Saharan dust in Central Europe: Impact on particulate matter characteristics in an urban and a natural locality

doi:10.21203/rs.3.rs-4885262/v1

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Saharan dust in Central Europe: Impact on particulate matter characteristics in an urban and a natural locality

https://doi.org/10.21203/rs.3.rs-4885262/v1

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At the turn of March and April 2024, most of mainland Europe was afflicted by sand dust particles carried by wind flow from the Sahara Desert. As a result, Central Europe experienced an exceptionally high increase in air pollution. In this work, the impact of this Saharan dust event on PM₁₀ characteristics in an urban and a natural locality in the Czech Republic was investigated. PM₁₀ concentrations before and during the Saharan dust event were measured using the Beta-Attenuation method. During the Saharan dust event, they were about 6–8 times higher than under normal atmospheric conditions, exceeding WHO guidelines by up to 2 times. A potential effect of terrain and altitude on the local concentrations of Saharan dust was observed. Airborne dust collected before and during the Saharan dust event was then studied using scanning electron microscopy combined with energy-dispersive spectroscopy (single-particle analysis of sizes and elemental composition) and X-ray diffractometry (mineralogical composition). Surprisingly, the particle size distribution was not significantly affected by the Saharan dust advection, but its dependency on the sampling locality was revealed. The dominant mineral in the Saharan dust was calcite, which substantially altered the local PM₁₀ composition. The studied Saharan dust probably originated from a natural area, as the amount of anthropogenic pollutants detected was negligible. Notably, its carbon content was lower compared with the usual local PM₁₀. The elevated PM₁₀ concentrations appear to be the most relevant risk associated with this Saharan dust event in Central Europe. The transported dust originated from the northern/north-western Sahara – probably from the Atlas region – which was verified by a backward trajectory analysis of air masses.

Earth and environmental sciences/Solid earth sciences/Mineralogy

Physical sciences/Physics/Techniques and instrumentation/Characterization and analytical techniques

Physical sciences/Physics/Techniques and instrumentation/Microscopy/Scanning electron microscopy

Earth and environmental sciences/Climate sciences/Atmospheric science

Saharan dust

Particulate matter

Air pollution

Single particle analysis

Elemental composition

Backward trajectory

Saharan dust is a type of particulate matter (PM) that originates from the Sahara Desert in the north of Africa. Sahara is a major source of aeolian soil dust in the world ^{1, 2, 3} with estimated production rates ranging between 400 · 10⁶ and 700 · 10⁶ tons per year. ⁴

The dust consists of small solid particles which are lifted by high-speed winds from arid land and can be transported by atmospheric flows thousands of kilometres away.¹ It is not uncommon for the Saharan dust to be carried by air masses around the globe to other continents. It has reached, among others, several regions across the American continent ⁵, the Middle East ⁴, and East Asia ⁶. In recent years, Saharan dust events (SDEs) have become quite common in the Mediterranean ⁷ and Europe (Croatia ⁸, Western and Central Europe ^{9, 10}, Scandinavia ¹, etc.)

Saharan dust has numerous environmental impacts around the world. For instance, it acts as a substantive carrier of inorganic nutrients, fertilizing remote ecosystems. Especially iron – a nutrient that plays a major role in the productivity of continental as well as marine ecosystems – is mentioned in this context. ^{11, 12, 7} Also phosphorus, contained in mineral dust aerosols, stimulates the productivity of ocean and terrestrial ecosystems. ¹²

Another environmental effect of Saharan dust is the long-distance transport of biological material (bacteria, fungal spores, pollen grains, etc.) to remote areas. Some of the microorganisms attached to dust particles can survive and be viable in local ecosystems where the aerosols are deposited. Such highly resistant microbial communities can contain potentially harmful pathogens. ^{3, 13}

Desert dust also poses a risk to human health because its particles suspended in the air can be breathed into the respiratory system. Particles smaller than 10 µm in aerodynamic diameter (PM₁₀ fraction) can enter the lower respiratory tract and induce respiratory problems. ¹⁴ Fine particulate matter with aerodynamic diameter below 2.5 µm (PM_2.5 fraction) can get to the bronchi and cause chronic bronchitis or asthma ¹⁵, while particles < 0.1 µm in aerodynamic diameter can even penetrate from alveoli into the bloodstream ¹⁶ and be transported by blood to other parts of the body which they can harm. ¹⁷ Besides particle size and morphology, its chemical composition influences the health effects, too. Among others, toxic metals (As, Cd, Cr, Hg, Pb, etc.) are of particular interest. ^{18, 19}

The presence of Saharan dust in the atmosphere is linked not only to the local air quality but also to the global climate. The impacts of mineral dust aerosols on climate change are discussed thoroughly by Kok et al. in ¹².

