Leipzig, 07.03.2025 – Holger Baars
NASA’s Earth Observatory highlighted colourful dust blowing out of Mauretania, as observed with its Moderate Resolution Imaging Spectroradiometer (MODIS) on 25 February 2025:
https://earthobservatory.nasa.gov/images/153998/colorful-dust-over-mauritania
Due to the trade winds, mineral dust from this region is typically transported towards Cabo Verde, where TROPOS operates an ACTRIS aerosol and cloud remote sensing facility.
And indeed, this special dust event with its striking different colours caused by different source regions of the dust was also observed from ground at the Ocean Science Centre Mindelo, as seen in the lidar observations made in the framework of PollyNET.
What do these Figures show (below)?
The attenuated backscatter coefficient (top and bottom plots) indicates the backscatter intensity in the atmosphere. Usually, higher backscatter indicates more particles in the atmosphere. The depolarization ratio (middle plot) instead is an indicator for the non-sphericity of the atmospheric particles. Mineral dust is considered non-spherical and is therefore indicated by increased depolarization ratio values (warmer colours), while other aerosol particles (marine, smoke etc.) are characterized by low depolarization values (blue colours).
On 25 February, a typical structure of the atmosphere over Cabo Verde was observed: a marine boundary layer, which is rather clean, reaching up top about 600 - 800 m and partly topped with clouds (whitish colours in the top panel). The low depolarization ratio (blue colours in the middle plot) indicates that no dust was present in the boundary layer. Above the boundary layer and the clouds, one can see a confined dust layer (indicated by enhanced depolarization) but "only" reaching up to 2 km.
However, the scenario changed overnight from 25 to 26 February.
On 26 February, Saharan dust was still above the marine boundary layer, but also partially mixed into the local boundary layer (see green colours in the depolarization in the lowermost 1 km). Such downmixing of dust is occasionally observed during winter months in this region.
We suppose, that the dust plumes observed by MODIS on 25 February have now reached the Cabo Verde Islands and did significantly contribute to a dust load in local boundary layer. This dust has the potential to impact local air quality and pose health risks, as it can act as a carrier for pathogens and heavy metals, a topic currently being investigated through the DUSTRISK Project.
The different colors of the dust may reflect differences in its source regions. Dust from the southern salt pan area tends to be rich in calcite, gypsum and kaolinite, while dust from the northern region contains higher concentrations of illite and hematite, with lower calcite and gypsum content. Iron oxides like hematite and iron rich minerals such as illite are associated with more reddish and orange colors, as visible in the satellite picture.
Further research combining of lidar derived aerosol profiles with mineral dust transport modeling, including back trajectories is needed to determine whether the dust reaching Cabo Verde originates from a mixture of these sources or predominantly from just one region. In-depth measurement analysis, supported by in-situ observations at the Cabo Verde Atmospheric Observatory will be key to conduct these investigations.
Satellite image of MODIS provided by NASA. Source: earthobservatory.nasa.gov
Lidar observations in Mindelo on 25 and 26 February 2025. Shown is the attenuated backscatter (top: at 532 nm, bottom: at 1064 nm) and the volume depolarization ratio at 532 nm.