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1.
Insights into the single-particle composition, size, mixing state, and aspect ratio of freshly emitted mineral dust from field measurements in the Moroccan Sahara using electron microscopy
Agnesh Panta, Konrad Kandler, Andrés Alastuey, Cristina González-Flórez, Adolfo Gonzalez-Romero, Martina Klose, Xavier Querol, Cristina Reche, Jesús Yus-Díez, Carlos Pérez García-Pando, 2023, original scientific article

Abstract: Abstract. The chemical and morphological properties of mineral dust aerosols emitted by wind erosion from arid and semi-arid regions influence climate, ocean, and land ecosystems; air quality; and multiple socio-economic sectors. However, there is an incomplete understanding of the emitted dust particle size distribution (PSD) in terms of its constituent minerals that typically result from the fragmentation of soil aggregates during wind erosion. The emitted dust PSD affects the duration of particle transport and thus each mineral's global distribution, along with its specific effect upon climate. This lack of understanding is largely due to the scarcity of relevant in situ measurements in dust sources. To advance our understanding of the physicochemical properties of the emitted dust PSD, we present insights into the elemental composition and morphology of individual dust particles collected during the FRontiers in dust minerAloGical coMposition and its Effects upoN climaTe (FRAGMENT) field campaign in the Moroccan Sahara in September 2019. We analyzed more than 300 000 freshly emitted individual particles by performing offline analysis in the laboratory using scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectrometry (EDX). Eight major particle-type classes were identified with clay minerals making up the majority of the analyzed particles both by number and mass, followed by quartz, whereas carbonates and feldspar contributed to a lesser extent. We provide an exhaustive analysis of the PSD and potential mixing state of different particle types, focusing largely on iron-rich (Fe oxide-hydroxides) and feldspar particles, which are key to the effects of dust upon radiation and clouds, respectively. Nearly pure or externally mixed Fe oxide-hydroxides are present mostly in diameters smaller than 2 µm, with the highest fraction below 1 µm at about 3.75 % abundance by mass. Fe oxide-hydroxides tend to be increasingly internally mixed with other minerals, especially clays, as particle size increases; i.e., the volume fraction of Fe oxide-hydroxides in aggregates decreases with particle size. Pure (externally mixed) feldspar represented 3.2 % of all the particles by mass, of which we estimated about a 10th to be K-feldspar. The externally mixed total feldspar and K-feldspar abundances are relatively invariant with particle size, in contrast to the increasing abundance of feldspar-like (internally mixed) aggregates with particle size with mass fractions ranging from 5 % to 18 %. We also found that overall the median aspect ratio is rather constant across particle size and mineral groups, although we obtain slightly higher aspect ratios for internally mixed particles. The detailed information on the composition of freshly emitted individual dust particles and quantitative analysis of their mixing state presented here can be used to constrain climate models including mineral species in their representation of the dust cycle.
