Aerosol Optical Properties and Types over Southern Africa and Reunion Island Determined from Ground-Based and Satellite Observations over a 13-Year Period (2008-2021)

Fires occur seasonally in Southern Africa, from June to November, increasing tropospheric aerosol loading and triggering harmful consequences for the environment and human health. This study aims to examine 13 years of aerosol optical characteristics and types over Southern Africa and Reunion Island. Using AERONET sun photometers and MODIS observations, we found that a high aerosol optical depth and Angstrom exponent are associated with two predominant types of aerosols (biomass burning/urban industrial and mixed type) throughout the spring season. According to CALIOP observations, the major aerosol types with occurrence frequencies above 10% are polluted continental/smoke, polluted dust, and elevated smoke, whereas dust, clean continental, and dusty marine have occurrence frequencies below 1%. In comparison to other seasons, the vertical profiles of elevated smoke have different shapes in spring, with a seasonal shift in the peak altitude (from 3-4 km), when fire activity is at its maximum. At these altitudes, the northern regions presented occurrence frequencies of 32% on average, while lower values were found for the southern or farthest regions (<10-20% on average). The Lagrangian HYSPLIT model back-trajectories demonstrated eastward transport, with air masses from South America and the Atlantic Ocean that recirculate around the study sites. The aerosols are mainly derived from active biomass burning areas near the study sites and, to a lesser extent, from remote sources such as South America.

Automated summary

Fires during the rainy season in Southern Africa increase the amount of tropospheric aerosols, causing negative effects on the environment and human health. This study examines the characteristics and types of aerosols over Southern Africa and Reunion Island for a period of 13 years. The results reveal that in spring, high aerosol optical depth and Angstrom exponent are associated with biomass burning and urban industrial aerosols, as well as a mixed type of aerosol. The aerosols mainly originate from biomass burning areas near the study sites and, to a lesser extent, from remote sources like South America.