In spring 2024, a strong Saharan dust event was observed across large areas of mainland Europe. The Czech Republic (Central Europe) was affected by the windblown Saharan dust between 30 March and 2 April 2024. During this period, the concentrations of PM₁₀ in the air were so high that smog situations were announced in the Czech Republic.

The present research aimed to study the impact of this SDE on the characteristics of PM₁₀ sampled at two localities – a medium-sized city and a natural place in the mountains – both situated on the territory of the Czech Republic. Particles trapped in air filters under normal atmospheric conditions and during the above-mentioned Saharan dust event were analysed.

Mass concentrations of PM₁₀ in the ambient air were measured using the Beta-Attenuation method. Single-particle analysis was carried out by scanning electron microscopy (SEM) combined with energy-dispersive spectroscopy (EDS) to evaluate the PM₁₀ size distributions, morphology, and elemental composition. These measurements were supplemented with X-ray diffraction (XRD) bulk analysis to reveal the prevailing mineral phases in the samples. Based on the results, the effect of Saharan dust on the properties of solid atmospheric aerosols in a typical Central European environment was assessed. The likely source region of the dust was identified from its mineralogical composition and verified by a backward trajectory analysis of air masses.

2.1. Particulate matter sampling

The studied PM₁₀ samples were collected in two measuring stations operated by the Czech Hydrometeorological Institute (CHMI): Hradec Králové and Polom. These sampling sites are shown on a map in Fig. 1.

Hradec Králové is a county city with 94,000 inhabitants, situated in a flat terrain of the Eastern Bohemian Region. Its altitude is around 230 m above sea level. The measuring station is located in a housing estate, about 25 meters from a 4-lane road with heavy traffic (50° 11' 43.3" N, 15° 50' 46.9" E). Hradec Králové does not have any industry which could emit a significant amount of PM.

The measuring station Polom is situated 38 km east-north-east from Hradec Králové in a natural locality in the Orlice Mountains (50° 21' 02.1" N, 16° 19' 22.5" E). It stands on a hillside (750 m above sea level) in a meadow close to the forest. The nearest populated area is a small village about 700 m downhill.

In both these stations, the sampling was carried out using a suspended particulate matter analyser MP101M, certified for the continuous monitoring of the PM₁₀ fraction. ²⁰ Each sample was obtained by sucking ambient air through a fibreglass filter tape for 24 hours. Two PM₁₀ samples were taken at each sampling site: one shortly before the onset of the SDE, the other during the highest SDE intensity. Table 1 summarizes the samples analysed. Meteorological conditions were recorded during the sampling, too.

Table 1

PM₁₀ samples taken at an urban locality (Hradec Králové) and a natural locality (Polom).
Sample	Locality	Sampling date	Before or during SDE
HK-A	Hradec Králové	27/03/2024	Before SDE
HK-B	Hradec Králové	30/03/2024	During SDE
Polom-A	Polom	28/03/2024	Before SDE
Polom-B	Polom	30/03/2024	During SDE

2.2. Mass concentration of PM₁₀

Mass concentrations of PM₁₀ in the ambient air were measured directly by the MP101M analysers using the Beta-Attenuation method. In this method, the filter with trapped particles is irradiated by beta radiation and its transmittance is evaluated. From the transmittance value, the mass of particles trapped in the filter is determined. Based on the known volume of air blown through the filter during the sampling period, the mass concentration of PM in the air is calculated. A more detailed description of the Beta-Attenuation method principle is given, e.g., in ²¹. Hourly PM₁₀ concentrations were determined in both sampling localities for the period from 27/03/2024 to 03/04/2024 and compared with the limits recommended by the World Health Organization (WHO).