Keywords: mineral dust, Moroccan Sahara, electron microscopy
Published in RUNG: 13.05.2024; Views: 124; Downloads: 3
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Absorption enhancement of black carbon particles in a Mediterranean city and countryside : effect of particulate matter chemistry, ageing and trend analysis
Jesús Yus-Díez, Marta Via, Andrés Alastuey, Angeliki Karanasiou, Maria Cruz Minguillon, Noemí Perez, Xavier Querol, Cristina Reche, Matic Ivančič, Martin Rigler, 2022, original scientific article

Abstract: Abstract. Black carbon (BC) is recognized as the most important warming agent among atmospheric aerosol particles. The absorption efficiency of pure BC is rather well-known, nevertheless the mixing of BC with other aerosol particles can enhance the BC light absorption efficiency, thus directly affecting Earth's radiative balance. The effects on climate of the BC absorption enhancement due to the mixing with these aerosols are not yet well constrained because these effects depend on the availability of material for mixing with BC, thus creating regional variations. Here we present the mass absorption cross-section (MAC) and absorption enhancement of BC particles (Eabs), at different wavelengths (from 370 to 880 nm for online measurements and at 637 nm for offline measurements) measured at two sites in the western Mediterranean, namely Barcelona (BCN; urban background) and Montseny (MSY; regional background). The Eabs values ranged between 1.24 and 1.51 at the urban station, depending on the season and wavelength used as well as on the pure BC MAC used as a reference. The largest contribution to Eabs was due to the internal mixing of BC particles with other aerosol compounds, on average between a 91 % and a 100 % at 370 and 880 nm, respectively. Additionally, 14.5 % and 4.6 % of the total enhancement at the short ultraviolet (UV) wavelength (370 nm) was due to externally mixed brown carbon (BrC) particles during the cold and the warm period, respectively. On average, at the MSY station, a higher Eabs value was observed (1.83 at 637 nm) compared to BCN (1.37 at 637 nm), which was associated with the higher fraction of organic aerosols (OA) available for BC coating at the regional station, as denoted by the higher organic carbon to elemental carbon (OC:EC) ratio observed at MSY compared to BCN. At both BCN and MSY, Eabs showed an exponential increase with the amount of non-refractory (NR) material available for coating (RNR-PM). The Eabs at 637 nm at the MSY regional station reached values up to 3 during episodes with high RNR-PM, whereas in BCN, Eabs kept values lower than 2 due to the lower relative amount of coating materials measured at BCN compared to MSY. The main sources of OA influencing Eabs throughout the year were hydrocarbon OA (HOA) and cooking-related OA (COA), i.e. primary OA (POA) from traffic and cooking emissions, respectively, at both 370 and 880 nm. At the short UV wavelength (370 nm), a strong contribution to Eabs from biomass burning OA (BBOA) and less oxidized oxygenated OA (LO-OOA) sources was observed in the colder period. Moreover, we found an increase of Eabs with the ageing state of the particles, especially during the colder period. This increase of Eabs with particle ageing was associated with a larger relative amount of secondary OA (SOA) compared to POA. The availability of a long dataset at both stations from offline measurements enabled a decade-long trend analysis of Eabs at 637 nm, that showed statistically significant (s.s.) positive trends of Eabs during the warmer months at the MSY station. This s.s. positive trend in MSY mirrored the observed increase of the OC:EC ratio over time. Moreover, in BCN during the COVID-19 lockdown period in spring 2020 we observed a sharp increase of Eabs due to the observed sharp increase of the OC:EC ratio. Our results show similar values of Eabs to those found in the literature for similar background stations.
Keywords: black carbomn, coating, organic aerosol, light absorption
Published in RUNG: 10.05.2024; Views: 108; Downloads: 2
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Increase in secondary organic aerosol in an urban environment : Increase in secondary organic aerosol in an urban environment
Marta Via, Maria Cruz Minguillon, Cristina Reche, Xavier Querol, Andrés Alastuey, 2021, original scientific article

