime, and radiative properties. Particularly significant are cirrus clouds, which form at high altitudes and contain ice crystals that strongly affect the planetary radiation budget. The study observed instances where cirrus clouds were located directly beneath or embedded within smoke layers identified by the fluorescence channel. This spatial co-location supports emerging hypotheses that smoke particles from wildfires might facilitate heterogeneous ice nucleation in cirrus clouds, potentially altering their microphysics and subsequent climate impacts.
Previous research had deemed forest fire smoke inefficient as ice nuclei at temperatures above -30°C, typically attributed to mineral dust and other aerosols. However, the fluorescence lidar observations from Leipzig provide empirical evidence suggesting that smoke aerosols can act as effective ice nuclei under certain conditions. This insight invites renewed scrutiny into aerosol-cloud interactions in the upper troposphere, emphasizing the necessity to reconsider wildfire smoke’s role in modulating cloud formation processes at large scales.
Technically, the MARTHA lidar system distinguishes itself through its multi-wavelength laser emissions at 355, 532, and 1064 nanometers, combined with an 80-centimeter diameter primary mirror that enhances signal collection efficiency. The backscattered light is analyzed through polarization and wavelength-dependent scattering characteristics to infer particle properties. Despite this advanced setup, differentiating between aerosol types like volcanic sulfates, urban pollution, or biomass smoke remained challenging due to overlapping scattering profiles. The addition of the fluorescence channel fills this critical gap, providing a molecular signature that elevates the classification accuracy of aerosol types remotely.
The incorporation of fluorescence lidar into routine atmospheric observations promises to revolutionize the detection of subtle but climatically significant aerosol layers. The Leipzig team’s case studies showcase how this method identifies aerosol structures associated with intense wildfire events far beyond regional boundaries. Their data suggests that the atmosphere over Europe’s upper troposphere may be more polluted than previously thought during wildfire seasons—a realization with far-reaching implications for climate modeling and air quality assessments.
Future developments are already underway to expand the capabilities and temporal coverage of fluorescence lidar observations. Since late 2023, the MARTHA system has been undergoing a comprehensive modernization, including the installation of a more powerful laser and a 32-channel spectrometer. These enhancements will enable higher spectral resolution and sensitivity, facilitating the measurement of aerosol layers extending into the lower stratosphere. According to Albert Ansmann of TROPOS, these improvements will allow for sustained, detailed aerosol monitoring over Central Europe, capturing both volcanic and wildfire aerosol trends vital for understanding climate evolution.
The ongoing research constitutes a central pillar of the Leibniz ScienceCampus ‘BioSmoke,’ an interdisciplinary initiative launched in autumn 2024 to unravel the complex interactions between biomass burning aerosols, biogenic particles, and atmospheric processes. The fluorescence lidar data serves as a cornerstone for this collaborative network, supporting studies on particle emission, long-range transport, and aerosol-cloud coupling mechanisms with unprecedented clarity and precision.
In conclusion, the integration of laser-induced fluorescence into ground-based lidar systems represents a transformative step forward in atmospheric science. By enabling the detection of invisible aerosol layers and unraveling their interplay with cirrus clouds, this technology equips scientists with a powerful toolset to decode aerosol-mediated climate effects. As wildfire activity intensifies globally due to climate change, understanding smoke aerosols’ nuanced roles in cloud physics and radiative forcing becomes ever more critical. TROPOS’s pioneering work thus not only sharpens our scientific lens but also enriches our predictive capabilities regarding climate dynamics in a changing world.
Subject of Research: Not applicable
Article Title: Invisible aerosol layers: improved lidar detection capabilities by means of laser-induced aerosol fluorescence
News Publication Date: 9-Apr-2025
Image Credits: Benedikt Gast, TROPOS
Keywords: fluorescence lidar, aerosol detection, biomass smoke, forest fires, atmospheric aerosols, cirrus clouds, ice nucleating particles, MARTHA lidar, aerosol-cloud interactions, upper troposphere, laser-induced fluorescence
