Henties Bay Research Articles

Fractional solubility of iron in mineral dust aerosols over coastal Namibia: a link to marine biogenic emissions?

Abstract. This paper presents the first investigation of the solubility of iron in mineral dust aerosols collected at the Henties Bay Aerosol Observatory (HBAO), in Namibia, from April to December 2017. During the study period, 10 intense dust events occurred. Elemental iron reached peak concentrations as high as 1.5 µg m−3, significantly higher than background levels. These events are attributed to wind erosion of natural soils from the surrounding gravel plains of the Namib desert. The composition of the sampled dust is found to be overall similar to that of aerosols from northern Africa but is characterized by persistent and high concentrations of fluorine which are attributed to local fugitive dust. The fractional solubility of Fe (%SFe) for both the identified dust episodes and background conditions ranged between 1.3 % and 20 % and averaged at 7.9 % (±4.1 %) and 6.8 (±3.3 %), respectively. Even under background conditions, the %SFe was correlated with that of Al and Si. The solubility was lower between June and August and increased from September onwards during the austral spring. The relation to measured concentrations of particulate MSA (methane sulfonic acid), solar irradiance, and wind speed suggests a possible two-way interaction whereby marine biogenic emissions from the coastal Benguela upwelling to the atmosphere would increase the solubility of iron-bearing dust according to the photo-reduction processes. This first investigation points to the western coast of southern Africa as a complex environment with multiple processes and active exchanges between the atmosphere and the Atlantic Ocean, requiring further research.

Variability of ambient particulate matter loading at Henties Bay, Namibia

The Namibian coast is one of the areas of international interest for aerosol studies. This is due to the region’s importance for the global radiation budget because of the presence of a semi-permanent stratocumulus cloud along the coast. Aerosol particles may scatter/absorb radiation and directly influence how long clouds last by modifying their properties. This is all dependent on the particles’ chemical and physical properties because of the sources they were emitted from. In this study, we identified and investigated episodes of high (HAE) and low (LAE) PM concentrations and the meteorology that may favour their occurrence. Here, we investigated PM2.5 (particles with an aerodynamic diameter of 2.5 µm or less) and PM10 (particles with an aerodynamic diameter of 10 µm or less) at Henties Bay, Namibia. Daily aerosol measurements were taken with E-samplers between 15 and 29 July 2019. The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model was used to investigate the long-range atmospheric transport of air masses that reached Henties Bay. The study found that during HAEs, the average PM2.5 concentration was 28.40 ± 18.10 µg/m3 and the average PM10 concentration was 68.20 ± 44.3 µg/m3. In contrast, during LAEs, the average PM2.5 concentration was 13.3 ± 9.52 µg/m3 and the average PM10 concentration was 30.00 ± 23.00 µg/m3. In both fractions, there was an observed dominant contribution from marine sources.

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

Abstract. Aerosol-aware microphysics parameterisation schemes are increasingly being introduced into numerical weather prediction models, allowing for regional and case-specific parameterisation of cloud condensation nuclei (CCN) and cloud droplet interactions. In this paper, the Thompson aerosol-aware microphysics scheme, within the Weather Research and Forecasting (WRF) model, is used for two fog cases during September 2017 over Namibia. Measurements of CCN and fog microphysics were undertaken during the AErosols, RadiatiOn and CLOuds in southern Africa (AEROCLO-sA) field campaign at Henties Bay on the coast of Namibia during September 2017. A key concept of the microphysics scheme is the conversion of water-friendly aerosols to cloud droplets (hereafter referred to as CCN activation), which could be estimated from the observations. A fog monitor 100 (FM-100) provided cloud droplet size distribution, number concentration (Nt), liquid water content (LWC), and mean volumetric diameter (MVD). These measurements are used to evaluate and parameterise WRF model simulations of Nt, LWC, and MVD. A sensitivity analysis was conducted through variations to the initial CCN concentration, CCN radius, and the minimum updraft speed, which are important factors that influence droplet activation in the microphysics scheme of the model. The first model scenario made use of the default settings with a constant initial CCN number concentration of 300 cm−3 and underestimated the cloud droplet number concentration, while the LWC was in good agreement with the observations. This resulted in droplet size being larger than the observations. Another scenario used modelled data as CCN initial conditions, which were an order of magnitude higher than other scenarios. However, these provided the most realistic values of Nt, LWC, MVD, and droplet size distribution. From this, it was concluded that CCN activation of around 10 % in the simulations is too low, while the observed appears to be higher reaching between 20 % and 80 %, with a mean (median) of 0.55 (0.56) during fog events. To achieve this level of activation in the model, the minimum updraft speed for CCN activation was increased from 0.01 to 0.1 m s−1. This scenario provided Nt, LWC, MVD, and droplet size distribution in the range of the observations, with the added benefit of a realistic initial CCN concentration. These results demonstrate the benefits of a dynamic aerosol-aware scheme when parameterised with observations.

