Deposition and mineralogy of atmospheric dust at Sierra Nevada and Granada (Andalusia, SE Spain): A single particle perspective

The transport of Saharan dust to the mountain range of Sierra Nevada occurs recurrently as well as episodically and represents an important source of nutrients for its ecosystem. At the same time, the mountain range is also vulnerable to the radiative effects, changing albedo and reduced snow covers attributed to the deposited particles from various natural and anthropogenic sources. A two-year campaign was conducted from June 2018 to August 2019 in the Sierra Nevada at 2500 m MSL (site: Albergue Universitario de Sierra Nevada) and simultaneously samples were also taken from Granada, a city lying in the foothills of Sierra Nevada (site: Faculty of Sciences, University of Granada, 670m MSL). The aim was to deduce the various sources acting over two distinct environments within a short-range distance (22 km). Deposition samples were collected on a carbon substrate using a flat plate sampler (passive method) with a sampling period of 40-72 hours. An average of 10 samples were collected per month from both sites and the current study focuses mainly on the summer months of June, July, and August when episodic dust events were observed. Along with the dust episodes, control days samples were also taken when no rain event or dust event occurred.

Single particle microscopy coupled with energy-dispersive X-ray analysis was utilized to get the chemical information on approx. 100,000 single particles over the size range of 0.5- 50 µm projected area diameter. The particles were classified according to a definite set of rules, and the main chemical classes as dust-like, sulphates, and salts-like and their mixtures were derived. Particles which didn’t fit into any of these classes were named ‘other’ classes. The focus of the results includes the relative abundance of these classes and their particle morphology (size and shape) over the two locations affected by different aerosol sources. Furthermore, the characteristics of iron-rich particles and iron content in the other dust particles were also studied given their importance in absorption properties and bioavailability for ecosystems. During the dust event days, the relative number abundance of chemical composition at Sierra Nevada showed >98 % of dust particles enriched in silicate type in the particle size range >1 µm while the <1 µm had a significant percentage in sulphates (>20%). Meanwhile at Granada, even though the dust events had an influence on the composition with higher dust content (>90%) for diameter >5µm modes, the lower size range presented higher fractions of anthropogenic particles consisting of sulphates and other particle types.