Chemical composition and source apportionment of atmospheric aerosols on the Namibian coast

Abstract. The chemical composition of aerosols is of particular importance to assess their interactions with radiation, clouds and trace gases in the atmosphere, and consequently their effects on air quality and the regional climate. In this study, we present the results of the first long-term dataset of the aerosol chemical composition at an observatory on the coast of Namibia, facing the southeast Atlantic Ocean. Aerosol samples in the mass fraction of particles smaller than 10 µm in aerodynamic diameter (PM10) were collected during 26 weeks between 2016 and 2017 at the ground-based Henties Bay Aerosol Observatory (HBAO; 22°6’ S, 14°30’ E, 30 m above mean sea level). The resulting 385 filter samples were analysed by X-ray fluorescence and ion-chromatography for 24 inorganic elements and 15 water-soluble ions. Statistical analysis by positive matrix factorization and back-trajectory modelling identified five major sources, sea salt (mass concentration: 70.8 ± 0.2 %), marine biogenic (13.5 ± 0.8 %), mineral dust (9.9 ± 0.1 %), secondary products (3.2 ± 1.0 %) and heavy metals (2.3 ± 2.5 %). While the contribution of sea salt aerosol was persistent, as the dominant wind direction was south-westerly and westerly from the open ocean, the occurrence of mineral dust was episodic and coincided with high wind speeds from the south-southeast and the north-northwest, along the coastline. Concentrations of heavy metals measured at HBAO were higher than reported in the literature from measurements over the open ocean. The heavy metals (V, Cr, Nd and Mn) measured at the site were attributed to mining activities and the combustion of heavy fuels in commercial ship traffic across the Cape of Good Hope sea route. Fluoride concentrations up to 25 µg m−3 were measured, as in heavily polluted areas in China. This is surprising and a worrisome result that has profound health implications and deserves further investigation. Although no clear signature for biomass burning could be determined, the source of secondary products identified by PMF was described by a mixture of aerosols typically emitted by biomass burning, but also by other biogenic activities. Episodic contributions with moderate correlations between NO3−, nss-SO42− (higher than 2 µg m−3) and nss-K+, were observed, further indicative of the potential for an episodic source of biomass burning. Sea salt accounted for up to 57 % of the measured mass concentrations of SO42− and the non-sea salt fraction contributed mainly to the secondary product and marine biogenic sources identified by PMF. The marine biogenic contribution is attributed to efficient oxidation in the moist marine atmosphere of sulphur-containing gas-phase emitted by marine phytoplankton in the fertile waters offshore in the Benguela Upwelling System. The data presented in this paper provide first-ever information on the temporal variability of aerosol concentrations in the Namibian marine boundary layer and the links to meteorological conditions shaping the transport patterns of aerosols from different sources. This data can be used to provide context for intensive observations in the area.