Abstract: The evolution of fine aerosol (PM1) species as well as the contribution of potential sources to the total organic aerosol (OA) at an urban background site in Barcelona, in the western Mediterranean basin (WMB) was investigated. For this purpose, a quadrupole aerosol chemical speciation monitor (Q-ACSM) was deployed to acquire real-time measurements for two 1-year periods: May 2014–May 2015 (period A) and September 2017–October 2018 (period B). Total PM1 concentrations showed a slight decrease (from 10.1 to 9.6 μgm�3 from A to B), although the relative contribution of inorganic and organic compounds varied significantly. Regarding inorganic compounds, SO42- , black carbon(BC) and NH4+ showed a significant decrease from period A to B (21 %, 18% and 9 %, respectively), whilst NO3- concentrations were higher in B (8 %). Source apportionment revealed OA contained 46% and 70% secondary OA (SOA) in periods A and B, respectively. Two secondary oxygenated OA sources (OOA) were differentiated by their oxidation status (i.e. ageing): less oxidized (LO-OOA) and more oxidized (MO-OOA). Disregarding winter periods, when LO-OOA production was not favoured, LO-OOA transformation into MO-OOA was found to be more effective in period B. The lowest LO-OOA-to-MO-OOA ratio, excluding winter, was in September–October 2018 (0.65), implying an accumulation of aged OA after the high temperature and solar radiation conditions in the summer season. In addition to temperature, SOA (sum of OOA factors) was enhanced by exposure to NOx-polluted ambient and other pollutants, especially to O3 and during afternoon hours. The anthropogenic primary OA sources identified, cooking-related OA (COA), hydrocarbon-like OA (HOA), and biomass burning OA (BBOA), decreased from period A to B in both absolute concentrations and relative contribution (as a whole, 44% and 30 %, respectively). However, their concentrations and proportion to OA grew rapidly during highly polluted episodes. The influence of certain atmospheric episodes on OA sources was also assessed. Both SOA factors were boosted with long- and medium-range circulations, especially those coming from inland Europe and the Mediterranean (triggering mainly MO-OOA) and summer breeze-driven regional circulation (mainly LO-OOA). In contrast, POA was enhanced either during air-renewal episodes or stagnation anticyclonic events.
Keywords: aerosol, organic aerosol, source apportionment, PM1, particulate matter
Published in RUNG: 10.05.2024; Views: 115; Downloads: 3
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Variability in sediment particle size, mineralogy, and Fe mode of occurrence across dust-source inland drainage basins : the case of the lower Drâa Valley, Morocco
Adolfo Gonzalez-Romero, Cristina González-Flórez, Agnesh Panta, Jesús Yus-Díez, Cristina Reche, Patricia Córdoba, Natalia Moreno, Andrés Alastuey, Konrad Kandler, Martina Klose, 2023, original scientific article

Abstract: The effects of desert dust upon climate and ecosystems depend strongly on its particle size and size-resolved mineralogical composition. However, there is very limited quantitative knowledge on the particle size and composition of the parent sediments along with their variability within dust-source regions, particularly in dust emission hotspots. The lower Drâa Valley, an inland drainage basin and dust hotspot region located in the Moroccan Sahara, was chosen for a comprehensive analysis of sediment particle size and mineralogy. Different sediment type samples (n= 42) were collected, including paleo-sediments, paved surfaces, crusts, and dunes, and analysed for particle-size distribution (minimally and fully dispersed samples) and mineralogy. Furthermore, Fe sequential wet extraction was carried out to characterise the modes of occurrence of Fe, including Fe in Fe (oxyhydr)oxides, mainly from goethite and hematite, which are key to dust radiative effects; the poorly crystalline pool of Fe (readily exchangeable ionic Fe and Fe in nano-Fe oxides), relevant to dust impacts upon ocean biogeochemistry; and structural Fe. Results yield a conceptual model where both particle size and mineralogy are segregated by transport and deposition of sediments during runoff of water across the basin and by the precipitation of salts, which causes a sedimentary fractionation. The proportion of coarser particles enriched in quartz is higher in the highlands, while that of finer particles rich in clay, carbonates, and Fe oxides is higher in the lowland dust emission hotspots. There, when water ponds and evaporates, secondary carbonates and salts precipitate, and the clays are enriched in readily exchangeable ionic Fe, due to sorption of dissolved Fe by illite. The results differ from currently available mineralogical atlases and highlight the need for observationally constrained global high-resolution mineralogical data for mineral-speciated dust modelling. The dataset obtained represents an important resource for future evaluation of surface mineralogy retrievals from spaceborne spectroscopy.