Corynosoma australe and Bolbosoma vasculosum (Polymorphidae: Acanthocephala) from Arctocephalus pusillus pusillus (Otariidae) and Argyrosomus spp. (Sciaenidae) from the Namibian Coast of Africa

Five Cape fur seals, Arctocephalus pusillus pusillus, 1 dusky kob, Argyrosomus coronus, and 55 silver kob, A. inodorus, collected from Cape Cross, Henties Bay, and Skeleton Coast Park, Namibia, were examined for acanthocephalans. Adults of Bolbosoma vasculosum and Corynosoma australe were recovered from 5 Cape fur seals, and cystacanths of C. australe from the single dusky kob and 3 of the silver kobs that were examined. This is the first record of B. vasculosum and the second of C. australe from the Namibian coast. Scanning electron microscopy of specimens of C. australe revealed features such as bursal papillae and micropores both on the proboscis hook surfaces and across the epidermal surface of the trunk. X-ray analyses of gallium cut hook, egg, and spines of C. australe are presented with analyses of spines of Corynosoma strumosum for comparison.

Halogenated monoterpenes from Namibian Plocamium rigidum: Accurate quantification and chemotaxonomic relevance

Recently, a Namibian Plocamium specimen that shares the characteristics of both P. suhrii and P. rigidum was described in terms of the chemistry of its secondary metabolite constituents. Three variants or forms of this species with similarities and subtle differences in terms of morphology were found in the intertidal zone of Henties Bay, Namibia. Without DNA analysis, there is a degree of uncertainty regarding which Plocamium species these samples belong to. However, tentative morphological identification of the samples suggests P. rigidum. Hence, detailed chemical profiles of these variants were investigated in order to determine if their chemical compositions are in agreement with their morphological and taxonomic similarities and differences. Twelve halogenated monoterpenes previously identified in Namibian Plocamium species were mostly found to be present in the crude MeOH/DCM extracts of the three variants in this study. The major compound, 1E,3R,4S,5E,7Z-1-bromo-3,4,8-trichloro-7-(dichloromethyl)-3-methylocta-1,5,7-triene, was previously identified as the major metabolite of P. suhrii, the Namibian Plocamium species and also mostly likely P. rigidum. Gas chromatography with flame ionisation detection (GC-FID) was used for the accurate determination of the absolute and relative quantities of the twelve metabolites in the extracts. This data revealed that two of the variants were chemically similar, while the quantities of the metabolites present in the remaining variant showed large variations. In particular, significant variations in the concentration of the major metabolite was observed in the latter variant. However, the relative abundance of this metabolite in the former variants was almost 90%, which is consistent with what was previously reported and hence these two variants most probably belong to the same Namibian Plocamium rigidum species. Hence it was concluded that the remaining sample could be an intraspecies variant, or even another species of Plocamium.

Evidence of the complexity of aerosol transport in the lower troposphere on the Namibian coast during AEROCLO-sA

Abstract. The evolution of the vertical distribution and optical properties of aerosols in the free troposphere, above stratocumulus, is characterized for the first time over the Namibian coast, a region where uncertainties on aerosol–cloud coupling in climate simulations are significant. We show the high variability of atmospheric aerosol composition in the lower and middle troposphere during the Aerosols, Radiation and Clouds in southern Africa (AEROCLO-sA) field campaign (22 August–12 September 2017) around the Henties Bay supersite using a combination of ground-based, airborne and space-borne lidar measurements. Three distinct periods of 4 to 7 d are observed, associated with increasing aerosol loads (aerosol optical thickness at 550 nm ranging from ∼0.2 to ∼0.7), as well as increasing lofted aerosol layer depth and top altitude. Aerosols are observed up to 6 km above mean sea level during the later period. Aerosols transported within the free troposphere are mainly polluted dust (predominantly dust mixed with smoke from fires) for the first two periods (22 August–1 September 2017) and smoke for the last part (3–9 September) of the field campaign. As shown by Lagrangian back-trajectory analyses, the main contribution to the aerosol optical thickness over Henties Bay is shown to be due to biomass burning over Angola. Nevertheless, in early September, the highest aerosol layers (between 5 and 6 km above mean sea level) seem to come from South America (southern Brazil, Argentina and Uruguay) and reach Henties Bay after 3 to 6 d. Aerosols appear to be transported eastward by the midlatitude westerlies and towards southern Africa by the equatorward moving cut-off low originating from within the westerlies. All the observations show a very complex mixture of aerosols over the coastal regions of Namibia that must be taken into account when investigating aerosol radiative effects above stratocumulus clouds in the southeast Atlantic Ocean.