A possible effect of the sampling locality on the PM₁₀ concentration was tested by the pairwise Kolmogorov-Smirnoff (K-S) test, which verifies the following null hypothesis (H₀):

H₀: F(x) = G(x)

where F and G are the distributions of values in two independent groups of x. Results with p < 0.05 were taken as statistically significant.

The test was applied for the period of the Saharan dust event (30/03–01/04) and for the days just before and after the SDE (27/03–29/03 and 02/04–03/04) to find out, whether the PM₁₀ concentrations at the sampling localities differ during the SDE, and whether they differ under normal atmospheric conditions.

2.3. Mineralogical analysis by X-ray diffraction

The mineralogic composition of the PM₁₀ samples was studied using a powder X-ray diffractometer MiniFlex 600 (Rigaku, Tokyo, Japan) equipped with a copper anode (CuK_⍺: λ = 1.5418 Å) and the HyPix-400 detector. The tube current and voltage were 15 mA and 40 kV, respectively. The diffraction patterns were measured in an angular range of 2θ = 10–51° (Bragg-Brentano geometry) with a 0.02° step. The data acquisition and analysis were carried out by the software SmartLab Studio II x64 v4.6.827.0. Particles trapped on the filters were directly analysed, and then the continuous signal background of the filter was subtracted. The sample height correction was made according to the position of the quartz peak at 2θ = 26.65° (planes (101) and (011), based on the RRUFF database).

To reveal and analyse the mineral phases present in the samples, the “Powder XRD v4.0.1.0” plugin was used. The XRD patterns were compared with reference standards from the RRUFF (https://rruff.info/) and COD (http://www.crystallography.net) databases. Semi-quantitative analysis of the dominant phases (i.e., quartz and calcite) was performed by the “RIR Quantification” plugin. The measurement principle implies that bulk composition was obtained.

2.4. Single-particle analysis of PM₁₀ by SEM-EDS

Bulk analytical techniques, such as the above-described X-ray diffraction, are limited in the sense that they provide only the average composition of particulate matter. Since atmospheric particles are heterogeneous, it is desirable to complement their bulk analysis with single-particle studies, highlighting the chemical and morphological characteristics of the individual particles. ^{22, 23} SEM combined with EDS is commonly used for this purpose.

In our case, PM₁₀ size distributions, morphology, and elemental composition were evaluated using a scanning electron microscope FlexSEM 1000 (Hitachi, Tokyo, Japan) equipped with an EDS detector (Oxford Instruments, detector type: SDD – Silicon Drift Detector, energy resolution: 137 eV at Mn Kα line, detection area: 30 mm²). Chemical elements from B to U can be measured by this detector.

Before the measurement, the particles were shaken out of the filters onto a conductive carbon tape and coated with a 7 nm thin golden layer using an EM ACE200 sputter coater (Leica Microsystems, Wetzlar, Germany) to avoid their charging in the SEM.

The microscopic imaging was carried out in a secondary electrons mode to capture the particle morphology and topographic details in high quality. EDS spectra of individual particles were acquired at the accelerating voltage of 15 kV and a working distance of 10 mm (recommended values); the spectrum acquisition time was 60 s. From the EDS spectra, the mass and atomic concentrations of chemical elements present in the individual particles were determined. More than 300 randomly selected particles from each sample were evaluated (307 particles from HK-A, 311 from HK-B, 303 from Polom-A, 308 from Polom-B).

Since the particles were attached to a carbon tape, a possible contribution of carbon signal from the tape to the EDS spectra had to be kept in mind when evaluating the EDS results. For this reason, absolute values of carbon concentrations were not considered when drawing conclusions.

Previous EDS measurements revealed that the fibreglass filters used to collect PM₁₀ contain a small portion of Ba (approx. 5 wt. %).²⁴ Based on this, Ba-containing particles were excluded from the evaluation as likely pieces of the filter.

The particle sizes were determined from the SEM images using ImageJ software. Feret’s diameter (F_d) of each particle was measured manually and coupled with the elemental composition data obtained by EDS. (By Feret’s diameter, we mean the longest straight distance between any two points along the boundary of the imaged particle ²⁵.)

