Heterogeneous ice nucleation ability of crystalline sodium chloride dihydrate particles

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH: ATMOSPHERES, VOL. 118, , doi: /jgrd.50325, 2013 Heterogeneous ice nucleation ability of crystalline sodium chloride dihydrate particles Robert Wagner 1 and Ottmar Möhler 1 Received 18 October 2012; revised 28 February 2013; accepted 9 March 2013; published 28 May [1] The aerosol and cloud chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) of the Karlsruhe Institute of Technology has been used to quantify the deposition mode ice nucleation ability of airborne crystalline sodium chloride dihydrate (NaCl 2H 2 O) particles with median diameters between 0.06 and 1.1 mm. For this purpose, expansion cooling experiments with starting temperatures from 235 to 216 K were conducted. Recently, supermicron-sized NaCl 2H 2 O particles deposited onto a surface have been observed to be ice-active in the deposition mode at temperatures below 238 K, requiring a median threshold ice saturation ratio of only 1.02 in the range from 238 to 221 K. In AIDA, heterogeneous ice nucleation by NaCl 2H 2 Owasfirst detected at a temperature of K with a concomitant threshold ice saturation ratio of Above that temperature, the crystallized salt particles underwent a deliquescence transition to form aqueous NaCl solution droplets upon increasing relative humidity. At nucleation temperatures below 225 K, the inferred threshold ice saturation ratios varied between 1.15 and The number concentration of the nucleated ice crystals was related to the surface area of the seed aerosol particles to deduce the ice nucleation active surface site (INAS) density of the aerosol population as a function of the ice supersaturation. Maximum INAS densities of about m 2 at an ice saturation ratio of 1.20 were found for temperatures below 225 K. These INAS densities are similar to those recently derived for deposition mode ice nucleation on mineral dust particles. Citation: Wagner, R., and O. Mo hler (2013), Heterogeneous ice nucleation ability of crystalline sodium chloride dihydrate particles, J. Geophys. Res. Atmos., 118, , doi: /jgrd Introduction [2] Recently, Wise et al. [2012] have unraveled a potential new mechanism for ice formation in the troposphere, namely depositional ice nucleation on crystalline hydrated sodium chloride particles. In the atmosphere, crystalline sodium chloride particles will form once an air parcel containing aqueous NaCl solution droplets is exposed to a relative humidity (RH) below 38 44%. The latter represents the range of the efflorescence relative humidity (ERH) of NaCl for temperatures between 249 and 298 K [Cziczo and Abbatt, 2000; Koop et al., 2000a]. Upon efflorescence, two crystalline species of NaCl might form, namely anhydrous NaCl and sodium chloride dihydrate (NaCl 2H 2 O). According to the bulk phase diagram for the sodium chloride-water system, NaCl 2H 2 O is the thermodynamically stable solid phase below K, whereas it is anhydrous NaCl above K [Martin, 2000]. Previous studies, however, have shown that 1 Karlsruhe Institute of Technology (KIT), Institute for Meteorology and Climate Research (IMK-AAF), Eggenstein-Leopoldshafen, Germany. Corresponding author: R. Wagner, Karlsruhe Institute of Technology (KIT), Institute for Meteorology and Climate Research (IMK-AAF), Eggenstein-Leopoldshafen, Germany. (robert.wagner2@kit.edu) American Geophysical Union. All Rights Reserved X/13/ /jgrd down to 253 K homogeneous crystallization of aqueous NaCl only leads to the formation of anhydrous NaCl [Cziczo and Abbatt, 2000], and that nucleation of the thermodynamically stable sodium chloride dihydrate is only triggered by heterogeneous crystallization on available surfaces like ice or oxalic acid dihydrate at such elevated temperatures [Koop et al., 2000a; Wagner et al., 2011]. Wise et al. [2012] have extended the temperature range for crystallization experiments with aqueous NaCl solution droplets down to about 221 K. The investigated droplets typically had a diameter of about 5 mm and were deposited onto a hydrophobic quartz disk that was placed into an environmental cell equipped with a combination of optical microscope and Raman spectrometer. The authors have detected that there is a transition regime between 252 and 236 K where the composition of the crystallized solution droplets changes from only anhydrous to only dihydrate NaCl particles, meaning that below a temperature of 236 K exclusively the formation of NaCl 2H 2 O was detected. [3] Once crystalline anhydrous and dihydrate NaCl particles are formed in the atmosphere at low RH, they could act as heterogeneous ice nuclei in the deposition mode when again exposed to an increasing relative humidity. This requires that the heterogeneous ice nucleation onset is below the deliquescence relative humidity (DRH) of the crystals, as otherwise aqueous NaCl solution droplets would be formed which can only freeze by homogeneous nucleation at elevated supersaturation levels. The DRH of anhydrous 4610

2 NaCl is rather insensitive to a change in temperature between 298 and 249 K and amounts to about 75% [Koop et al., 2000a]. For NaCl 2H 2 O, the deliquescence curve is obtained from the sodium chloride-water phase diagram shown in the relative humidity temperature space by connecting the NaCl/NaCl 2H 2 O peritectic at K and 75% RH with the ice/nacl 2H 2 O eutectic at K and 81% RH [Koop et al., 2000a]. In a previous study [Wagner et al., 2011], we have detected a DRH value of 82% for crystalline NaCl 2H 2 O at 242 K which is in good agreement with the extrapolation from the phase diagram. Wise et al. [2012] have found DRH values for NaCl 2H 2 O between 76.6 and 93.2% RH in the temperature range from 235 to 257 K. [4] Concerning the competition between deliquescence and heterogeneous ice nucleation, Wise et al. [2012] have shown that above a temperature of 239 K, only deliquescence of the deposited NaCl 2H 2 O crystals occurred, whereas below 235 K, the crystals only heterogeneously nucleated ice in the deposition mode. In the narrow temperature regime from 235 to 239 K, both processes were observed. The NaCl 2H 2 O crystals thereby proved to be remarkably efficient ice nuclei in the temperature range from 221 