Keywords: mineral dust, aerosols, geology
Published in RUNG: 12.01.2024; Views: 673; Downloads: 3
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Insights into the size-resolved dust emission from field measurements in the Moroccan Sahara
Cristina González-Flórez, Martina Klose, Andrés Alastuey, Sylvain Dupont, Jerónimo Escribano, Vicken Etyemezian, Adolfo Gonzalez-Romero, Yue Huang, Konrad Kandler, Jesús Yus-Díez, 2023, original scientific article

Abstract: Atmospheric mineral dust consists of tiny mineral particles that are emitted by wind erosion from arid regions. Its particle size distribution (PSD) affects its impact on the Earth's system. Nowadays, there is an incomplete understanding of the emitted dust PSD and a lot of debate about its variability. Here, we try to address these issues based on the measurements performed during a wind erosion and dust emission field campaign in the Moroccan Sahara within the framework of FRAGMENT project.
Keywords: aerosol particles, mineral dust, emission processes, climate
Published in RUNG: 23.10.2023; Views: 950; Downloads: 5
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Source apportionment study on particulate air pollution in two high-altitude Bolivian cities: La Paz and El Alto
Valeria Mardoñez, Marco Pandolfi, Lucille Joanna S. Borlaza, Jean-Luc Jaffrezo, Andrés Alastuey, Jean-Luc Besombes, Isabel R. Moreno, Noemí Perez, Griša Močnik, Patrick Ginot, 2023, original scientific article

Abstract: La Paz and El Alto are two fast-growing, high-altitude Bolivian cities forming the second-largest metropolitan area in the country. Located between 3200 and 4050 m a.s.l. (above sea level), these cities are home to a burgeoning population of approximately 1.8 million residents. The air quality in this conurbation is heavily influenced by urbanization; however, there are no comprehensive studies evaluating the sources of air pollution and their health impacts. Despite their proximity, the substantial variation in altitude, topography, and socioeconomic activities between La Paz and El Alto result in distinct sources, dynamics, and transport of particulate matter (PM). In this investigation, PM10 samples were collected at two urban background stations located in La Paz and El Alto between April 2016 and June 2017. The samples were later analyzed for a wide range of chemical species including numerous source tracers (OC, EC, water-soluble ions, sugar anhydrides, sugar alcohols, trace metals, and molecular organic species). The United States Environmental Protection Agency (U.S. EPA) Positive Matrix Factorization (PMF v.5.0) receptor model was employed for the source apportionment of PM10. This is one of the first source apportionment studies in South America that incorporates an extensive suite of organic markers, including levoglucosan, polycyclic aromatic hydrocarbons (PAHs), hopanes, and alkanes, alongside inorganic species. The multisite PMF resolved 11 main sources of PM. The largest annual contribution to PM10 came from the following two major sources: the ensemble of the four vehicular emissions sources (exhaust and non-exhaust), accountable for 35 % and 25 % of the measured PM in La Paz and El Alto, respectively; and dust, which contributed 20 % and 32 % to the total PM mass. Secondary aerosols accounted for 22 % (24 %) in La Paz (El Alto). Agricultural smoke resulting from biomass burning in the Bolivian lowlands and neighboring countries contributed to 9 % (8 %) of the total PM10 mass annually, increasing to 17 % (13 %) between August–October. Primary biogenic emissions were responsible for 13 % (7 %) of the measured PM10 mass. Additionally, a profile associated with open waste burning occurring from May to August was identified. Although this source contributed only to 2 % (5 %) of the total PM10 mass, it constitutes the second largest source of PAHs, which are compounds potentially hazardous to human health. Our analysis additionally resolved two different traffic-related factors, a lubricant source (not frequently identified), and a non-exhaust emissions source. Overall, this study demonstrates that PM10 concentrations in La Paz and El Alto region are predominantly influenced by a limited number of local sources. In conclusion, to improve air quality in both cities, efforts should primarily focus on addressing dust, traffic emissions, open waste burning, and biomass burning.
Keywords: source apportionment, particular air pollution, high altitude, positive matrix factorization, PMF
Published in RUNG: 15.09.2023; Views: 838; Downloads: 5
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