2.5. Evaluation of the particles’ sizes and elemental composition

Elemental concentrations and Feret’s diameters of individual particles from each sample were statistically evaluated. Differences among the samples in the distributions of these parameters were tested by the K-S test, again. Results with p < 0.05 were taken as statistically significant. The content of elements most abundant in the particles (C, O, Si, Al, Fe, Ca, Mg, and K) was compared among the samples and related to mineralogical results from XRD. The data evaluation was carried out using MS Excel 2019 and MATLAB R2021a.

2.6. Identification of the Saharan dust provenance

Based on the elemental and mineralogical composition of the HK-B and Polom-B samples, the likely source area of the Saharan dust was proposed. For both samples, the weight ratios Si/Al, Ca/Al, and (Ca + Mg)/Fe were calculated and compared with the reference values provided in ²⁶ and ²⁷. The results of this comparison indicated the dust provenance. The content of calcite was used as an additional identifier.

To verify the proposed provenance of the Saharan dust and estimate its contribution to PM₁₀ concentrations at the sampling sites, the long-range transport of PM₁₀ was investigated by calculating backward trajectories of air masses. For this purpose, the HYbrid Single-Particle Lagrangian Integrated Trajectory HYSPLIT_4 model ²⁸ was used. The backward trajectories lasted 48 hours – starting on March 29 at 0:00 UTC and ending on March 30 (i.e., this computation is related to the day of sampling HK-B and Polom-B samples). The backward trajectories were recalculated every 2 hours with the Global Data Assimilation System (GDAS) meteorological data; the grid resolution was 1° × 1°. Only Hradec Králové was selected as the receptor site (target location) since the spatial resolution of the HYSPLIT model is smaller than the geographical distance between the sites Hradec Králové and Polom. The receptor height was set to 500 m above ground level. Four averaged air mass clusters representing air masses of different origins were computed from 25 backward trajectories.

3.1. Mass concentrations of PM₁₀

Mass concentration of PM is commonly used as the main criterion to assess air quality. It strongly depends on the meteorological conditions: the washout effect of rainfall typically decreases the PM concentration ²⁹, while the impacts of air temperature and humidity are variable, depending on the local conditions. ^{29, 30, 31} The meteorological conditions in the period of interest were fairly stable and similar at both sampling localities (Hradec Králové: humidity 28–94%, temperature 1.1–23.5°C, wind velocity 0.0–9.0 m s^− 1; Polom: humidity 39–98%, temperature 2–18°C, wind velocity 1.2–7.7 m s^− 1). During 30/03, i.e. the day of Saharan dust sampling, there was a southeast light wind of velocity around 3 m s^− 1 in Hradec Králové and 4 m s^− 1 in Polom, and there was no rainfall.

Figure 2 shows hourly PM₁₀ concentrations at both sampling sites before, during and after the Saharan dust event. The SDE substantially increased PM₁₀ concentrations compared with the adjacent days with normal atmospheric conditions: the total amount of dust captured on the HK-B filter (i.e., in Hradec Králové during the SDE) was 5.6 times higher than the total amount captured on the HK-A filter (in Hradec Králové before the SDE). The Polom-B filter captured even 8.4 times more dust than the Polom-A filter.

From the time course of the concentrations, it was estimated that the onset of Saharan dust (to reach the first maximum) took approx. 5 hours in Polom and 7 hours in Hradec Králové. During the onset, the concentrations were increasing linearly with time, a bit faster in Polom. Then the concentrations somewhat fluctuated, suggesting spatial inhomogeneity of the dust cloud density.

Before and after the SDE, the 24-hour mean concentrations were below the WHO air quality guideline value (45 µg m^− 3). ³² This recommended value was substantially exceeded during the SDE both in Hradec Králové (76.4 µg m^− 3 on 30/03, 99.4 µg m^− 3 on 31/03) and in Polom (85.2 µg m^− 3 on 30/03, 79.3 µg m^− 3 on 31/03).