to 238 K, yielding an average value of for the ice saturation ratio, S ice, at the onset of ice formation. For anhydrous NaCl particles, a slightly higher average value for S ice of was found. [5] The high ice nucleation ability of NaCl 2H 2 O might represent an example for the chemical bond requirement in heterogeneous ice nucleation [Pruppacher and Klett, 1997], meaning that the water molecules in the crystal are preferential sites for the further deposition of water vapor from the gas phase. Also a chemically different hydrate species, namely oxalic acid dihydrate, has recently been observed to be a partly very efficient ice nucleus in the deposition mode [Kanji et al., 2008; Wagner et al., 2010]. Additionally, the microscope images recorded by Wise et al. [2012] show that the dihydrate particles have a higher degree of surface roughness compared to the anhydrous crystals which is another factor that could add to the particular ice nucleation efficiency of NaCl 2H 2 O. Based on a model simulation where the trajectories of air parcels with initially aqueous NaCl were tracked through the tropical upper troposphere, Wise et al. [2012] have estimated that dihydrate crystals are present 40 80% of the time in the upper troposphere at temperatures below 220 K and could therefore play a role in cirrus cloud formation in view of their high ice nucleation ability. [6] Motivated by the Wise et al. s [2012] study and based on previous experiments at our own facility with NaCl and internally mixed NaCl/oxalic acid particles [Wagner et al., 2011], we have performed a dedicated series of crystallization and ice nucleation experiments with airborne NaCl particles of median diameters between 0.06 and 1.1 mm in the temperature range from 235 to 216 K. The experiments were part of a measurement campaign that was conducted at the coolable aerosol and cloud chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) of the Karlsruhe Institute of Technology. Each experiment at a given temperature was started with the addition of aqueous NaCl solution droplets to the AIDA chamber. Crystallization of the injected droplets was then monitored at constant temperature and relative humidity via in situ depolarization and infrared extinction spectroscopy measurements over a timescale of several hours. After crystallization was complete, the particle ensemble was probed on its deliquescence and ice nucleation ability in an expansion cooling experiment during which ice supersaturated conditions were established in the chamber interior. We chose to present the results from these measurements in two separate articles. In a previous article, we have already analyzed the temperature-dependent partitioning between anhydrous and dihydrate particles upon crystallization of the injected aqueous NaCl solution droplets [Wagner et al., 2012]. A brief summary of our major findings is given in the next paragraph. The present article describes the additionally performed ice nucleation experiments with the crystalline NaCl and NaCl 2H 2 O particles. [7] For exploring the crystallization behavior of the aqueous NaCl solution droplets, we have quantitatively analyzed the infrared extinction spectra of the crystallized particles to infer the relative number fractions of NaCl and NaCl 2H 2 O particles. Extinction spectra of anhydrous NaCl particles at mid-infrared wavelengths solely reveal a structureless scattering signature, whereas NaCl 2H 2 O particles additionally give rise to characteristic absorption bands due to the vibrational modes of the water molecules in the crystals. As a prerequisite for the quantitative analysis of the temperature-dependent partitioning between the two solid phases of sodium chloride from the infrared spectra, we have additionally deduced the so far unknown infrared optical constants of NaCl 2H 2 O. In agreement with Wise et al. [2012], the analysis of the AIDA experiments also showed that there is a narrow temperature range where the composition of the crystallized particles almost completely changes from anhydrous to dihydrate NaCl particles. Under our experimental conditions, this transition regime was however shifted to lower temperatures by an amount of about 13 K when comparing the temperatures where 50% of the particles have crystallized as NaCl 2H 2 O. This means, e.g., that at a temperature of 235 K where Wise et al. [2012] have exclusively detected the formation of NaCl 2H 2 O, in AIDA only about 7% of the injected NaCl solution droplets have crystallized as the dihydrate. This different trend is in accordance with a former AIDA crystallization experiment with aqueous NaCl particles at 244 K [Wagner et al., 2011], in which dihydrate formation could not be detected at all. The temperature shift of the NaCl-NaCl 2H 2 O transition regime between the AIDA and the Wise et al. s [2012] experiments could be related to the difference in the size of the investigated particles or to the fact that the particles were either airborne or deposited onto a surface. [8] Note that Wise et al. [2012] in their article do not explicitly refer to sodium chloride dihydrate as the lowtemperature phase of crystalline NaCl which forms below 236 K. Instead, they introduce the general term hydrated form of NaCl. This is because they have observed slight spectral discrepancies in their recorded Raman intensities in comparison with the spectrum from a preceding work which supposedly was due to NaCl 2H 2 O[Dubessy et al., 1982; Wise et al., 2012]. They also underline, however, that this might simply be due to the different temperatures at which the Raman spectra were collected. Our recorded infrared spectra showed good agreement with those previously published for NaCl 2H 2 O[Mutter et al., 1959; Schiffer and Hornig, 1961], indicating that it is indeed the dihydrate 4611

