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چکیده
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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 for 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 stepwise stripped 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, and comparing these with reference suspensions of microcline and quartz, exposed to salt concentrations and pH 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 positive
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