Temporal characteristics of columnar aerosol optical properties and radiative forcing (2011-2015) measured at AERONET's Pretoria_CSIR_DPSS site in South Africa

Ground-based observations of the spectral aerosol optical depths (AODs) revealed that the northwest of South Africa (SA) possessed large aerosol loading and still remained unexplored as none of the authors have been extensively studied. The characteristics of aerosol optical, physical, and radiative properties, as well as their relationships presented in this paper, were derived from the direct sun and sky radiances measured at Pretoria during August 2011-December 2015 using the AERONET's (CE-318) automatic sun/sky radiometer. The annual mean AOD(440), AE(440-870), and SSA-T-440 estimated at Pretoria during the study period were 0.23 +/- 0.13, 1.50 +/- 0.26, and 0.91 +/- 0.04, respectively. The mean AOD(440) (AE(440-870)) for the study period appeared higher during the spring and summer seasons (summer), suggest dominance of fine mode particles attributed to biomass burning activities and seasonal influence of meteorology. Analysis of frequency occurrences of AOD and AE also indicate that this region is richly populated with fine mode particles. Further, the AOD-AE relationship was studied at Pretoria and the result concluded that the mixed type aerosols contributed more among the others followed by the urban/industrial-biomass burning and clean continental (background) aerosols. The high summertime SSA-T-440 and fine mode radius of AVSD could be associated with the hygroscopic growth of water-soluble aerosols under high water vapor (absorbing aerosols). The positive (negative) values of aerosol radiative forcing (ARF) were observed in all the months, an indication of significant heating (cooling) within the atmosphere (top of the atmosphere (TOA) and bottom of the atmosphere (BOA)) were due to strong absorption (scattering) of radiation. Further, the efficiency derived between ARF and AOD(440) indicated that ARF is a strong function of AOD at the BOA noted with a high degree of correlation coefficient (r = 0.93). (C) 2017 Elsevier Ltd. All rights reserved.