Aerosol optical properties over three cities of Angola using long term AERONET data

Aerosols can influence the climate, by scattering or absorbing solar radiation. The presence of aerosols may also affect human health as their exposure has been linked to pulmonary and cardiovascular diseases. This paper aims to investigate the spatiotemporal trends of aerosol optical properties, the major types of aerosols, the possible sources of the aerosols, and the aerosol radiative forcing at three locations in Angola, namely, Huambo, Lubango, and Namibe, using the Aerosol Robotic Network (AERONET) dataset from 2016 to 2021. The results revealed that at the three sites, there was a decrease in the aerosol optical depth (AOD) during the warm months, a period characterized by rainfall, and the minimum loadings of AOD varied between 0.073 and 0.132. Huambo registered the highest percentages of biomass-burning aerosols (79 %), followed by Lubango (56 %) and Namibe (21 %). The Ångström exponent (AE) at Lubango ranged from 1.48 to 1.52, denoting the existence of fine-mode aerosol particles. The annual mean of the volume size distribution (VSD) showed that among the three sites, Huambo registered the highest concentration of fine-mode aerosols particles (0.053 μm³/μm²) when compared with Namibe (0.034 μm³/μm²) and Lubango (0.030 μm³/μm²). The mean peak radius of the fine particle over the three sites was about 0.15 μm, while the mean peak radius of the coarse particle registered at both Huambo and Lubango (5.06 μm) was greater than the one at Namibe (3.86 μm). Namibe had the highest annual mean of VSD for coarse-mode aerosol particles among all sites. Further, an investigation revealed that the annual mean aerosol radiative forcing (ARF) at the bottom of the atmosphere (BOA) was the highest at Huambo (−53.63 Wm^-2), while the ARFs at the top of the atmosphere (TOA) varied between −13.91 Wm^-2 and -9.59 Wm^-2, among the three sites. The annual mean ARF efficiencies (ARFE) showed a higher value at BOA at Lubango (−214.57 Wm^-2 per AOD), whilst at TOA, it was recorded at Huambo (−60.27 Wm^-2 per AOD). The findings obtained from the current study provide a summary of the aerosol optical properties at three sites in Angola, which could enrich the knowledge of the influence of aerosol direct radiative impact over parts of the country and so enhance future climate models of this region.