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

Laakso L, Vakkari V, Virkkula A, Laakso H, Backman J, Kulmala M, Beukes J, Van Zyl P, Tiitta P, Josipovic M, Pienaar J, Chiloane K, Gilardoni S, Vignati E, Wiedensohler A, Tuch T, Birmili W, Piketh S, Collett K, Fourie G, Komppula M, Lihavainen H, De Leeuw G, Kerminen V ...see all

Atmospheric Chemistry and Physics, vol. 12, issue 4 (2012) pp. 1847-1864

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Abstract

In this paper we introduce new in situ observations of atmospheric
aerosols, especially chemical composition, physical and optical
properties, on the eastern brink of the heavily polluted Highveld area
in South Africa. During the observation period between 11 February 2009
and 31 January 2011, the mean particle number concentration (size range
10-840 nm) was 6310 cm(-3) and the estimated volume of sub-10 mu m
particles 9.3 mu m(3) m(-3). The aerosol absorption and scattering
coefficients at 637 nm were 8.3 Mm(-1) and 49.5 Mm(-1), respectively.
The mean single-scattering albedo at 637 nm was 0.84 and the Angstrom
exponent of scattering was 1.5 over the wavelength range 450-635 nm. The
mean O-3, SO2, NOx and H2S-concentrations were 37.1, 11.5, 15.1 and 3.2
ppb, respectively. The observed range of concentrations was large and
attributed to the seasonal variation of sources and regional
meteorological effects, especially the anticyclonic re-circulation and
strong winter-time inversions. In a global context, the levels of gases
and particulates were typical for continental sites with strong
anthropogenic influence, but clearly lower than the most polluted areas
of southeastern Asia. Of all pollutants observed at the site, ozone is
the most likely to have adverse environmental effects, as the
concentrations were high also during the growing season. The
measurements presented here will help to close existing gaps in the
ground-based global atmosphere observation system, since very little
long-term data of this nature is available for southern Africa.

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