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Title: A comprehensive laboratory study on the immersion freezing behavior of illite NX particles: a comparison of 17 ice nucleation measurement techniques

Immersion freezing is the most relevant heterogeneous ice nucleation mechanism 3 through which ice crystals are formed in mixed-phase clouds. In recent years, an increasing 4 number of laboratory experiments utilizing a variety of instruments have examined immersion 5 freezing activity of atmospherically relevant ice nucleating particles (INPs). However, an 6 inter-comparison of these laboratory results is a difficult task because investigators have used 7 different ice nucleation (IN) measurement methods to produce these results. A remaining 8 challenge is to explore the sensitivity and accuracy of these techniques and to understand how 9 the IN results are potentially influenced or biased by experimental parameters associated with 10 these techniques. 11 Within the framework of INUIT (Ice Nucleation research UnIT), we distributed an 12 illite rich sample (illite NX) as a representative surrogate for atmospheric mineral dust 13 particles to investigators to perform immersion freezing experiments using different IN 14 measurement methods and to obtain IN data as a function of particle concentration, 15 temperature (T), cooling rate and nucleation time. Seventeen measurement methods were 16 involved in the data inter-comparison. Experiments with seven instruments started with the 17 test sample pre-suspended in water before cooling, while ten other instrumentsmore » employed 18 water vapor condensation onto dry-dispersed particles followed by immersion freezing. The 19 resulting comprehensive immersion freezing dataset was evaluated using the ice nucleation 20 active surface-site density (ns) to develop a representative ns(T) spectrum that spans a wide 21 temperature range (-37 °C < T < -11 °C) and covers nine orders of magnitude in ns. 22 Our inter-comparison results revealed a discrepancy between suspension and dry-23 dispersed particle measurements for this mineral dust. While the agreement was good below ~-24 26 °C, the ice nucleation activity, expressed in ns, was smaller for the wet suspended samples 25 and higher for the dry-dispersed aerosol samples between about -26 and -18 °C. Only 26 instruments making measurement techniques with wet suspended samples were able to 27 measure ice nucleation above -18 °C. A possible explanation for the deviation between -26 28 and -18 °C is discussed. In general, the seventeen immersion freezing measurement 29 techniques deviate, within the range of about 7 °C in terms of temperature, by three orders of 30 magnitude with respect to ns. In addition, we show evidence that the immersion freezing 31 efficiency (i.e., ns) of illite NX particles is relatively independent on droplet size, particle 32 mass in suspension, particle size and cooling rate during freezing. A strong temperature-33 2 dependence and weak time- and size-dependence of immersion freezing efficiency of illite-34 rich clay mineral particles enabled the ns parameterization solely as a function of temperature. 35 We also characterized the ns(T) spectra, and identified a section with a steep slope between -36 20 °C and -27 °C, where a large fraction of active sites of our test dust may trigger immersion 37 freezing. This slope was followed by a region with a gentler slope at temperatures below -27 38 °C. A multiple exponential distribution fit is expressed as ns(T) = exp(23.82 × exp(-exp(0.16 × 39 (T + 17.49))) + 1.39) based on the specific surface area and ns(T) = exp(25.75 × exp(-exp(0.13 40 × (T + 17.17))) + 3.34) based on the geometric area (ns and T in m-2 and °C, respectively). 41 These new fits, constrained by using an identical reference samples, will help to compare IN 42 measurement methods that are not included in the present study and, thereby, IN data from 43 future IN instruments.« less
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Journal Article
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Journal Name: Atmospheric Chemistry and Physics, 15(5):2489-2518
Research Org:
Pacific Northwest National Laboratory (PNNL), Richland, WA (US)
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Country of Publication:
United States