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 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.

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).…”

Aerosol and cloud properties in the Namibian region during AEROCLO-sA field campaign: 3MI airborne simulator and sun-photometer measurements

“…The mean AEROCLO-sA instantaneous DRE value is +85 W m -2 for AAC. Our observations are in agreement with previous studies indicating a strong positive aerosol forcing over the region (De Graaf et al, 2019b, 2020 with possible feedbacks on cloud development due to both aerosol and water vapour combined 440 radiative effects (Deaconu et al, 2019). However, the aerosol DRE on the AEROCLO-sA region is higher than the mean DRE observed in the southern Atlantic region, mainly because of the exceptional atmospheric conditions sampled during the flights (high loads of absorbing aerosols and high cloud albedo).…”

“…The real part of the 320 refractive index at 675 nm ranges from 1.41 at Henties Bay to 1.54 at Bonanza, and the imaginary part at 675 nm ranges from 0.008 at Henties Bay to 0.032 at Bonanza. The known environmental characteristics of Henties Bay, a coastal site with high content of sea salt and sulphate aerosols, frequent fog and a persistent as well as elevated relative humidity (Formenti et al, 2019;Klopper et al, 2020), support the low values of the real and imaginary refractive indices. Figure 4 also shows different behaviours on the spectral variation of the imaginary part of the 325 refractive index k in the South-Eastern region.…”