Measurement report: The Urmia playa as a source of airborne dust and ice-nucleating particles - Part 1: Correlation between soils and airborne samples

The emergence of desiccated lake bed sediments and their exposure to winderosion as a consequence of climate change and drought in arid and semiaridregions of the world poses a growing hazard. Airborne dust originating fromsuch soils can create health and environmental issues due to their high saltcontent and the presence of toxic elements. The aim of the present study istwofold, namely to investigate the newly emerged playa surfaces ofwestern Lake Urmia (LU) in Iran and their contribution to aerosol in theregion by means of physicochemical, mineralogical, and elemental analysesand to study the ice nucleation (IN) activity of both surface-collected soiland airborne dust samples. The playa surfaces created by desiccation of LUon the western shores were mapped and sampled at 130 locations. Soil sampleswere subjected to physicochemical analyses, and their erodible fraction wasdetermined. Based on these analyses, four highly erodible playa surfacesfrom the northwest to the south of LU were selected as sites for collection ofdust by impaction and soil samples from the uppermost surface. Theirparticle physicochemical properties (size distribution, elemental andmineralogical composition) were compared with their IN activity determinedby emulsion freezing experiments in a differential scanning calorimeter(DSC) in two suspension concentrations of 2 wt % and 5 wt %. Thephysicochemical soil properties differed significantly between the differentplaya surfaces, which affects their susceptibility to wind erosion. Sandsheets and sandy salt crusts were the most erodible playa surfaces due totheir high sand fraction and low organic matter and clay content, favouring thepresence of small aggregates. Mineralogical analyses document the prevalenceof quartz, carbonates, and clay minerals, such as kaolinite, palygorskite, andchlorite in all of the samples. The predominant elements in the samples areCa, Fe, Al, Si, and Na (and in some cases Ba, Sr, and Zn). The correlationbetween soil and dust samples based on mineralogical composition, elementalenrichment factors, and physicochemical properties confirm that the playasurfaces are the major contributors to dust in the region. IN activity withonset temperatures ranging from 245 to 250 K demonstrates the high potentialof dust blown from Urmia playa surfaces to affect cloud properties andprecipitation. Freezing onset temperatures and the fraction ofheterogeneously frozen droplets in the emulsions reveal variations in INactivity depending on the mineralogical composition of the samples but which are alsoinfluenced by organic matter, salinity, and pH. Specifically, IN activitycorrelates positively with organic matter and clay minerals and negativelywith pH, salinity, and (surprisingly) K-feldspar andquartz content. The high wind erodibility and dust production of the LUplaya surfaces together with their high IN activity can play an importantrole in the climate of the region and thus needs careful monitoring andspecific attention.

Automated summary

The emergence of desiccated lake bed sediments and their exposure to wind erosion is a growing hazard in arid and semiarid regions. This study investigates the newly emerged playa surfaces of Lake Urmia in Iran and their contribution to aerosol in the region, as well as their ice nucleation activity. The physicochemical properties of the soil samples from different playa surfaces varied significantly, affecting their susceptibility to wind erosion. The high wind erodibility and dust production of these playa surfaces, combined with their high ice nucleation activity, can play an important role in the climate of the region.