3 that forms in the investigated temperature range. It is therefore reasonable to assume that also Wise et al. [2012] have observed the formation of NaCl 2H 2 O, and we therefore chose to use this assignment when referring to their results in our manuscript. [9] Concerning the subject of the present article, namely the heterogeneous ice nucleation ability of the crystallized NaCl and NaCl 2H 2 O particles, both the experimental approach from Wise et al. [2012] and the AIDA expansion cooling technique have their inherent advantages and can thus be considered as complementary methods to fully explore the ice nucleation behavior in this system. The appeal of the procedure by Wise et al. [2012] lies in the chemical identification of the particle that has acted as the ice nucleus. The AIDA chamber experiments probe an ensemble of airborne particles with a well-characterized size distribution which allows a more quantitative analysis of the heterogeneous ice nucleation ability with respect to the surface area of the seed aerosol particles, as, e.g., in the framework of the ice nucleation active surface site (INAS) density concept [Hoose and Möhler, 2012]. Moreover, not only the onset of ice nucleation on the most ice-active seed aerosol particles can be determined, but the data can additionally be analyzed to yield spectra of the activated fraction of the aerosol particles as a function of the supersaturation at different temperatures. Our article will first introduce the key aerosol and ice particle measurement techniques of the AIDA chamber which are used to quantitatively deduce the depositional ice nucleation ability of crystalline NaCl and NaCl 2H 2 O (section 2). The results of this analysis are then presented in section 3 in terms of temperaturedependent nucleation onsets, activated fractions, and INAS densities, including a comparison with the ice nucleation ability of different atmospheric aerosol types which also promote ice nucleation in the deposition mode, like mineral dust and volcanic ash particles. A summary in section 4 concludes our article. Here we also discuss whether our results on pure sodium chloride particles can directly be transferred to predict the ice nucleation ability of actual sea salt aerosol (SSA) particles, which, apart from NaCl as their primary constituent, contain further ions like SO 4 2 and Mg 2+ as minor components [Koop et al., 2000a; Wise et al., 2012]. 2. Experimental 2.1. General Setup and Instrumentation [10] The ice nucleation experiments were performed in the 84 m 3 sized aerosol and cloud chamber AIDA at the Karlsruhe Institute of Technology (Figure 1) [Wagner et al., 2006b]. The aerosol vessel is located in an isolating housing and its temperature can be controlled between ambient and about 183 K. The temperature inhomogeneity is typically less than 0.3 K throughout the chamber volume, as ensured by ventilating air around the aerosol vessel and continuously operating a mixing fan inside the chamber during the experiments. The chamber can be evacuated with two vacuum pumps, which allows an efficient cleaning of the vessel and yields background particle number concentrations of less than 1 cm 3. Purified water is evaporated into the evacuated chamber in such an amount that ice coverage of the inner chamber walls is achieved. Afterward, the chamber is refilled to ambient pressure with particle-free synthetic air and the NaCl seed aerosol particles, whose ice nucleation ability is to be investigated, are injected into the chamber as detailed in section 2.2. [11] To probe heterogeneous ice nucleation by NaCl and NaCl 2H 2 O crystals in the deposition mode, the particles must be exposed to an environment that is supersaturated with respect to ice. The ice layer on the chamber walls controls the initial relative humidity at the start of an ice nucleation experiment to almost 100% with respect to ice. Ice supersaturated conditions are then established by expansion cooling, i.e., by lowering the pressure inside the aerosol vessel at controllable rates with the vacuum pumps. The relative humidity during expansion cooling as well as the microphysical properties of the nucleated ice crystals are measured by a comprehensive set of instruments [Wagner et al., 2009], whose characteristics are briefly described in the following. [12] The relative humidity is measured in situ by tunable diode laser (TDL) absorption spectroscopy with a time resolution of 1 s. A selected ro-vibrational water vapor absorption line at 1.37 mm is scanned to deduce the water vapor pressure, p w (T ), at the prevailing AIDA temperature with an estimated uncertainty of 5%. For the same temperature, the saturation water vapor over ice, p w,ice (T), is computed [Murphy and Koop, 2005]. The quotient of p w (T )andp w,ice (T ) then yields the ice saturation ratio, S ice (T ). As a major advantage, this technique is applicable to both cloud-free and in-cloud conditions. [13] Two optical particle counters (OPC1 and OPC2, type WELAS, Palas GmbH) are connected to the bottom of the AIDA chamber to measure time series of the number concentration of the nucleated ice crystals, N ice, during the expansion cooling experiments. The detection ranges of the instruments (for a refractive index of 1.33) are mm (OPC1) and mm (OPC2). This implies that, depending on their sizes (section 2.2), also a fraction of the seed aerosol particles might be detected by these instruments. The nucleated ice crystals, however, rapidly grow to much larger sizes compared to the NaCl and NaCl 2H 2 O crystals. Therefore, an optical threshold size can be applied to separately count the ice particles. Based on previous intercomparisons of the OPC1 and OPC2 records for the ice particle number concentration with the results from different instruments, we assume that the maximum uncertainty in N ice is 20% [Möhler et al., 2006]. [14] The number concentration and size of the nucleated ice crystals can additionally be retrieved from their infrared extinction signatures, which are measured at 4 cm 1 resolution between 6000 and 800 cm 1 with an FTIR (Fourier transform infrared) spectrometer (IFS66v, Bruker) that is connected to an open-path multiple reflection cell inside the AIDA chamber [Wagner et al., 2006a]. Infrared extinction spectroscopy is also employed to identify the phase of the NaCl seed aerosol population. The same twofold area of application, i.e., characterization of both the seed aerosol particles and the ice crystals, also holds for in situ laser light scattering and depolarization measurements with the SIMONE instrument [Schnaiter et al., 2012]. This device measures the intensity of laser light that is scattered from the particles in 178 backward direction. Formation of large ice crystals on a subset of the aerosol population by deposition mode nucleation is detected by a strong increase in the 4612

