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

Abstract

The emergence of desiccated lake bed sediments and their exposure to wind erosion as a consequence of climate change and drought in arid and semiarid regions of the world poses a growing hazard. Airborne dust originating from such soils can create health and environmental issues due to their high salt content and the presence of toxic elements. The aim of the present study is twofold, namely to investigate the newly emerged playa surfaces of western Lake Urmia (LU) in Iran and their contribution to aerosol in the region by means of physicochemical, mineralogical, and elemental analyses and to study the ice nucleation (IN) activity of both surface-collected soil and airborne dust samples. The playa surfaces created by desiccation of LU on the western shores were mapped and sampled at 130 locations. Soil samples were subjected to physicochemical analyses, and their erodible fraction was determined. Based on these analyses, four highly erodible playa surfaces from the northwest to the south of LU were selected as sites for collection of dust by impaction and soil samples from the uppermost surface. Their particle physicochemical properties (size distribution, elemental and mineralogical composition) were compared with their IN activity determined by emulsion freezing experiments in a differential scanning calorimeter (DSC) in two suspension concentrations of 2 wt % and 5 wt %. The physicochemical soil properties differed significantly between the different playa surfaces, which affects their susceptibility to wind erosion. Sand sheets and sandy salt crusts were the most erodible playa surfaces due to their high sand fraction and low organic matter and clay content, favouring the presence of small aggregates. Mineralogical analyses document the prevalence of quartz, carbonates, and clay minerals, such as kaolinite, palygorskite, and chlorite in all of the samples. The predominant elements in the samples are Ca, Fe, Al, Si, and Na (and in some cases Ba, Sr, and Zn). The correlation between soil and dust samples based on mineralogical composition, elemental enrichment factors, and physicochemical properties confirm that the playa surfaces are the major contributors to dust in the region. IN activity with onset temperatures ranging from 245 to 250 K demonstrates the high potential of dust blown from Urmia playa surfaces to affect cloud properties and precipitation. Freezing onset temperatures and the fraction of heterogeneously frozen droplets in the emulsions reveal variations in IN activity depending on the mineralogical composition of the samples but which are also influenced by organic matter, salinity, and pH. Specifically, IN activity correlates positively with organic matter and clay minerals and negatively with pH, salinity, and (surprisingly) K-feldspar and quartz content. The high wind erodibility and dust production of the LU playa surfaces together with their high IN activity can play an important role in the climate of the region and thus needs careful monitoring and specific attention.