Three years of measurements of light-absorbing aerosols over coastal Namibia: seasonality, origin, and transport

Abstract. Continuous measurements between July 2012 and December 2015 at the Henties Bay Aerosol Observatory (HBAO; 22∘ S, 14∘05′ E), Namibia, show that, during the austral wintertime, transport of light-absorbing black carbon aerosols occurs at low level into the marine boundary layer. The average of daily concentrations of equivalent black carbon (eBC) over the whole sampling period is 53 (±55) ng m−3. Peak values above 200 ng m−3 and up to 800 ng m−3 occur seasonally from May to August, ahead of the dry season peak of biomass burning in southern Africa (August to October). Analysis of 3-day air mass back-trajectories show that air masses from the South Atlantic Ocean south of Henties Bay are generally cleaner than air having originated over the ocean north of Henties Bay, influenced by the outflow of the major biomass burning plume, and from the continent, where wildfires occur. Additional episodic peak concentrations, even for oceanic transport, indicate that pollution from distant sources in South Africa and maritime traffic along the Atlantic ship tracks could be important. While we expect the direct radiative effect to be negligible, the indirect effect on the microphysical properties of the stratocumulus clouds and the deposition to the ocean could be significant and deserve further investigation, specifically ahead of the dry season.

English

For many years, the generation of nitrates from organic sources in order to create nutrient solutions for hydroponics had proved a challenge until lately when microorganisms were introduced to perform this task. The objectives of the current study therefore were to use local microbial consortium to nitrify goat manure in water and to determine microbial diversity in the inoculated consortium. Therefore, microorganisms were sourced from garden soil and natural compost at the Sam Nujoma Marine resources Research Centre (SANUMARC) in Henties Bay Namibia to convert organic nitrogen in goat manure from Utuseb farm near Walvis Bay into nitrates. Results show that microbial consortium from the compost source produced significantly (P< 0.05) more nitrates followed by the garden-soil source, suggesting that it is necessary to add inoculum in order to generate nitrate from goat manure. The ammonia oxidising bacteria (AOB) community from the compost sample’s was dominated by uncultured ammonia-oxidising species followed by uncultured bacterium (both not identified), with the least being Nitrosomonas species. The AOB community from the garden source was dominated by uncultured bacterium, followed by uncultured ammonia-oxidising species and the least being Nitrosomonas species. NOB community from the compost source was dominated by uncultured bacterium, followed by Nitrobacter winogradskyi and Nitobacter vulgaris with the least being Nitrococcus mobilis and Nitrospira moscovensis. For the garden soil microbial source, uncultured nitrite-oxidising bacteria dominated followed by uncultured bacterium, whereas the least species were N. moscovensis and Nitrobacter alkalicus. Moreover, community composition of the compost sample was more diverse than the community from the garden sample. These results maintain that there are other unculturable yet important microbes doing the same job if not better than the known ones, in this case suggesting that there may be other local nitrite-oxidizing bacteria responsible for oxidizing ammonia other than the traditionally known Nitrobacter, Nitrospira and Nitrococcus species.   Key words: Organic hydroponics, ammonification, nitrification, ammonia-oxidizing bacteria, nitrite-oxidizing bacteria.

Assessment of natural radionuclide distribution in shore sediment samples collected from the North Dune beach, Henties Bay, Namibia

The activity concentrations of 238U, 232Th and 40K were determined using gamma spectrometry (HPGe detector) and their radiological hazards are presented in this study for shore sediment samples from the North Dune beach of Henties Bay, Namibia. The activity concentrations were found to range from 25.32 ± 1.24 Bq.kg−1 to 232.33 ± 0.62 Bq.kg−1 with an average value of 175.59 ± 0.92 Bq.kg −1 for 238U, BDL to 77.99 ± 45 Bq.kg−1 with an average value of 40.17 ± 27 Bq.kg−1 for 232Th and 222.39 ± 8 Bq.kg−1 to 482.16 ± 10 Bq.kg - 1 with an average value of 349.66 ± 8 Bq.kg−1 for 40K. In order to assess the radiological hazard of the shore sediment samples, the radiological hazard indices such as absorbed dose rates (ADR), radium equivalent (Req) annual effective dose equivalent (AEDE), the hazard indices (Hex and Hin), and the excess lifetime cancer risk (ELCR) were calculated. The values obtained in the present study were compared with world acceptable limits and it was found that the average values of radium equivalent (Req) and external hazard index (Hex) were below the world allowable limits. However, the average values of absorbed dose rates (ADR), annual effective dose equivalent (AEDE) (outdoor and indoor), internal hazard index (Hin) and excess lifetime cancer risk (ELCR) exceeded the world acceptable limits.

