The Urmia playa as a source of airborne dust and ice-nucleating particles – Part 2: Unraveling the relationship between soil dust composition and ice nucleation activity

• Published in: Atmospheric chemistry and physics Vol. 22; no. 22; pp. 14931 - 14956
• Main Authors: Hamzehpour, Nikou, Marcolli, Claudia, Klumpp, Kristian, Thöny, Debora, Peter, Thomas
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 23-11-2022; Copernicus Publications
• Subjects: Aerosols; Agricultural land; Analysis; Arid regions; Arid zones; Atmospheric particulates; Calorimetry; Carbonates; Clay; Clay minerals; Climate change; Cloud properties; Components; Composition; Correlation; Deserts; Differential scanning calorimetry; Dust; Dust composition; Emulsions; Feldspars; Freezing; Freezing temperatures; Global temperature changes; Heat; Ice; Ice nucleation; Ice removal; Laboratories; Mineralogy; Minerals; Nucleation; Organic compounds; Organic matter; Organic soils; pH effects; Playas; Quartz; Removal; Saline soils; Salts; Seeds; Semi arid areas; Semiarid lands; Semiarid zones; Soils, Salts in; Vegetation; Vegetation cover; Water scarcity; Weather stations; X ray powder diffraction
• DOI: 10.5194/acp-22-14931-2022
• EISSN: 1680-7324
• ISSN: 1680-7324; 1680-7316

Ice-nucleating particles (INPs) originating from deserts, semi-arid regions,
and dried lakebeds may cause heterogeneous ice nucleation, impacting cloud
properties. Recently, due to climate change and water scarcity, abandoned
agricultural lands with little surficial crust and negligible vegetation
cover have become an increasing source of atmospheric dust worldwide.
Unlike deserts, these areas are rich in soluble salt and (bio-)organic
compounds. Using soil samples from various sites of the Lake Urmia playa
(LUP) in northwestern Iran and airborne dusts collected at nearby
meteorological stations, we elucidate how minerals, soluble salts, and organic
matter interact to determine the IN activity of saline soils and dust. X-ray
powder diffraction shows that the mineralogical composition is dominated by
K-feldspars (microcline), quartz, carbonates, and clay minerals. The samples
were stripped stepwise of organic matter, carbonates, and soluble salts.
After each removal step, the ice nucleation (IN) activity was quantified in
terms of onset freezing temperatures (Thet) and heterogeneously frozen
fractions (Fhet) by emulsion freezing experiments using differential
scanning calorimetry (DSC). We examined the influence of soluble salts and
pH on microcline and quartz in emulsion freezing experiments, comparing
these with reference suspensions of microcline and quartz exposed to salt
concentrations and pH levels characteristic of the LUP samples. These analyses,
combined with correlations between Thet and Fhet, allow us to
identify the components that contribute to or inhibit IN activity. The LUP
dusts turn out to be very good INPs, with freezing onset temperatures around
248 K in immersion freezing experiments. Interestingly, their IN activity
proves to be dominated by the relatively small share of (bio-)organic matter
(1 %–5.3 %). After organic matter removal, the remaining IN activity (Thet≈244 K) can be traced back to the clay fraction, because Thet and Fhet correlate positively with the clay mineral content but negatively with quartz and microcline. We attribute the inability of quartz and microcline to act as INPs to the basic pH of the LUP samples as well as to the presence of soluble salts. After additionally removing soluble salts and carbonates, the IN activity of the samples increased again significantly (Thet≈249 K), and the negative correlation with quartz and microcline turned into a slightly positive one. Removing carbonates and salts from the natural samples leads to an increase in Thet and Fhet as well, indicating that their presence also suppresses the IN activity of the (bio-)organic INPs. Overall, this study demonstrates that mineral and organic INPs do not just add up to yield the IN activity of soil dust but that the freezing behavior is governed by inhibiting and promoting interactions between the components.