For the days with normal atmospheric conditions (27/03–29/03 and 02/04–03/04), the K-S test did not confirm a statistically significant difference between the concentration distributions in Hradec Králové and Polom (p = 0.122). On the contrary, a statistically significant difference (p = 0.030) between the concentrations in Hradec Králové and Polom was found for the SDE period (30/03–01/04). In view of the geographical proximity of these two sites (38 km) and differences in their altitude and terrain profile, a certain effect of terrain/altitude on the amount of Saharan dust fallout can be assumed, in addition to the potential inhomogeneity of the dust cloud.

3.2. Mineralogical analysis by X-ray diffraction

A thorough X-ray diffraction analysis allowed us to identify the main mineral phases present in the airborne dust before and during the Saharan dust event (Fig. 3). The ambient dust background before the onset of the SDE (samples HK-A, Polom-A) was similar in Hradec Králové and Polom, containing mainly quartz, calcite, and kaolinite, with a smaller proportion of gypsum. The diffraction intensity of these samples is low compared with the intensities of the Saharan dust-containing samples (HK-B, Polom-B) due to the low PM₁₀ concentrations in the pre-SDE days and, therefore, a small amount of dust trapped in the HK-A and Polom-A filters.

The samples collected during the SDE (HK-B and Polom-B) were also mutually similar. Their prevailing mineral phase was calcite, followed by quartz, kaolinite, gypsum, and – unlike the pre-SDE samples – also muscovite and a little magnetite. A small amount of dolomite was found in the samples HK-B and Polom-B, too. (The presence of dolomite in the pre-SDE samples HK-A and Polom-A could not be confirmed due to the low signal intensity of these samples.) The calcite-to-quartz ratio clearly increased in the samples with Saharan dust compared with the pre-SDE samples (calcite-to-quartz ratios: HK-A: 0.4, HK-B: 0.7; Polom-A: 0.3, Polom-B: 1.1). These values are only very approximate, but still confirm the trend visible in the diffractograms in Fig. 3, i.e., they are consistent with the ratios of the heights of the most pronounced calcite and quartz peaks. This dependency of the calcite-to-quartz ratio on the day of sampling (before or during the SDE) indicates a relatively high content of calcite in the Saharan dust. Gypsum, which was found in all the samples, probably originates from local sources.

3.3. Single particle analysis of PM₁₀

3.3.1. Particle sizes

The particles collected in Hradec Králové were on average bigger compared with those from Polom. Figure 4 provides density plots (histograms) and mean values of particle Feret’s diameters for all samples. Using the K-S test, a statistically significant difference in F_d distributions was found for the samples HK-A vs. Polom-A (p < 0.001) as well as for the samples HK-B vs. Polom-B (p < 0.001). On the contrary, the null hypothesis stating F_d distributions equality could not be rejected for the sample pairs HK-A vs. HK-B (p = 0.101) and Polom-A vs. Polom-B (p = 0.340). These results imply that the particle size distribution in PM₁₀ fraction was mostly determined by the sampling locality, not by the presence of Saharan dust. A possible reason is the difference in altitude of the sampling localities: in a stable atmosphere, coarse particles are preferentially present at lower altitudes due to gravity (gravitational settling is more important for them), ^{33, 34} while fine particles tend to remain suspended at higher altitudes.

3.3.2. Elemental composition

Chemical elements most abundant in PM₁₀, as determined from EDS spectra, were C, O, Si, Al, Fe, Ca, Mg, and K. Figure 5 shows their average content per particle for all samples. All the samples contained predominantly carbon and oxygen. It should be mentioned here that carbon content may be somewhat overestimated in Fig. 5 due to a possible contribution of carbon signal from the tape on which the particles were attached during the EDS measurement.

The K-S test has shown that the concentration distributions of elements contained in the particles significantly differ between the HK-A and HK-B samples for all evaluated elements: C (p < 0.001), O (p = 0.004), Si (p = 0.001), Al (p < 0.001), Fe (p < 0.001), Ca (p < 0.001), Mg (p < 0.001), and K (p < 0.001). It means that – within statistical accuracy – the incoming Saharan dust changed the distributions of these elements in PM₁₀ in Hradec Králové. As can be seen in Fig. 5, the content of C and Fe per particle decreased, while the content of the other elements increased a bit during the SDE.

