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Remote Sensing

Remote sensing of atmospheric aerosols in South Africa

Optical and physical properties of aerosols are measured using a sun photometer to help understand the meteorology of an aerosol hotspot region.
30 May 2013, SPIE Newsroom. DOI: 10.1117/2.1201306.004893

Aerosols are liquid or solid particles suspended in the air, and have received significant attention in the past few years.1 They are known to significantly influence the radiative budget of the Earth's atmosphere, both directly—by scattering and absorbing radiation—and indirectly, by affecting cloud properties. The impact of aerosols, particularly on climate systems, is currently incomplete. Systematic observational evidence is required to study the highly variable characteristics of atmospheric aerosols in time and space.2

Measurements of aerosols have progressed significantly since the establishment of ground-based monitoring networks, such as the AErosol RObotic NETwork (AERONET). AERONET sun photometers provide a continuous-time series of solar radiation measurements with high temporal resolution. The locations of AERONET stations are chosen to provide the most complete coverage of different climatic zones: there are currently more than 700 stations (see Figure 1) and the network continues to expand rapidly.

Figure 1. Locations of stations in the AErosol RObotic NETwork (AERONET).

We use AERONET sun photometer data from the Skukuza site (latitude: 24°59'S; longitude: 31°35'E; elevation: 150m) within the Kruger National Park of the Mpumalanga province (see Figure 2) to study the atmosphere over South Africa (SA). The subtropical atmospheric circulation conditions of this region make it particularly relevant for aerosol studies. Several sources of aerosols in this region include aeolian dust, and also local anthropogenic pollution from industrial, urban, maritime, and biomass-burning activities. The diversity in sources causes high variability in the type and properties of the aerosols.3

Figure 2. The location of the Skukuza AERONET station (indicated by the solid black circle) within the Mpumalanga province of South Africa.

Previous studies indicated that large quantities of aerosols and trace gases from the industrialized Mpumalanga Highveld region, and other sources, are injected into the atmosphere over SA every year.3, 4 Pollutants from the Zambian Copperbelt and emissions from biomass burning, especially during the late winter and spring, are even stronger contributions to the total aerosol loading over SA than previously thought.4, 5 The emissions from savanna fires in SA and their transportation across the continent have been studied through campaigns of the Southern African Fire Atmospheric Research Initiative (e.g., SAFARI-92 and SAFARI-2000).6

The sun photometer instrument we use (see Figure 3) measures the direct sun radiances in eight spectral channels (340, 380, 440, 500, 675, 870, 940, and 1020nm). The 940 and 1020nm channels are used to estimate the columnar water vapor content and the remaining channels are used to retrieve the spectral aerosol optical depth (AOD), a measure of the light that is not scattered by the aerosols. We make sky radiance measurements (in the horizontal and principal planes) in four spectral channels (440, 675, 870, and 1020nm) that are used to calculate the size distribution, single scattering albedo (SSA), and refractive indices of the aerosols.7 The processed aerosol-related data are available from the AERONET website.8 We used the final product, quality assured (Level 2.0) AERONET data from 2005–2006 for our study. The total uncertainty and errors in the parameters determined have been discussed previously.3

Figure 3. An AERONET CIMEL Electronique (CE-318) sun photometer used to study direct Sun and diffuse sky radiances.

We use two parameters to realistically characterize aerosol properties. The Angstrom exponent depends on particle size and the AOD depends mainly on aerosol column density. Our results are illustrated as contour maps in Figure 4 that help determine and discriminate between different aerosol types.9 The different sections in these maps denote clean maritime, biomass burning/urban-industrial, desert dust, and mixed type aerosols. The boundaries between the sections correspond to the selected threshold values of the AOD and the Angstrom exponent, the precise values of which are very important. Areas of greater density represent different aerosol types. The 2005–2006 data indicates that the primary source of aerosols in our study region was the biomass-burning/urban industry type.

Figure 4. Contour density plot of the Angstrom Exponent versus the aerosol optical depth (AOD) at 500nm for the Skukuza region.

The global coverage of ground-based AERONET stations means that our results can be used to compare and validate aerosol types that are derived from different models and satellite studies. Our findings can also be used to evaluate the characteristics of aerosol types on regional and global scales to identify where satellite observations need to be improved. Our future research will involve comparing the aerosol optical properties at Skukuza to other AERONET sites in SA to help understand radiative forcing in the region.

The authors thank the principal investigators and other staff members of the Skukuza AERONET site for maintaining the instrument and collecting the data for this analysis.

Raghavendra Kumar Kanike, Venkataraman Sivakumar
University of KwaZulu-Natal (UKZN)
Durban, South Africa

Raghavendra Kumar Kanike conducted his PhD research with Rajuru Ramakrishna Reddy at Sri Krishnadevaraya University, India, and received his degree in 2010. He is currently working as a postdoctoral research fellow with Venkataraman Sivakumar at the UKZN where he is actively engaged in optical remote sensing of atmospheric aerosols using AERONET sun photometers and validation techniques using satellite data.

Rajuru Ramakrishna Reddy, Kotalo Rama Gopal
Sri Krishnadevaraya University
Anantapur, India

1. H. Seinfeld, S. N. Pandis, Atmospheric Chemistry and Physics, from Air Pollution to Climate Change, John Wiley, New York, 1998.
2. Intergovernmental Panel on Climate Change (IPCC), Climate change. The physical science basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007. Chapter 2, pp. 128
3. A. J. Queface, S. J. Piketh, T. F. Eck, S. C. Tsay, A. F. Mavume, Climatology of aerosol optical properties in Southern Africa, Atmos. Environ. 45, p. 2910-2921, 2011.
4. M. Tesfaye, V. Sivakumar, J. Botai, G. M. Tsidu, Aerosol climatology over South Africa based on 10 years of Multiangle Imaging Spectroradiometer (MISR) data, J. Geophys. Res. 116, p. D20216, 2011. doi:10.1029/2011JD016023
5. S. J. Piketh, H. J. Annegarn, P. D. Tyson, Lower tropospheric aerosol loadings over South Africa: The relative contribution of aeolian dust, industrial emissions, and biomass burning, J. Geophy. Res. 104, p. 1597-1607, 1999.
6. B. N. Holben, T. F. Eck, I. Slutsker, D. Tanré, J. P. Buis, A. Setzer, E. Vermote, AERONET—a federated instrument network and data archive for aerosol characterization, Rem. Sens. Environ. 66, p. 1-16, 1998.
7. B. N. Holben, D. Tanré, A. Smirnov, T. F. Eck, I. Slutsker, N. Abuhassan, W. W. Newcomb, An emerging ground-based aerosol climatology: Aerosol optical depth from AERONET, J. Geophys. Res. 106, p. 12067-12097, 2001.
8. http://aeronet.gsfc.nasa.gov/  AERONET website.
9. D. G. Kaskaoutis, H. D. Kambezidis, N. Hatzianastassiou, P. G. Kosmopoulos, K. V. S. Badarinath, Aerosol climatology: On the discrimination of aerosol types over four AERONET sites, Atmospheric Chem. Phys. Discussions 7, p. 6357-6411, 2007.