4 Figure 1. Scheme of the AIDA aerosol and cloud chamber facility with the relevant scientific instrumentation used for the NaCl crystallization and ice nucleation experiments. TDL: tunable diode laser, FTIR: Fourier transform infrared, OPC: optical particle counter, CPC: condensation particle counter, SMPS: scanning mobility particle sizer, APS: aerodynamic particle spectrometer, SIMONE: scattering intensity measurements for the optical detection of ice, I: scattering intensity. back-scattering intensity. Additionally, the back-scattered laser light is polarization-resolved to determine the backscattering linear depolarization ratio, d, of the aerosol particles. As d is zero for light scattering by spheres but different from zero when aspherical particles are present, the depolarization measurements are a powerful tool to detect phase transitions (deliquescence and efflorescence) in the NaCl particle ensemble Aerosol Generation and Characterization [15] As outlined in section 1 and detailed in our previous article [Wagner et al., 2012], ensembles of crystalline NaCl and NaCl 2H 2 O particles were generated in situ by first injecting aqueous NaCl solution droplets into the AIDA chamber, and then observing their crystallization at constant temperature and relative humidity over timescales of several hours. An ultrasonic nebulizer (GA2400 Sinaptec), filled with a 5 wt% (weight percent) aqueous NaCl solution (NaCl, Merck, >99.5%), was typically used for aerosol injection. Four individual experiments with using the nebulizer for aerosol generation were conducted at 235.7, 230.7, 225.7, and K (Table 1, Experiments 1 to 4). These values also correspond to the starting temperatures of the subsequent expansion cooling runs. In all experiments, the ultrasonic nebulizer was operated with identical settings and with a portion of the same NaCl solution. [16] The gradual crystallization of the particle ensemble of initially spherical aqueous NaCl solution droplets was evidenced by a continuous increase in the depolarization ratio. After the entire particle population had crystallized, d remained at a constant value. The infrared extinction spectra of the crystallized particle ensembles were then used in an optimization scheme to retrieve the number fractions of anhydrous and dihydrate NaCl crystals [Wagner et al., 2012]. The key step of this approach was to fit simulated spectra to the measured infrared extinction spectra. These simulated Table 1. Parameters of Individual Ice Nucleation Experiments With Crystallized NaCl and NaCl 2H 2 O Aerosol Particles a Exp. T/K Aeros. Gen. f NaCl2H 2O N total /cm 3 d mean /mm A total /mm 2 cm 3 A NaCl2H 2O/mm 2 cm UN/5 wt % UN/5 wt % UN/5 wt % UN/5 wt % AM/4 wt % , AM/1 wt % , a T: the mean AIDA gas temperature at aerosol injection and during the crystallization period until the start of pumping. For aerosol generation (third column), either an ultrasonic nebulizer (UN) or an atomizer (AM) filled with an aqueous NaCl solution of the specified wt % was used. f NaCl2H 2O: the number fraction of particles that have crystallized as NaCl 2H 2 O. N total : the overall number concentration of the NaCl and NaCl 2H 2 O particles, d mean : the mean diameter of the number size distribution measured at ambient temperature, A total : the overall surface area concentration of the NaCl and NaCl 2H 2 O particles, including a size correction for the dihydrate particle fraction, A NaCl2H 2O: the surface area concentration of only the dihydrate particle fraction. See text for details. 4613

5 dn/dlogd p / cm Exp. 1, K Exp. 4, K d p / µm Figure 2. Number size distributions of the crystallized NaCl and NaCl 2H 2 O particle ensembles during Experiment 1 (black) and Experiment 4 (gray). The solid lines show the raw size spectra as measured by the SMPS and the APS at ambient temperature. The dashed lines are obtained after a size correction for the NaCl 2H 2 O particle fraction as present in AIDA at low temperatures. See text for details. spectra were computed as a superposition of extinction due to NaCl 2H 2 O and scattering due to anhydrous NaCl particles. The size distribution parameters of both the dihydrate and the anhydrous particle fraction were optimized to get best agreement between the measured and the computed infrared spectra. The temperature-dependent partitioning between the two solid phases was then obtained from the optimized size distribution parameters. As summarized in Table 1, the percentage of dihydrate particles, f NaCl2H 2 O, increased from 7% at K to 88% at K. Table 1 additionally includes the total particle number concentrations, N total, and the mean diameters (arithmetic averages) [Hinds, 1999], d mean, of the number size distributions of the crystallized aerosol populations, as both measured immediately before the start of the expansion cooling experiments. N total was measured with two condensation particle counters (CPC3010 and 3775, TSI), and d mean was calculated from the number size distributions that were measured by combining the size spectra from a scanning mobility particle sizer (SMPS, TSI, mobility diameter range: mm) and an aerodynamic particle spectrometer (APS, TSI, aerodynamic diameter range: mm). Two representative number size distributions from Experiments 1 and 4 are shown in Figure 2 as a function of the volume-equivalent sphere diameter, d p (solid lines). To convert the mobility diameter from the SMPS into d p, a dynamic shape factor, w, of1.08asrepresentative for cubic anhydrous NaCl particles was employed. The same value for w as well as the particle density of anhydrous NaCl, r(nacl)=2.165gcm 3, were chosen to transform the aerodynamic diameter from the APS into d p [Hinds, 1999]. [17] As already explained in Wagner et al. [2012], anhydrous NaCl was chosen as the reference for the d p conversion because the size distributions were measured outside the isolating housing at ambient temperature by sampling air from the cold chamber interior. It is therefore reasonable to assume that the dihydrate particle fraction from AIDA will transform upon warming and be detected as anhydrous