Chemical composition and source apportionment of atmospheric aerosols on the Namibian coast
Authors: Klopper Danitza , Formenti Paola , Namwoonde Andreas , Cazaunau Mathieu , Chevaillier Servanne , Feron Anaïs , Gaimoz Cécile , Hease Patrick , Lahmidi Fadi , Mirande-Bret Cécile , Triquet Sylvain , Zeng Zirui , Piketh Stuart J. 
Published: December 2020

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 south-eastern 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 factorisation (PMF) identified five major components, sea salt (mass concentration: 74.7±1.9 %), mineral dust (15.7±1.4 %,), ammonium neutralised (6.1±0.7 %), fugitive dust (2.6±0.2 %) and industry (0.9±0.7 %). 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-south-east and the north-north-west, along the coastline. Concentrations of heavy metals measured at HBAO were higher than
reported in the literature from measurements over the open ocean. V, Cd, Pb
and Nd were attributed to fugitive dust emitted from bare surfaces or mining
activities. As, Zn, Cu, Ni and Sr were attributed to the combustion of
heavy oils in commercial ship traffic across the Cape of Good Hope sea route, power generation, smelting and other industrial activities in the
greater region. 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 PMF ammonium-neutralised component 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 was contributed mainly by the ammonium-neutralised component and small contributions from the mineral dust component. The marine biogenic
contribution to the ammonium-neutralised component is attributed to efficient oxidation in the moist marine atmosphere of sulfur-containing gas phase emitted by marine phytoplankton in the fertile waters offshore in the Benguela Upwelling
System. The data presented in this paper provide the first ever information on the temporal variability of aerosol concentrations in the Namibian marine
boundary layer. This data also provide context for intensive observations in
the area.