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Heterogeneous freezing of droplets with immersed mineral dust particles – measurements and parameterization

机译:浸没的矿物粉尘颗粒对液滴的异质冻结–测量和参数化

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摘要

During the measurement campaign FROST (FReezing Of duST), LACIS(Leipzig Aerosol Cloud Interaction Simulator) was used toinvestigate the immersion freezing behavior of size selected, coatedand uncoated Arizona Test Dust (ATD) particles with a mobilitydiameter of 300 nm. Particles were coated with succinic acid(CHO), sulfuric acid (HSO) and ammonium sulfate((NH)SO). Ice fractions at mixed-phase cloudtemperatures ranging from 233.15 K to 239.15 K (±0.60 K) weredetermined for all types of particles. In this temperature range,pure ATD particles and those coated with CHO or smallamounts of HSO were found to be the most efficient icenuclei (IN). ATD particles coated with (NH)SO were themost inefficient IN. Since the supercooled droplets were highlydiluted before freezing occurred, a freezing point suppression dueto the soluble material on the particles (and therefore in thedroplets) cannot explain this observation. Therefore, it isreasonable to assume that the coatings lead to particle surfacealterations which cause the differences in the IN abilities. Twodifferent theoretical approaches based on the stochastic and thesingular hypotheses were applied to clarify and parameterize thefreezing behavior of the particles investigated. Both approachesdescribe the experimentally determined results, yielding parametersthat can subsequently be used to compare our results to those fromother studies. However, we cannot clarify at the current state whichof the two approaches correctly describes the investigated immersionfreezing process. But both approaches confirm the assumption thatthe coatings lead to particle surface modifications lowering thenucleation efficiency. The stochastic approach interprets thereduction in nucleation rate from coating as primarily due to anincrease in the thermodynamic barrier for ice formation (i.e.,changes in interfacial free energies). The singular approachinterprets the reduction as resulting from a reduced surface densityof active sites.
机译:在测量活动FROST(dust的FReezing)期间,使用LACIS(莱比锡气溶胶云相互作用模拟器)研究了流动直径为300 nm的选定大小,涂覆和未涂覆的Arizona测试粉尘(ATD)颗粒的浸没冷冻行为。颗粒涂有琥珀酸(CHO),硫酸(HSO)和硫酸铵((NH)SO)。对于所有类型的颗粒,确定了在233.15 K至239.15 K(±0.60 K)的混合相云温度下的冰分数。在此温度范围内,发现纯ATD颗粒和涂有CHO或少量HSO的颗粒是最有效的冰核(IN)。涂有(NH)SO的ATD颗粒是效率最低的IN。由于过冷的液滴在冻结发生之前已被高度稀释,因此,由于颗粒上(以及液滴中)的可溶性物质而导致的凝固点抑制无法解释这一现象。因此,有理由认为涂层会导致颗粒表面变质,从而引起IN能力的差异。基于随机和奇异假设的两种不同的理论方法被用于澄清和参数化所研究颗粒的冻结行为。两种方法都描述了实验确定的结果,并产生了可随后用于将我们的结果与其他研究的结果进行比较的参数。但是,我们无法在当前状态下澄清两种方法中的哪一种正确地描述了所研究的浸入冷冻过程。但是两种方法都证实了这样的假设:涂层导致颗粒表面改性,从而降低了成核效率。随机方法将涂层成核速率的降低解释为主要是由于形成冰的热力学势垒的增加(即界面自由能的变化)。奇异方法解释了减少是由于活性部位的表面密度降低引起的。

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