Scientists are using new techniques to identify previously undetected layers of cloud in the atmosphere, layers which will influence cloud formation and climate.
This week we have been seeing satellite images of wildfire smoke from Canada crossing the Atlantic and arriving in western Europe. The smoke can be tracked because this is an extreme and clearly visible event but, since 2022, Scientists at the Leibniz Institute for Tropospheric Research (TROPOS) have been using fluorescence lidar technology to detect wildfire smoke in Europe that would be invisible to any other method.
Lidar stands for Light Detection and Ranging and is usually mentioned within these pages in relation to its use in automated vehicles. In this application it works by sending laser pulses into the atmosphere and measuring the light that bounces back. Traditional lidar can detect particles, but it often can’t say what kind they are, which is where fluorescence come in.
Fluorescence is a process where certain substances absorb light (often ultraviolet or laser light) and then emit light at a longer wavelength. Think Ross’s teeth.
In fluorescence lidar, when the laser hits the particles in the atmosphere some, such as biological materials, wildfire smoke or pollution from certain sources, emit fluorescent light.
The lidar system can pick up this faint fluorescent glow, separate it from the normal backscatter, and analyse its wavelength. This can help distinguishing between, say wildfire smoke and volcanic ash.
The team at TROPOS have a lidar known as MARTHA (Multiwavelength Atmospheric Raman Lidar for Temperature, Humidity, and Aerosol Profiling) which received an upgrade in 2022, allowing it to see aerosol particles more clearly, especially at night.
Between August 2022 and October 2023, about 50 nighttime measurements were made. Several of these captured smoke layers from major Canadian wildfires, which had drifted over Europe. One standout event occurred over 4th -5th July 2023, when a 2 km thick smoke layer was observed over Leipzig, showing strong fluorescence.
Traditional lidar often misses thin, high-altitude aerosol layers but the fluorescence technique revealed several such layers at heights up to 10 km. This is significant as such smoke layers can influence cirrus cloud formation – high-altitude clouds made of ice crystals. One case, observed on 29th-30th May 2023, showed cirrus clouds forming just below a smoke layer, suggesting the smoke particles may have triggered the ice crystal formation.
This implies that the atmosphere may contain more smoke than previously believed, especially during wildfire seasons. While these thin smoke layers might not block much sunlight, they may still affect the climate by altering cloud formation.
The researchers suggest that more long-term data could help confirm whether wildfire smoke regularly acts as a source of ice nucleating particles, potentially influencing climate on a broader scale.
To that end the MARTHA system is being equipped with a more powerful laser and a 32-channel spectrometer which will enable detailed aerosol measurements.
Albert Ansmann from TROPOS says: ‘This will allow us to precisely document all trends in the climate system – caused by forest fire aerosol and volcanic aerosol in the upper troposphere and lower stratosphere – over Saxony and Central Europe in the coming decades.’
Image caption as supplied by TROPOS: At the beginning of June 2025, smoke from Canadian forest fires was again observed in the atmosphere over Leipzig. The image shows a measurement from the night of 2/3 June 2025. In the lidar signal in the infrared range (upper plot) we see extended aerosol layers of 4.5-6.5 km and 8-10 km. From about 3 o’clock local time (= 01:00 UTC), cirrus clouds begin to form in the upper layer. The clouds can be recognised by the high signals (dark red colours) due to the strong backscattering from the ice crystals.
The measurement of the laser-induced fluorescence (lower plot) identifies the aerosol layers as smoke due to their strong fluorescence (again recognisable by the red colours). Using backward trajectories, the observed smoke layers can be traced back to Canada, where strong forest fires are currently raging again. The ice clouds can be recognised in the lower plot by their low fluorescence capacity, as the ice crystals in the clouds themselves do not fluoresce. Since the clouds form in the smoke layer, this is another exciting case for the investigation of aerosol-cloud interactions and the question of the extent to which smoke is suitable as an ice nucleus.
Above image: Benedikt Gast, TROPOS
Main photo: Pawel Czerwinski
Leave a Reply