Is There a Relationship between Lake Urmia Saline Lakebed Emissions and Wet Deposition Composition in the Caucasus Region?

One of the most significant environmental catastrophes throughout the Middle East and Caucasus region has been the desiccation of Lake Urmia, the world second-largest hypersaline lake. This lake has lost approximately two-thirds of its water volume since 1999, resulting in increased exposure of lacustrine deposits that are considered as a new aerosol source. This in situ study investigates for the first time the spatial distribution of water-soluble ion concentrations (Cl-, Br-, NO2-, NO3-, SO42-, methanesulfonate (MSA), pyruvate, oxalate, adipate, Na+, K+, Ca2+, Mg2+, and NH4+) in wet deposition samples collected around the Lake Urmia region to characterize relationships between rainwater composition and location relative to the lake. Rainwater collection was performed for 21 rain events from September 2017 to September 2018 across 13 sampling stations, providing representative coverage of the region surrounding Lake Urmia. The most dominant ions are as follows: Ca2+ > Cl- > SO42- > Na+ > NO3-. Organic acids and MSA contribute negligibly to the total ion concentrations. Principal component analysis (PCA) and correlation coefficient (CC) analyses show that the majority of ions throughout the region are associated with salt and crustal particles comprised of Cl-, Br-, SO42-, Na+, Ca2+, and Mg2+, with the minority of ions (e.g., NH4+ and NO3-) stemming from anthropogenic emissions. Concentrations of salt and crustal tracer species (i.e., Na+, Cl-, Br-, Mg2+, and SO42-) were inversely related to the distance from the Lake Urmia shoreline, suggestive of potential subcloud scavenging (washout) of freshly emitted aerosols. Backward trajectory analysis (HYSPLIT) shows that most of the rain events were linked to air masses originating in areas far upwind of Lake Urmia. Results of this study emphasize that wet deposition data provide support for effective scavenging of salt and crustal emissions as a function of distance from a desiccated lakebed.