To evaluate the impact of the Saharan dust event on PM₁₀ chemistry in Polom, the K-S test was performed in the same way on the pair of samples Polom-A vs. Polom-B. The result is that the SDE changed the distributions of all the elements evaluated, except for Al (p = 0.337) and Fe (p = 0.961). The content of C per particle decreased during the SDE, while the content of O, Si, Ca, Mg, and K increased.

The iron content in the HK-B and Polom-B samples was about 2.2 wt. % per particle, which falls into the 1–5 wt. % range typical for Northern Africa. ²⁶ It suggests that this SDE may have delivered some Fe-based nutrients to Central Europe. Considering that the Saharan dust concentrations at the sampling localities were several times higher than usual PM₁₀ concentrations at these sites, a non-negligible contribution of Saharan dust to Fe-based nutrients is conceivable. On the other hand, almost no phosphorus detected in the particles implies that P-based nutrients were not delivered.

Five particles containing Na and Cl together were found in both HK-B and Polom-B samples (unlike the pre-SDE samples), showing the possibility of minor mixing of sea salt with the Saharan dust. ²⁷

The content of chemical elements in the samples is consistent with the results of XRD analyses. The substantially increased proportion of silicon, calcium, aluminium and potassium during the SDE is related to quartz, calcite and aluminosilicates (e.g., muscovite) transported with the Saharan dust. Magnesium, detected by the EDS method, can be contained in calcite ³⁵, or it may originate from dolomite (CaMg(CO₃)₂) or clay minerals. ²⁶

The decrease in carbon content in HK-B and Polom-B samples may be explained by the lower proportion of anthropogenic carbonaceous particles and biological particles in Saharan dust, which is dominated by mineral components. It also supports the idea that the Saharan dust did not contribute to the content of polycyclic aromatic hydrocarbons (PAHs) in the PM₁₀. It would be favourable because PAHs are known for their carcinogenicity and mutagenicity. ³⁶

The presence of toxic elements in the PM₁₀ samples was investigated, too. As, Cd, Co, Cr, Cu, Ni, and Pb were examined as carcinogenic elements. ^{37, 38} A few copper-containing particles were found: 6 in HK-A, 2 in HK-B, 1 in Polom-A, and 1 in Polom-B sample. One lead-containing particle was found in each of HK-B, Polom-A, and Polom-B. No arsenic, cadmium, cobalt, chromium or nickel was detected by the EDS method. These results indicate a low content of the studied toxic elements in the sampled PM₁₀ and no significant contribution of Saharan dust to heavy metal pollution.

It is worth noting that the mentioned toxic elements were found in Saharan dust in Poland by Szuszkiewicz et al.¹⁰ after an SDE in 2021 and attributed to human activities such as combustion of fossil fuels, biomass or waste, and road traffic. This suggests that the Saharan dust investigated in the present study probably originates from a natural area without significant sources of anthropogenic pollution.

3.3.3. Particle morphology

The morphology of most of the observed airborne particles is typical of crustal mineral dust. Particles are often rough and sharp-edged, with smaller particles attached to their surface (Fig. 6a). Needle-shaped formations were seen on a calcium-rich particle, possibly calcite (Fig. 6b). Such “fibrous” calcite is a form of secondary calcites, which are widespread in arid and desert environments ³⁹. A distinct fracture surface is visible on the quartz particle in Fig. 6c. The gypsum grain in Fig. 6d has more rounded morphology, likely due to the aeolian erosion of this soft material. ³⁹

3.4. Potential source regions of the Saharan dust

3.4.1. Source identification based on the dust composition

The mineral and elemental composition of particles collected during the SDE corresponds well to atmospheric mineral dust in general ⁴⁰ and to dust typical of North Africa. ^{27, 26} Several regions in North Africa are considered to be significant sources of windblown dust. The literature points out especially north-west Africa (Atlas region – Morocco, northern Algeria, Tunisia; central and southern Algeria, ^{27, 1} Mali, Mauritania ²⁶), Chad (in particular Bodélé depression), northern Sudan,²⁶ Libya, and Egypt ^27,41. The sub-Saharan region is also a significant source of atmospheric dust. ^{27, 26}

The composition of North African dust varies to some extent in different regions, thus it can indicate its provenance. According to Scheuvens et al. ²⁶, the most indicative mineral source tracers for northern African dust are calcite and – to a lesser extent – dolomite. They propose, among others, to use the weight ratios Si/Al, Ca/Al, (Ca + Mg)/Fe, and calcite content to distinguish the main source areas of dust in North Africa. These parameters were determined for our Saharan dust-containing samples (HK-B and Polom-B) and are provided in Table 2. The comparison of these parameters with the reference values provided in ²⁶ and ²⁷ suggests that the African dust studied in the present work originates from the northern/north-western part of the Sahara Desert.

