title: Direct radiative forcing of urban aerosols over Pretoria (25.75°S, 28.28°E) using AERONET Sunphotometer data: first scientific results and environmental impact.
abstract: The present study uses the data collected from Cimel Sunphotometer of Aerosol Robotic Network (AERONET) for the period from January to December, 2012 over an urban site, Pretoria (PTR; 25.75°S, 28.28°E, 1449 m above sea level), South Africa. We found that monthly mean aerosol optical depth (AOD, τ(a)) exhibits two maxima that occurred in summer (February) and winter (August) having values of 0.36 ± 0.19 and 0.25 ± 0.14, respectively, high-to-moderate values in spring and thereafter, decreases from autumn with a minima in early winter (June) 0.12 ± 0.07. The Angstrom exponents (α440-870) likewise, have its peak in summer (January) 1.70 ± 0.21 and lowest in early winter (June) 1.38 ± 0.26, while the columnar water vapor (CWV) followed AOD pattern with high values (summer) at the beginning of the year (February, 2.10 ± 0.37 cm) and low values (winter) in the middle of the year (July, 0.66 ± 0.21 cm). The volume size distribution (VSD) in the fine-mode is higher in the summer and spring seasons, whereas in the coarse mode the VSD is higher in the winter and lower in the summer due to the hygroscopic growth of aerosol particles. The single scattering albedo (SSA) ranged from 0.85 to 0.96 at 440 nm over PTR for the entire study period. The averaged aerosol radiative forcing (ARF) computed using SBDART model at the top of the atmosphere (TOA) was -8.78 ± 3.1 W/m², while at the surface it was -25.69 ± 8.1 W/m² leading to an atmospheric forcing of +16.91 ± 6.8 W/m², indicating significant heating of the atmosphere with a mean of 0.47K/day.
authors.affiliation: Discipline of Physics, School of Chemistry and Physics, College of Agriculture, Engineering and Science, Westville Campus, University of KwaZulu-Natal, Durban 4000, South Africa. Electronic address: jadesina173@gmail.com.
authors.forename: Ayodele Joseph
authors.initials: AJ
authors.lastname: Adesina
authors.affiliation: Discipline of Physics, School of Chemistry and Physics, College of Agriculture, Engineering and Science, Westville Campus, University of KwaZulu-Natal, Durban 4000, South Africa; Key Laboratory for Aerosol-Cloud-Precipitation of China Meteorological Administration, School of Atmospheric Physics, Nanjing University of Information Science and Technology, Nanjing 210044, China. Electronic address: krkumar@nuist.edu.cn.
authors.forename: Kanike Raghavendra
authors.initials: KR
authors.lastname: Kumar
authors.affiliation: Discipline of Physics, School of Chemistry and Physics, College of Agriculture, Engineering and Science, Westville Campus, University of KwaZulu-Natal, Durban 4000, South Africa.
authors.forename: Venkataraman
authors.initials: V
authors.lastname: Sivakumar
authors.affiliation: Optronic Sensor Systems, Council for Scientific and Industrial Research (CSIR)-DPSS, Pretoria 0001, South Africa.
authors.forename: Derek
authors.initials: D
authors.lastname: Griffith
mesh_terms: D000336:Aerosols / Q000032:analysis* / Q000737:chemistry; D001272:Atmosphere / Q000737:chemistry*; D004777:Environment; D055620:Optical Phenomena; D058998:Remote Sensing Technology; D013019:South Africa; D014887:Weather
chemical_list: D000336:Aerosols
keywords: AERONET; Aerosol optical depth; Pretoria; Radiative forcing; Single scattering albedo