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

Lauri Laakso ,E Vignati,G De Leeuw,V.-M Kerminen,H Laakso,W Birmili,S Piketh,P G Van Zyl,K Chiloane,J P Beukes,Thomas Tuch ,Jacobus J Pienaar ,Heikki Lihavainen ,Markku Kulmala ,John Backman ,Gerhard Fourie ,Aki Virkkula ,Kirsten S Collett ,Stefania Gilardoni ,Ville Vakkari ,Alfred Wiedensohler ,Mika Komppula ,Miroslav Josipovic ,Petri Tiitta

doi:10.5194/acp-12-1847-2012

Abstract

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 cm3 and the estimated volume of sub-10 μm particles 9.3 μm3 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 Ångström exponent of scattering was 1.5 over the wavelength range 450–635 nm. The mean O3, 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 south-eastern 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.

Highlights

The direct and indirect radiative effects of aerosol particles constitute the largest uncertainty in current radiative forcing estimates of the Earth’s climatic system (Foster et al, 2007; Hansen et al, 2007)
Over the continental Southern Hemisphere, excluding equatorial Amazonia (Guyon et al, 2003), long-term studies of atmospheric aerosols are limited compared to the vast area it covers (Jayaratne and Verma, 2001; Swap et al, 2003; Ross et al, 2003; Laakso et al, 2006; Suni et al, 2008; Rissler et al, 2006; Vakkari et al, 2011; Queface et al, 2011; Hirsikko et al, 2012; Venter al., 2012)
Laakso et al.: South African EUCAARI measurements especially optical properties of submicron aerosol particles have mainly been investigated during the SAFARI 1992 and SAFARI 2000 measurement campaigns (Swap et al, 2003; Ross et al, 2003; Eck et al, 2003)

Summary

Introduction

The direct and indirect radiative effects of aerosol particles constitute the largest uncertainty in current radiative forcing estimates of the Earth’s climatic system (Foster et al, 2007; Hansen et al, 2007). In order to reduce the uncertainties associated with atmospheric aerosols in climate systems, detailed information on the temporal and spatial variability of different aerosol properties is required. Such information can be obtained from a combination of model simulations, remote sensing and continuous in-situ aerosol measurements. On a long-term basis, the South African Weather Service and NOAA-ESRL have conducted observations of aerosol number concentrations and optical properties such as light scattering and absorption as part of the Global Atmosphere Watch (GAW) program at the station in Cape Point At the time of writing, no publications on the aerosol measurements at Cape Point were available

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