NaCl in the size distribution measurements. The actual size of the NaCl 2H 2 O crystals as present in AIDA at low temperatures will be larger, both due to the inclusion of the water molecules in the crystals and due to the lower particle density of NaCl 2H 2 O compared to NaCl [Light et al., 2009]. Therefore, a correction factor for the sizes of the dihydrate particle fraction has been applied according to equation (4) in Wagner et al. [2012] for the accurate computation of the total surface area concentration, A total, of the particle ensemble, which is required as input for analyzing the ice nucleation ability within the INAS concept. [18] The two dashed lines in Figure 2 show the estimated actual number size distributions of the NaCl and NaCl 2H 2 O particle ensembles as prevalent at AIDA conditions, i.e., after applying the size correction to the dihydrate particle fraction. These number size distributions were then converted into surface area size distributions and integrated to yield A total of the aerosol populations, as summarized in Table 1. In lack of any information about the actual particle habit of NaCl 2H 2 O, we assumed that the particles were spherical in shape. A further table entry shows the calculated values for A NaCl2H 2 O, which represents the surface area concentration of the subset of particles that have crystallized as NaCl 2H 2 O. The distinction between A total and A NaCl2H 2 O is necessary because it is the dihydrate particle fraction that triggers heterogeneous ice nucleation in the expansion cooling experiments. [19] Table 1 additionally includes two experiments (Experiments 5 and 6) where an atomizer with a 4 and 1 wt % NaCl solution was used for aerosol generation, respectively. Those were dedicated experiments to generate smaller sized particle ensembles with mean diameters less than 0.1 mm (Experiment 6), which were explicitly needed to deduce the infrared optical constants of NaCl 2H 2 O [Wagner et al., 2012]. These smaller sized ensembles of crystallized NaCl and NaCl 2H 2 O particles, however, were probed on their heterogeneous ice nucleation ability as well and are therefore also included in our analysis. Only from their infrared extinction spectra, one cannot retrieve the number fractions of dihydrate and anhydrous NaCl particles as for Experiments 1 4. This is because for small particle sizes, the scattering contribution to extinction vanishes so that anhydrous NaCl crystals become invisible in the infrared spectra recordings, meaning that their contribution cannot be quantified. For our analysis, we have assumed that f NaCl2H 2 O for Experiment 5 is identical to Experiment 3 because it was conducted at the same temperature. For Experiment 6 that was conducted at an intermediate temperature, the dihydrate particle fraction was estimated by a linear interpolation of the f NaCl2H 2 O values from Experiments 3 and Results and Discussion 3.1. General Overview of the Temperature-Dependent Heterogeneous Ice Nucleation Ability [20] Before turning to the quantitative analysis of the ice nucleation data in section 3.2, we want to give a general overview of the temperature-dependent heterogeneous ice nucleation ability of the crystallized NaCl and NaCl 2H 2 O particle ensembles. We present the AIDA measurements from three selected expansion cooling experiments which underline that there are two competing processes upon increasing relative humidity, namely deliquescence and heterogeneous ice nucleation. A particularly interesting example is the 4614

6 p / hpa S ice, S liq N -3 ice / cm δ WAGNER AND MÖHLER: NACL 2H 2 O AS A HETEROGENEOUS ICE NUCLEUS a b c d Exp T / K optical depth X Y time / s wavenumber / cm e Y X Schiffer and Hornig [1961] Figure 3. (a d) Time series of various AIDA records during the expansion cooling cycle with an ensemble of crystalline NaCl and NaCl 2H 2 O particles at K (Experiment 2). (a) AIDA pressure (black line) and mean gas temperature (gray line). (b) Saturation ratio with respect to ice (S ice, black line) and liquid supercooled water (S liq, gray line). (c) Ice particle number concentration, N ice. (d) Back-scattering linear depolarization ratio, d. (e) Infrared extinction spectra of the aerosol population at two different times of the experiment, as shown by the correspondingly labeled arrows on the timescale in the bottom left panel (black lines). Additionally shown in gray is a literature infrared spectrum of NaCl 2H 2 OintheO Hstretchingand bending mode regimes, obtained by digitizing Figure 1 from Schiffer and Hornig [1961]. crystallization experiment at K where anhydrous and dihydrate NaCl particles have formed with roughly equal number fractions (Experiment 2, Table 1). [21] Figures 3a 3d show time series of relevant AIDA records for the expansion cooling cycle performed with the crystallized aerosol population from Experiment 2. The two arrows on the timescale labeled X and Y denote the times when the two infrared extinction spectra that are shown in Figure 3e were recorded. Spectrum X was monitored after the entire population of injected aqueous NaCl solution droplets had crystallized. As described in section 2.2, this was confirmed by reaching a constant level of the back-scattering linear depolarization ratio d. The fraction of solution droplets that have crystallized as anhydrous NaCl contribute to the spectral habitus with a featureless scattering signature, whereas the water molecules in the NaCl 2H 2 O particle fraction give rise to prominent extinction bands in the O H stretching (~3400 cm 1 ) and the O H bending mode (~1600 cm 1 ) regime [Wagner et al., 2012]. As already indicated in section 1, spectrum X compares well with a literature spectrum for NaCl 2H 2 O. For comparison, the latter is also plotted in Figure 3e (gray lines). From top to bottom, Figures 3a 3d show the AIDA pressure and mean gas temperature (black and gray line in Figure 3a, respectively), the saturation ratios with respect to ice and liquid supercooled water (S ice, S liq =RH w /100) as inferred from the TDL water vapor absorption measurements (black and gray line in Figure 3b, respectively), the number concentration of nucleated ice crystals, N ice, as measured with the optical particle counters (Figure 3c), and the back-scattering