An AMS study of magma transport and emplacement mechanisms in mafic dykes from the Etendeka Province, Namibia

The Henties Bay Outjo dyke swarm (HOD) in NW Namibia is part of the early Cretaceous Paraná-Etendeka Large Igneous Province. The dykes are dominantly doleritic, compositionally equivalent to the erupted lava series and thus the HOD provides a look at the feeder systems of a flood basalt province. The subvertical dykes mostly strike NE-SW and minor NW-SE, parallel or perpendicular to the Damara Belt in which they intruded. We present a magnetic fabric study using the anisotropies of low-field magnetic susceptibility (AMS) and of anhysteretic remanent magnetization (AARM) with the aim to derive magma flow directions and better constrain emplacement mechanisms within the dyke swarm.Magnetic susceptibility and its anisotropy in the dykes is mainly controlled by distribution anisotropy of titanomagnetite that mimics the flow-oriented silicate fabric. The anisotropy is low in most samples, supporting a magmatic origin. In 66 of 110 investigated samples the AMS fabric is “normal”, with the κmax axis (inferred flow orientation) within the dyke plane. Most samples yielded vertical to subvertical flow orientations regardless of location near or distant from the former rifted margin. The “anomalous” magnetic fabrics, where κmax is inclined to the dyke plane, are attributed to two mechanisms. One is the single-domain effect of titanomagnetite, which was found by unequal orientations of AMS and AARM fabrics.The other case anomalous fabric is rotation of the AMS axes by shear within the magma, producing symmetric imbrication of AMS fabric on opposite dyke walls; or more commonly, asymmetric magnetic fabrics, which we relate to tectonic shear at the dyke walls during emplacement. Field support for syn-emplacement shear is given by dyke segmentation geometries including locally curved segment tips, en-echelon arrangements and left/right-stepping displacements. Regionally, syn-emplacement shear is consistent with the observed reactivation during Gondwana breakup of transcrustal shear zones in the Damara mobile belt.

Origin of CFB Magmatism: Multi-tiered Intracrustal Picrite-Rhyolite Magmatic Plumbing at Spitzkoppe, Western Namibia, during Early Cretaceous Etendeka Magmatism

Early Cretaceous tholeiitic picrite-to-rhyolite dykes around Spitzkoppe, western Namibia, are part of the extensive Henties Bay–Outjo swarm, penecontemporaneous with ∼132 Ma Etendeka lavas ∼100 km to the NW. Although only intermediate to rhyolitic dykes contain clinopyroxene phenocrysts, the behaviour of Ca, Al and Sc in the dyke suite shows that liquidus clinopyroxene—together with olivine—was a fractionating phase when MgO fell to ∼9 wt %. Both a plot of CIPW normative di–hy–ol–ne–Q and modelling using (p)MELTS show that a mid-crustal pressure of ∼ 0·6 GPa is consistent with this early clinopyroxene saturation. Sr, Nd, Hf and Pb isotope variations all show trends consistent with AFC contamination (assimilation linked to fractional crystallization), involving Pan-African Damara belt continental crust. The geochemical variation, including isenthalpic AFC modelling using (p)MELTS, suggests that the picrites (olivine-rich cumulate suspensions) were interacting with granulite-facies metamorphic lower crust, the intermediate compositions with amphibolite-facies middle crust, and the rhyolitic dykes (and a few of the basalts) with the Pan-African granites of the upper crust. The calculated densities of the magmas fall systematically from picrite to rhyolite and suggest a magmatic system resembling a stack of sills throughout the crust beneath Spitzkoppe, with the storage and fractionation depth of each magma fraction controlled by its density. Elemental and isotopic features of the ∼20 wt % MgO picrites (including Os isotopes) suggest that their parental melts probably originated by fusion of mid-ocean ridge basalt (MORB) source convecting mantle, followed by limited reaction with sub-continental lithospheric mantle metasomatized just prior to the formation of the parental magmas. Many of the distinctive features of large-volume picritic–basaltic magmas may not be derived from their ultimate mantle sources, but may instead be the results of complex polybaric fractional crystallization and multi-component crustal contamination.