The comparison of ice nucleation ability of sand with silt-clay fraction of mineral particles and the role of carbonate coatings in ice formation temperature

Mineral dust particles play an important role in the formation of ice and ultimately the amount of precipitation, especially in the middle latitudes. Therefore, understanding the role of each component of dust and their contribution in ice nucleation activity in clouds is of particular importance. Therefore, the objectives of the present research were 1. to compare the ice nucleation activity of sand-sized particles with silt-clay and 2. to determine the role of carbonates in soils in the cloud formation temperature (Thet) and in the percentage of heterogeneous nucleation (Fhet). For this purpose, sixteen soil and dust samples were collected from highly sensitive playa surfaces which are exposed to wind erosion as a consequence of Lake Urmia (LU) recession. The sand particles were separated from the silt-clay section by a 50 μm sieve. Then, carbonates and soluble solutes were removed in each sample and each particle size, and the nucleation activity of the samples was determined by immersion freezing method. Fourier transform infrared spectrum (FTIR) was used to determine both the type of carbonates in natural samples and to confirm the removal of the corresponding bands in carbonate-removed samples. The results showed that in most samples, silt-clay particles have stronger nucleation activity than sand particles. This could be due to their smaller size with higher specific surface area which lead to the presence of more active sites in ice nucleation. The comparison of carbonate removal samples with soluble solute removal samples showed that although solute washing from the samples alone increases the ice nucleation activity, this change is more significant in the carbonate removal samples. Thet and Fhet parameters increased in carbonate removal samples, except for the sample with low carbonate and salt content. The average increase of Thet and Fhet in the sand component was 0.7 Kelvin and 9%, and in the silt-clay component was 1 Kelvin and 15%, respectively. The highest increase in Thet after carbonate removal in both particle sizes was observed in D4 and S5 samples with carbonate content of 0.43% and aragonite as dominant mineral in D4 and 0.41% and the dominant mineral of calcite in S5. The increase in Thet in comparison to the natural samples was 3.2 Kelvin in D4 and 3.3 Kelvin in S5. However, the highest increase in Fhet was related to samples D1 to D4, which were rich in aragonite mineral. Therefore, it seems that the abundance of aragonite mineral in the studied samples is one of the important factors in reducing the ice nucleation activity of dust samples originated from the LU playa surfaces.