Table 2

Parameters indicating the likely source area of the Saharan dust. The potential regions of origin are suggested according to Scheuvens et al. ²⁶ and Kandler and Scheuvens ²⁷.
Parameter	Sample		Potential provenance
Parameter	HK-B	Polom-B	Potential provenance
Si/Al	2.9	3.1	North Africa, Bodélé depression in Chad
Ca/Al	2.1	2.5	Atlas region, Egypt
(Ca + Mg)/Fe	2.7	2.2	Atlas region, central Algeria, Libya, and Egypt
Calcite [wt. %]	25	17	northern Algeria, Moroccan Atlas

The idea of the northern/north-western origin of the studied dust is supported by Scheuvens’s statement that mineral dusts with high carbonate contents (> 10 wt. %, calcite > dolomite) mainly originate from the northern part of North Africa (the increased carbonate content there stems from the dominating limestones and carbonate-rich soils). For both our Saharan dust-containing samples, the calcite content (determined from XRD analysis by the RIR method) was considerably above the “threshold” value of 10 wt. %, while the content of dolomite was around 1 wt. %. Considering this and looking at the potential source regions in Table 2, the most likely provenance of the studied dust seems to be the Atlas region on the north-western edge of the Sahara.

It should be noted here that due to its large particle size, quartz is quickly depleted from the aerosol during its advection. ²⁷ Similarly, calcite removal processes can occur, reducing its content in long-range transported aerosols. ²⁶ For these reasons, the source sediments may have contained even higher proportions of quartz and calcite. Therefore, the parameters given in Table 2 may be underestimated. The values of these parameters could actually be a bit higher and thus more supportive of the proposed region of origin.

On the other hand, the relatively high content of calcite in the Saharan dust implies that this mineral was not removed (dissolved or transformed into different substances) on a large scale from the aerosol during its transport to Central Europe. It is not a matter of course because, e.g., Meinander et al. ¹ reported that in Saharan dust transported towards Finland in February 2021, carbonate minerals (calcite, dolomite) were completely missing. The authors surmise that carbonate particles were destroyed by atmospheric processes in that case.

3.4.2. Backward trajectory analysis of air masses

The four calculated air mass clusters were of different origins, as depicted in Fig. 7a. Clusters 1 and 2 were marine-originating air masses (cluster 1 was a fresh air mass), while clusters 3 and 4 were continental-originating air masses.

Figure 7b shows PM₁₀ concentrations calculated for the individual clusters by the HYSPLIT model. The highest concentrations were observed in the cluster 4, the second-highest concentrations in the cluster 3. Both of these air masses originated from the African continent, indicating that the Saharan dust transport substantially contributed to the increased PM₁₀ concentrations at the sampling sites. Looking at the map in Fig. 7a, the likely provenance is the Atlas region. Thus, the backward trajectory analysis indicates the same region of origin as identified from the dust composition.

The PM₁₀ median concentrations in cluster 2 were low at both sampling sites, the interquartile span shows considerable variability of measured concentrations (Fig. 7b). This cluster was likely partly influenced by the long-range transport from Africa (Fig. 7a). Finally, the fresh marine air mass (cluster 1) did not significantly contribute to PM₁₀ concentrations.

The Saharan dust event at the turn of March and April 2024 considerably affected several characteristics of solid atmospheric aerosols in the Czech Republic (Central Europe). Changes in the PM₁₀ properties were evidenced both in a lowland urban locality and in a natural mountain locality. PM₁₀ mass concentrations in the air were about 6–8 times higher during the Saharan dust event than under normal atmospheric conditions, exceeding the WHO guideline value by up to 2 times. The terrain and altitude may have some effect on the Saharan dust concentration at a given locality. The particle size distribution was not significantly influenced by the arrival of Saharan dust, but its dependency on the sampling locality was revealed.

