Five years of aerosol remote sensing in Mindelo – a milestone in atmospheric research in the Atlantic

Mindelo (São Vicente). For five years now, a distinctive green laser beam has been shining at night up to 30 km above the harbour of the island’s capital. It forms part of a high-energy lidar with which the Leibniz Institute for Atmospheric Research (TROPOS) operates continuous aerosol and cloud measurements at the Ocean Science Centre Mindelo (OSCM).

The lidar is part of PollyNET, a global network of fixed and mobile lidar systems coordinated by TROPOS. This network enables the detection of airborne particles (aerosols) – such as desert dust, forest fire smoke, industrial pollution or even sea salt – as well as their trajectories across our planet. The remote sensing station in Mindelo complements the dust samples that have been collected of the island of São Vicente for almost 30 years in Calhau on the north-east coast.

To mark the Lidar’s anniversary, the researchers involved have taken stock of the first few years. The focus over the past five years has been on the remote sensing of aerosols, in particular the study of the vertical structure of Saharan dust, its influence on cloud formation in the region, as well as the temporal variability of aerosol concentrations and the optical properties of the atmosphere. The station in Mindelo has thus become an important reference point for atmospheric observations in the tropical Atlantic.

The measurement station was set up in stages: since 2021, aerosol remote sensing using lidar systems has been in continuous operation, whilst instruments for cloud remote sensing were added later, primarily to better investigate aerosol–cloud interactions and the associated precipitation formation. In addition, a radiation measurement station has been in operation since 2024 to determine the influence of atmospheric conditions on the radiation balance at ground level.

A first, notable highlight of the measurements was the volcanic eruption of Cumbre Vieja on La Palma in September 2021. Aerosol particles of volcanic origin were detected by the lidar over Mindelo. The event underscored the capabilities of the remote sensing systems under exceptional atmospheric conditions.

The establishment and expansion of the measuring station were accompanied by a total of three international measurement campaigns: two campaigns named ASKOS (2021, 2022), during which measurement data were collected for comparison and calibration with the AEOLUS space lidar, and ORCESTRA/CLARINET (2024), which investigated tropical weather systems in the Atlantic between West Africa and the Caribbean. These campaigns have significantly complemented the on-site measurements and contributed to embedding the observations within a broader understanding of the processes involved. The TROPOS station received special recognition with a visit from the President of the Republic of Cabo Verde, José Maria Neves, and the Federal President of Germany, Frank-Walter Steinmeier, on 4 October 2023. On that day, the complete TROPOS suite of instruments for remote sensing and radiation measurement was put into operation.

Long-term measurements using the lidar clearly reveal pronounced seasonal Saharan dust events, strong vertical stratification of the aerosols, and high variability in dust intensity over the course of the year. At the same time, outside the dust season, stable marine salt particles occur, which characterise the natural background signal of the aerosols in the region.

To carry out these measurements on a long-term basis, scientific and technical staff from TROPOS visit Mindelo at least twice a year to carry out major maintenance work. The daily minor tasks are handled by staff from the OSCM and IMAR (IMar – Instituto do Mar). “We would like to express our sincere thanks to the residents and friends of Mindelo, the staff at OSCM and IMAR, and also the residents of the neighbourhood around the OSCM for their help and acceptance of our work,” says Prof. Andreas Macke from TROPOS. “We would also like to thank our colleagues at the GEOMAR Research Centre, without whose ongoing logistical support at the OSCM the construction and operation of the TROPOS station would not have been possible, and of course the INMG for its continued logistical support.”

The Cape Verde Atmospheric Observatory (CVAO) is now one of the long-term atmospheric observation sites worldwide that are essential for monitoring climate change. The measurements form part of the European research infrastructure ACTRIS and now provide a vital basis for understanding the atmosphere over the tropical Atlantic. Through OSCM and CVAO, the Helmholtz Centre for Ocean Research (GEOMAR) and TROPOS, together with partners such as the Instituto do Mar (IMar) and the Instituto Nacional de Meteorologia e Geofísica (INMG) of the Republic of Cape Verde, are investigating changes in the ocean and atmosphere of the tropical Atlantic off the coast of West Africa.