linear depolarization ratio d (Figure 3d). [22] Pumping is started at time zero and immediately leads to an increase in RH w.atrh w = 73%, as highlighted by the vertical gray line, d drops from 0.31 to about 0.10, indicating a deliquescence transition in the aerosol population. In accordance with the deliquescence relative humidities reported in the literature, this is due to deliquescence of the fraction of particles that have crystallized as anhydrous NaCl. Because d does not drop to the background value that is observed for an entirely liquid particle ensemble, aspherical crystals are still present, namely those particles that have crystallized as NaCl 2H 2 O. At the time when recording the extinction spectrum Y, we thus have a mixture of aqueous NaCl solution droplets and crystalline NaCl 2H 2 O particles. This is nicely reflected in the spectrum that shows the broad extinction signatures due to liquid water, with the characteristic narrow peaks due to NaCl 2H 2 O still superimposed on them. [23] Before surpassing the deliquescence relative humidity of NaCl 2H 2 O at about 82% RH w, the NaCl 2H 2 O crystals act as heterogeneous ice nuclei in the deposition mode. As marked by the first vertical black line, ice formation initiates at a threshold ice saturation ratio of S ice = 1.25 and at a temperature of K. We define the nucleation onset as the time when the ice particle number concentration has exceeded 1 cm 3, because modes with N ice 1cm 3 could also be due to nucleation on impurities. During continued pumping, S ice reaches a peak value of 1.33 before the increasing number of nucleated ice crystals starts to deplete the excess of water vapor in the gas phase, leading to a decrease in S ice. As soon as S ice again drops below the nucleation threshold of 1.25 (second vertical black line), no 4615

7 T / K Ice hom. freezing NaCl NaCl*2H 2 O Exp. 1 Exp. 2 Exp RH w / % Figure 4. Experimental trajectories of expansion cooling cycles 1, 2, and 5 in the temperature-relative humidity space. The trajectories, shown as the gray lines, cover the period from the start of expansion cooling at ice subsaturated condition until the time when the maximum ice supersaturation was reached. Squares denote observed deliquescence points of anhydrous NaCl particles, and circles represent the detected onsets of ice formation. The NaCl/NaCl 2H 2 O peritectic at K and the ice/nacl 2H 2 O eutectic at K are marked by the upward and downward triangles, respectively. The different black lines denote the following: NaCl: deliquescence relative humidities of anhydrous NaCl according to the parameterization by Tang and Munkelwitz [1993] (extrapolation below the peritectic shown as dashed line), NaCl 2H 2 O: deliquescence relative humidities of sodium chloride dihydrate, obtained by connecting the peritectic and the eutectic (extrapolation below the eutectic shown as dashed line), Ice, hom. freezing: ice melting and homogeneous freezing temperatures according to Koop et al. [2000b]. further ice crystals nucleate and pumping is stopped shortly thereafter. The ice particle number concentration at S ice =1.33 amounts to about 17 cm 3. In relation to the NaCl 2H 2 Oseed aerosol number concentration, as obtained from the data in Table 1 and corrected for pumping dilution, this corresponds to an ice active fraction of 9.6%. This example underlines that knowledge of the partitioning between anhydrous NaCl and NaCl 2H 2 O is a prerequisite to predict the behavior of the aerosol population upon increasing relative humidity: a particle ensemble of only anhydrous NaCl crystals probed in an expansion cooling cycle started at K would deliquesce and then homogeneously nucleate ice at elevated supersaturated levels. In contrast, crystalline NaCl 2H 2 O particles are able to act as heterogeneous ice nuclei in the deposition mode at that temperature before surpassing their deliquescence relative humidity, thereby inducing ice formation at a much lower supersaturation level compared to homogeneous freezing. [24] The expansion cooling cycles can further be illustrated by plotting their experimental trajectories in the temperaturerelative humidity space (Figure 4). For Experiment 2, it becomes obvious that the value for the initial deliquescence transition of anhydrous NaCl at 73% RH w, as shown by the square, is slightly below that expected from the extrapolation of the Tang and Munkelwitz s [1993] parameterization, even when taking into account an uncertainty of 5% for our humidity measurements. This can be explained, on the one hand, by the inhomogeneity in the gas temperature throughout the chamber volume. The value of 73% RH w refers to the mean gas temperature, but deliquescence will initiate in the coldest parts of the vessel with concomitantly locally higher RH w values compared to the mean. On the other hand, there are also differences between various formulations for the saturation water vapor pressure over supercooled water at such low temperatures [Murphy and Koop, 2005], which increases the uncertainty of the determined RH w value. Heterogeneous ice formation during Experiment 2, as marked by the circle, then starts clearly before surpassing the deliquescence relative humidity of NaCl 2H 2 O and well below the homogeneous freezing limit. [25] We now turn to Figure 5 that shows the AIDA records during the expansion cooling cycles from Experiments 1 (235.7 K, left panel) and 5 (225.9 K, right panel). The respective experimental trajectories in the T-RH w space are plotted in Figure 4. In Experiment 1, the crystallized aerosol population is dominated by anhydrous NaCl particles. During the expansion cooling cycle, these particles first deliquesce at RH w = 73% (vertical gray line) and then homogeneously freeze at S ice = 1.38 (vertical black line). Temperature inhomogeneities, which could account for the difference between the measured and expected deliquescence relative humidity for anhydrous NaCl as outlined above, might explain the deviation of our recorded freezing onset from the Koop et al. s [2000b] homogeneous freezing parameterization. The fate of the minor percentage of NaCl 2H 2 O crystals cannot be exactly traced. After the deliquescence step of the anhydrous crystals, the depolarization ratio almost drops