A team of scientists conducted an analysis of the aerosol composition over the Namibian coast. The atmosphere contains smoke coming from biomass fires in Angola and also South America.
The study, published in the Atmospheric Chemistry and Physics Journal also shows evidence of a great variability in the atmospheric aerosol composition over Namibia.
Based in Henties Bay, on the Namibian coast, scientists carried multiples measurements out to understand the chemical composition of the troposphere (from the Earth surface to about 6 km above sea level) from 22 August to 12 September 2017. A lidar, which measures the aerosols in the atmospheric column using a laser, and a sun photometer were used to look into aerosols from the ground. Other measurements were made possible thanks to the French airborne lidar system LNG, used during two operational flights, and space-borne instruments.
Natural and anthropogenic traces in the atmosphere
The region is characterized by a complex aerosol composition. Man-set fires and wildfires in neighbouring countries create biomass-burning aerosols such as smoke. Natural aerosols include mineral dust from the Namibian coast, the Etosha Pan and the Makgadikgadi Pan in Botswana, as well as marine sea spray. Aerosols are also man-made, as it is the case with pollution coming from industries all over Southern Africa, port activities and ship emissions.
“The aerosol chemical composition depends on their sources of emission and rules their radiative effects in the Atlantic Ocean and the impact on climate: it is thus a crucial parameter to understand”, argued Dr. Paola Formenti, CNRS specialist of southern African aerosols.
This study is the first to show a high temporal variability in the aerosol composition, brought by different air trajectories. During the short measurement campaign, three periods were identified with distinct compositions and an increasing optical thickness.
The first period shows a low thickness, associated with air masses from Angola travelling along the Namibian and Angolan coasts. Intermediate thickness shown in the second period are due to dust from the Etosha Pan mixed with biomass burning aerosols from Angola, which circulated above the ocean. During the third period, air masses from Angola were transported directly to Namibia, thus creating a high aerosol thickness.
Fire smoke from as far as South America
The publication highlights the contribution of forest fire from southern Brazil, Argentina and Uruguay to the aerosol composition over Henties Bay. Dr. Formenti said: “on 6 and 7 September, we observed a high density of smoke, which may be associated with biomass burning over Angola but also with fires occurring from 1 to 4 September in southern Brazil, Argentina and Uruguay”. Present in the highest aerosol layers (between 5 and 6 km above sea level), this smoke travels in 3 to 6 days, following four transport routes across the Atlantic Ocean.
Patrick Chazette et al., Evidence of the complexity of aerosol transport in the lower troposphere on the Namibian coast during AEROCLO-sA, Atmos. Chem. Phys., 19, 14979–15005, 2019.