Multi-year analysis of aerosol optical properties and implications to radiative forcing over urban Pretoria, South Africa

The present study focused on investigating the aerosol optical properties and direct aerosol radiative forcing (DARF) using the AERONET Sunphotometer measurements conducted during 2011-2017 over an urban-industrial city, Pretoria (25.75 degrees S, 28.28 degrees E) located in the Northwest of South Africa (SA). Results revealed high aerosol optical depth (AOD(440)) and angstrom ngstrom exponent (AE(440-870)) during SON (0.28 +/- 0.09, 1.46 +/- 0.16) and DJF (0.24 +/- 0.07, 1.48 +/- 0.18) indicating the dominance of anthropogenic fine-mode aerosols from biomass burning. The single scattering albedo (SSA(440)) reached a maximum of 0.90 +/- 0.05 in DJF and minimum (0.85 +/- 0.04) during JJA. The source analysis from the concentration-weighted trajectory (CWT) model exhibited large heterogeneity in all seasons. However, the region is influenced by distinct aerosol types with the abundance of anthropogenic fine absorbing aerosols (86%) and the invisible ratio of dust particles. In addition, the aerosol volume size distribution (VSD) increased with an increasing AOD, exhibiting a bimodal lognormal structure, with a more pronounced peak in fine- relative to coarse-mode particles. Further, the inversion products showed a strong spectral dependence in SSA with substantial heterogeneity in all seasons. At last, the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model showed that the DARF within the atmosphere was more pronounced during SON (46.71 W m(-2)) and JJA (46.39 W m(-2)) due to significant differences in AOD and SSA, with an annual mean value of 33.16 W m(-2), and the corresponding atmospheric heating rate of 0.96 K day(-1). The study provides information on the existing aerosol distribution and their potential impact on climatic change over an urban city in the Northwest of SA and could form a basis for policymaking over the region.