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

Abstract

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.

Butene Emissions From Coastal Ecosystems May Contribute to New Particle Formation

“…In this campaign, entrainment of particles from the free troposphere can be excluded as long‐range transport of biomass burning aerosol was observed in the free troposphere during the campaign (Chazette et al., 2019; Formenti et al., 2019), but no evidence of biomass burning reaching the ground‐based site was observed from VOCs (such as acetonitrile) and aerosol composition measurements. This is in agreement with previous findings in Henties Bay (Formenti et al., 2018; Klopper et al., 2020) and with other studies (Haywood et al., 2021; Redemann et al., 2021; Zuidema et al., 2018), which showed that during the austral spring the entrainment of biomass burning emissions in the marine boundary layer occurs well offshore of the subcontinent over the central southern Atlantic Ocean. The NPF events occurring at the stratocumulus cloud deck could rather contribute to the increase in particle number concentrations observed in the Aitken mode (Figure 3).…”

Vertical structure of biomass burning aerosol transported over the southeast Atlantic Ocean

“…in Box E and in the lower layers of the aerosol plume in Box D, where the plume most likely has components of continental aerosol typical of the nearby Namibian coast (Klopper et al, 2020). The most striking feature of the results are the near constant values of these parameters in the upper two kilometers of the plume over the course of several days as evident from the range of values in the 25-75 percentile shaded grey in Figs.…”

Sensitivity analysis of an aerosol-aware microphysics scheme in Weather Research and Forecasting (WRF) during case studies of fog in Namibia

“…The area is interesting for studying fog formation as the contribution of anthropogenic sources to the background aerosol concentration is limited, meaning that pollution is minimal. Namibia has a population density of 3 people km −2 (Statista, 2020), and previous research has shown that the influence of anthropogenic activities is minor (Formenti et al, 2019;Klopper et al, 2020).…”