to the background value that is observed in the presence of only spherical particles, meaning that a potential further deliquescence step due to NaCl 2H 2 O at about RH w = 82% cannot be resolved. This is because the anhydrous particles, in addition to their much higher number fraction compared to the NaCl 2H 2 O crystals, also have a larger size after deliquescence due to the water uptake and therefore dominate the intensity of the backscattering signal. Before the homogeneous freezing onset, a small heterogeneous ice mode with N ice 1cm 3 is formed. This could be related to deposition mode nucleation on the minor percentage of NaCl 2H 2 O crystals, but as explained above, nucleation on impurities cannot be excluded. [26] In a former AIDA expansion run started at K, we have probed the ice nucleation ability of crystallized aqueous NaCl solution droplets that had contained a solid inclusion of oxalic acid [Wagner et al., 2011]. These solid inclusions have triggered the precipitation of sodium chloride dihydrate in a much larger subset of the NaCl solution droplets than observed in Experiment 1 from our present study. We cannot completely rule out that the presence of oxalic acid modifies the behavior of the particles compared to pure NaCl. The most likely morphology of the particles after crystallization, however, can be envisaged as a solid oxalic core surrounded by an almost pure matrix of solid NaCl or NaCl 2H 2 O, which is available for deposition mode ice nucleation. Only the dihydrate particle fraction clearly underwent a deliquescence transition in the course of this expansion cooling experiment at RH w = 82% (S ice = 1.21) and T = K. Prior to deliquescence, heterogeneous ice formation by deposition mode nucleation on NaCl 2H 2 O was not observed. This experiment from our 4616

8 p / hpa S ice, S liq / cm -3 N ice δ Exp. 1 time / s Exp time / s T / K Figure 5. Time series of various AIDA records during the expansion cooling cycles from Experiment 1 (235.7 K) and Experiment 5 (225.9 K). The individual panels contain the same measurements as in Figures 3a 3d. previous work in combination with Experiment 2 from our present study thus localizes the transition regime of the two competing processes deliquescence and heterogeneous ice nucleation for NaCl 2H 2 O. At and above K, deliquescence occurs; at and below K, heterogeneous ice nucleation in the deposition mode takes place. [27] The AIDA records of our third selected expansion cooling cycle (Experiment 5, Figure 5) are representative for all experiments that were started below 226 K. At these lower temperatures, the crystallized particle ensembles are dominated by NaCl 2H 2 O. During Experiment 5, we observe heterogeneous ice nucleation starting at a threshold of S ice = 1.20 (RH w = 75%, vertical black line). Just before the strong increase in the ice particle number concentration, there is a slight decrease in the trace of the depolarization ratio, which indicates the deliquescence transition of the small number fraction of anhydrous crystals. This deliquescence step, however, is not as clearly resolved as at higher temperatures where larger fractions of anhydrous particles are present, and we therefore did not include this point into the trajectory of Experiment 5 in Figure 4. At starting temperatures even lower than in Experiment 5, heterogeneous ice formation initiates before the deliquescence relative humidity of anhydrous NaCl is surpassed. Therefore, we cannot unambiguously resolve whether ice formation is solely due to deposition mode nucleation on the NaCl 2H 2 O particle fraction or, at temperatures below 220K, additionally involves the minor percentage of anhydrous NaCl crystals that are also present in the particle ensemble. We will further discuss this issue in the next section Quantitative Analysis and Discussion Onset Conditions for Ice Nucleation [28] In Figure 6, we have summarized the temperaturedependent onsets for deposition mode nucleation on crystalline sodium chloride dihydrate particles from all conducted expansion cooling cycles. Different symbols are used to differentiate between the experiments where the ultrasonic nebulizer (squares) and the atomizer (circles) were used for aerosol generation, which strongly affects the mean diameter of the number size distribution of the aerosol population. Filled symbols denote the nucleation onset for the first expansion cooling cycle performed with the crystallized particle ensemble. In most experiments, we have probed S ice hom. freezing UN,1 st UN, 2 nd AM,1 st AM,2 nd Wise et al. anhydrous Wise et al. dihydrate T / K Figure 6. Ice saturation ratio versus temperature for the onset of heterogeneous ice nucleation on the crystallized NaCl and NaCl 2H 2 O particle ensembles, as derived for a threshold ice particle number concentration of 1 cm 3. Different symbols are used to distinguish between the experiments where the ultrasonic nebulizer (UN) and the atomizer (AM) were used for aerosol generation. Filled symbols denote the onset conditions for the first expansion cooling runs, whereas open symbols correspond to those from the second expansion cooling cycles where the same aerosol particles were probed again on their ice nucleation ability after the ice crystals from the preceding run had sublimed. The two solid lines denote the average S ice values for the onset of depositional ice nucleation on NaCl (gray) and NaCl 2H 2 O (black) particles from the Wise et al. s [2012] study. The dashed black line denotes the homogeneous freezing threshold [Koop et al., 2000b]. 4617

