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)

Abstract

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 Angström 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.

Characteristics of Absorbing and Non-Absorbing Aerosol over South Africa using MODIS and OMI satellite measurements

“…Furthermore, a comparison of the seasonal AAOD showed that the least value is almost half of the highest value. It should be recalled that the peak of AAOD corresponds with the biomass-burning period (SON) in South Africa (see Kumar et al 2014;Ranaivombola et al 2023). This implies that more than 50% of the absorbing aerosols may be attributed to biomass-burning related.…”

Comment on egusphere-2024-921

“…CC BY 4.0 License. et al, 2017;Ranaivombola et al, 2023), India (Kaskaoutis et al, 2009;Patel et al, 2017), Buenos Aires (Cúneo et al, 2022) and Australia (Yang et al, 2021). In this study, we classified aerosols by using the following parameters: AOD, AE, EAE, SSA, and FMF, and a variety of threshold values (Figure 2) and all-point data to have the largest amount of data.…”

“…This method was already used in several works over South Africa (Kumar et al, 2017(Kumar et al, , 2020Ranaivombola et al, 2023). The BB/UR aerosol types can be distinguished using SSA, in combination with the EAE size indicator.…”

“…We have chosen to standardize, with BB/UR from the AOD-AE method, according to the work of Ranaivombola et al (2023).…”

“…The calculation of aerosol frequencies is based on the equation 2 in Ranaivombola et al (2023), where N s (type), is the number of occurrences for a given aerosol type and period (i.e., September-October-November (SON) 2022, September, October, Event 1, and Event 2). Sun-photometer data for Skukuza and Maputo ranges from 1998 to 2022 and from 2019 to 2022, respectively, although Skukuza sun-photometer was out of operation from 2012 to March 2016 owing to calibration and maintenance issues (Adesina and Piketh, 2016).…”