The mineralogical composition of Saharan dust – with calcite and quartz being the most abundant components – substantially altered the overall PM₁₀ composition at both sampling sites. While the prevailing mineral phase under normal (pre-SDE) atmospheric conditions was quartz, the main component of PM₁₀ during the SDE was calcite. The studied Saharan dust may have brought some iron-based nutrients (but not phosphorus-based nutrients) to the region of sampling. It probably originates from a natural area, as the amount of anthropogenic pollutants detected was negligible. The likely source region of the studied dust is the Atlas region on the north-western edge of the Sahara Desert, which was verified by the high PM₁₀ concentrations in the clusters 4 and 3 from the backward trajectory analysis.

Since neither the particle size distribution nor the content of toxic elements in the analysed PM₁₀ was substantially affected by the Saharan dust advection, the elevated levels of PM₁₀ concentrations appear to be the most relevant risk associated with this SDE in Central Europe.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Author Contribution

J.L. conceptualization, methodology, formal analysis, investigation, data curation, visualization, writing – original draft, writing – review & editing, project administration. D.J. conceptualization, methodology, formal analysis, investigation, validation, writing – review & editing. M.N. methodology, investigation, writing – review & editing. A.H.Š. methodology, investigation, writing – original draft. D.H. investigation, formal analysis. J.K. conceptualization, resources, validation, writing – review & editing, supervision. All authors approved the final manuscript.

Acknowledgement

The authors would like to thank the Faculty of Science, University of Hradec Králové for the support of this research.

Data Availability

The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

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No competing interests reported.

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You are reading this latest preprint version

Saharan dust in Central Europe: Impact on particulate matter characteristics in an urban and a natural locality

Status:

Version 1

Abstract

Figures

1. Introduction

2. Materials and Methods

2.1. Particulate matter sampling

2.2. Mass concentration of PM₁₀

2.3. Mineralogical analysis by X-ray diffraction

2.4. Single-particle analysis of PM₁₀ by SEM-EDS

2.5. Evaluation of the particles’ sizes and elemental composition

2.6. Identification of the Saharan dust provenance

3. Results and Discussion

3.1. Mass concentrations of PM₁₀

3.2. Mineralogical analysis by X-ray diffraction

3.3. Single particle analysis of PM₁₀

3.3.1. Particle sizes

3.3.2. Elemental composition

3.3.3. Particle morphology

3.4. Potential source regions of the Saharan dust

3.4.1. Source identification based on the dust composition

3.4.2. Backward trajectory analysis of air masses

4. Conclusions

Declarations

Declaration of competing interest

Author Contribution

Acknowledgement

Data Availability

References

Additional Declarations

Status:

Version 1

Saharan dust in Central Europe: Impact on particulate matter characteristics in an urban and a natural locality

Status:

Version 1

Abstract

Figures

1. Introduction

2. Materials and Methods

2.1. Particulate matter sampling

2.2. Mass concentration of PM10

2.3. Mineralogical analysis by X-ray diffraction

2.4. Single-particle analysis of PM10 by SEM-EDS

2.5. Evaluation of the particles’ sizes and elemental composition

2.6. Identification of the Saharan dust provenance

3. Results and Discussion

3.1. Mass concentrations of PM10

3.2. Mineralogical analysis by X-ray diffraction

3.3. Single particle analysis of PM10

3.3.1. Particle sizes

3.3.2. Elemental composition

3.3.3. Particle morphology

3.4. Potential source regions of the Saharan dust

3.4.1. Source identification based on the dust composition

3.4.2. Backward trajectory analysis of air masses

4. Conclusions

Declarations

Declaration of competing interest

Author Contribution

Acknowledgement

Data Availability

References

Additional Declarations

Status:

Version 1

2.2. Mass concentration of PM₁₀

2.4. Single-particle analysis of PM₁₀ by SEM-EDS

3.1. Mass concentrations of PM₁₀

3.3. Single particle analysis of PM₁₀