9 the aerosol load again in a second expansion cooling cycle at the same starting temperature and initial relative humidity, which was conducted after the ice cloud from the preceding expansion run had sublimed. This was done to investigate whether the ice nucleation ability of the NaCl 2H 2 O particles changes after they have already been involved in ice cloud formation. The nucleation onsets from the second expansion cooling cycles are shown as the open symbols. As a comparison, the average nucleation onsets for anhydrous and dihydrate NaCl from the Wise et al. s [2012] study are also shown. Please note that we have defined the onsets as the times when 1 cm 3 ice particles have nucleated. For the experiments with the atomizer (AM, Experiments 5 and 6) where on the order of 10 4 cm 3 seed aerosol particles were present, we are thus sensitive to an about 0.01% number fraction of the aerosol particles becoming ice-active at the nucleation onset. For the experiments with the ultrasonic nebulizer (UN, Experiments 1 4) with a seed aerosol number concentration of the order of 10 2 cm 3, the nucleation onsets correspond to an ice-active fraction of already about 1%. At first glance, this seems to make the comparison between the results from the AM and the UN experiments less meaningful. The AIDA data for Experiment 5 (Figure 5), however, reveal that the ice particle number concentration rapidly increases after the nucleation onset from 1 to above 100 cm 3. This indicates that the nucleation onset for this AM experiment, corresponding to only a 0.01% active fraction, does not just represent a minor percentage of the aerosol population with particularly high ice nucleation ability. Instead, a larger fraction of the NaCl 2H 2 O particle ensemble starts to become ice active within a narrow range of S ice values above this nucleation threshold, justifying that the latter can indeed be compared to those inferred from the UN experiments. The application of the INAS concept (section 3.2.2) will further illustrate the good agreement between the AM and the UN experiments. [29] The AIDA results for the nucleation onsets feature the following characteristics: [30] 1. The threshold ice saturation ratios are rather insensitive to temperature. The highest onset value of S ice = 1.25 is observed for the highest nucleation temperature of K (Experiment 2). At colder temperatures, the S ice values vary between 1.15 and [31] 2. The nucleation thresholds do not depend on the size of the seed aerosol particles. The smaller sized NaCl 2H 2 O particle ensembles generated with the atomizer (AM) show the same onset conditions for deposition mode nucleation as the larger sized aerosol populations generated with the ultrasonic nebulizer (UN). [32] 3. The ice nucleation ability does not notably change in repeated expansion runs performed with the same aerosol load. This excludes any type of preactivation mechanism, representing a memory effect by which seed aerosol particles that already have participated once in ice crystal formation maintain a higher ice nucleation ability for succeeding nucleation events [Pruppacher and Klett, 1997]. [33] Note again that only the nucleation onsets above 220 K can unambiguously be related to NaCl 2H 2 O, because the deliquescence transition of the anhydrous particle fraction was detected before the ice nucleation onset, and therefore this particle fraction can be excluded from contribution to heterogeneous ice nucleation. We assume, however, that this also holds for the experiments below 220 K, given that the aerosol population is dominated by the dihydrate particles and that Wise et al. [2012]have always observed ice formation to initiate on NaCl 2H 2 O in a mixture where both anhydrous and dihydrate NaCl crystals were present. In comparison with the Wise et al. s [2012] study, the AIDA experiments yield higher nucleation onsets for deposition mode nucleation on NaCl 2H 2 O. In addition, the transition regime of the two competing processes deliquescence and heterogeneous ice nucleation is shifted to lower temperatures. In AIDA, we still have detected deliquescence of NaCl 2H 2 O to occur at K, whereas exclusively heterogeneous ice nucleation was observed in the Wise et al. s [2012] experiments at that temperature. [34] The ensemble of deposited NaCl 2H 2 O crystals in the Wise et al. s [2012] experiments thus contained at least one extremely efficient ice nucleus that triggered deposition mode nucleation at only 2% supersaturation. Such efficient ice nuclei were either not present in AIDA, or their number concentration was simply too low to become detectable as a distinct ice crystal mode by our instrumentation. At least for Experiments 5 and 6 with seed aerosol number concentrations of about 10 4 cm 3, we had the same sensitivity to freezing compared to the Wise et al. s [2012] study. The airborne particles probed in AIDA, however, had smaller sizes than those investigated by Wise et al. [2012], thereby reducing the probability for the existence of an ice-active site that could promote ice formation at very low supersaturation levels. Moreover, the dihydrate crystals probed in AIDA could have had a lower degree of surface roughness compared to those investigated by Wise et al. [2012], thereby featuring a lower ice nucleation ability. Concerning the Wise et al. s [2012] experiments, one cannot tell whether the low reported nucleation onsets are representative for a larger subset of the deposited NaCl 2H 2 O crystals because only the nucleation of the first ice crystal was monitored. Ice active fractions or INAS densities as a function of S ice could not be determined. Note that we also cannot completely exclude the possibility that Wise et al. [2012], instead of the dihydrate, have probed a different hydrated species of NaCl. Another difference between both studies is the magnitude of the humidification rate, which in the AIDA expansion cooling experiments is above the upper threshold value of 10% RH per minute reported for the Wise et al. s [2012] experiments. The higher humidification rate in AIDA, however, would only affect our results if there is a significant time delay between the actual nucleation onset and the time when the pristine deposition ice germs have grown to ice crystals of sizes detectable by the optical particle counters. For the temperatures covered by our experiments, Möhler et al. [2006] have estimated this time delay to be about 4 s (223 K). The absolute difference between the S ice values for our inferred nucleation onsets to those measured 4 s earlier is only about This bias is much smaller than the difference between the S ice values for the dihydrate from the Wise et al. [2012] and our study. We are therefore strongly inclined to rule out that the difference in humidification rate explains the different results for the freezing onsets. [35] Notwithstanding the somewhat higher nucleation onsets for NaCl 2H 2 O as inferred from the AIDA 4618