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South African EUCAARI measurements: Seasonal variation of trace gases and aerosol optical properties

by L. Laakso, V. Vakkari, A. Virkkula, H. Laakso, J. Backman, M. Kulmala, J. P. Beukes, P. G. Van Zyl, P. Tiitta, M. Josipovic, J. J. Pienaar, K. Chiloane, S. Gilardoni, E. Vignati, A. Wiedensohler, T. Tuch, W. Birmili, S. Piketh, K. Collett, G. D. Fourie, M. Komppula, H. Lihavainen, G. De Leeuw, V. M. Kerminen show all authors
Atmospheric Chemistry and Physics ()
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In this paper we introduce new in situ observations of atmospheric\naerosols, especially chemical composition, physical and optical\nproperties, on the eastern brink of the heavily polluted Highveld area\nin South Africa. During the observation period between 11 February 2009\nand 31 January 2011, the mean particle number concentration (size range\n10-840 nm) was 6310 cm(-3) and the estimated volume of sub-10 mu m\nparticles 9.3 mu m(3) m(-3). The aerosol absorption and scattering\ncoefficients at 637 nm were 8.3 Mm(-1) and 49.5 Mm(-1), respectively.\nThe mean single-scattering albedo at 637 nm was 0.84 and the Angstrom\nexponent of scattering was 1.5 over the wavelength range 450-635 nm. The\nmean O-3, SO2, NOx and H2S-concentrations were 37.1, 11.5, 15.1 and 3.2\nppb, respectively. The observed range of concentrations was large and\nattributed to the seasonal variation of sources and regional\nmeteorological effects, especially the anticyclonic re-circulation and\nstrong winter-time inversions. In a global context, the levels of gases\nand particulates were typical for continental sites with strong\nanthropogenic influence, but clearly lower than the most polluted areas\nof southeastern Asia. Of all pollutants observed at the site, ozone is\nthe most likely to have adverse environmental effects, as the\nconcentrations were high also during the growing season. The\nmeasurements presented here will help to close existing gaps in the\nground-based global atmosphere observation system, since very little\nlong-term data of this nature is available for southern Africa.

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