Ice Formation by Sulfate and Sulfuric Acid Aerosol Particles under Upper-Tropospheric Conditions

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1 3752 JOURNAL OF THE ATMOSPHERIC SCIENCES Ice Formation by Sulfate and Sulfuric Acid Aerosol Particles under Upper-Tropospheric Conditions YALEI CHEN, PAUL J. DEMOTT, SONIA M. KREIDENWEIS, DAVID C. ROGERS, AND D. ELI SHERMAN Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado (Manuscript received 7 October 1999, in final form 21 March 2000) ABSTRACT Ice formation in ammoniated sulfate and sulfuric acid aerosol particles under upper-tropospheric conditions was studied using a continuous flow thermal diffusion chamber. This technique allowed for particle exposure to controlled temperatures and relative humidities for known residence times. The phase states of (NH 4 ) 2 SO 4 and NH 4 HSO 4 particles were found to have important impacts on their ice formation capabilities. Dry (NH 4 ) 2 SO 4 particles nucleated ice only at high relative humidity (RH 94%) with respect to water at temperatures between 40 and 60 C. This result suggested either an impedance or finite time dependence to deliquescence and subsequent homogeneous freezing nucleation. Ammonium sulfate particles that entered the diffusion chamber in a liquid state froze homogeneously at relative humidities that were 10% lower than where ice nucleated on initially dry particles. Likewise, crystalline or partially crystallized (as letovicite) NH 4 HSO 4 particles required higher relative humidities for ice nucleation than did initially liquid bisulfate particles. Liquid particles of size 0.2 m composed of either ammonium sulfate or bisulfate froze at lower relative humidity at upper-tropospheric temperatures than did m sulfuric acid aerosol particles. Comparison of calculated homogeneous freezing point depressions suggest that size effects on freezing may be more important than the degree of ammoniation of the sulfate compound. 1. Introduction Aerosol particles play an important role in the formation of clouds. In the presence of aerosol particles, condensation and freezing nucleation occur more readily, at conditions that actually exist in the earth s atmosphere. The conditions for formation of cirrus clouds in the upper troposphere (UT) are of great interest in atmospheric research. Cirrus clouds are composed of ice crystals, and form at temperatures from 30 to 70 C (Warneck 1988). There are two possible ways that cirrus clouds impact global climate change. They reflect incoming solar radiation energy back to space, which results in a cooling effect. However, cirrus clouds also reduce the infrared longwave emission from the earth s surface to space and thereby have a warming influence. The net effect of cirrus clouds is still uncertain in quantitative models of radiation and global warming. Therefore it is of interest to understand the conditions that lead to their formation, microphysical properties, and persistence. Some field measurements suggest that sulfuric acid Corresponding author address: Sonia M. Kreidenweis, Department of Atmospheric Science, Colorado State University, Fort Collins, CO droplets are the most abundant particulate substances in the UT (Brock et al. 1995). During the National Aeronautics and Space Administration (NASA) Subsonic Aircraft: Contrail and Cloud Effects Special Study (SUCCESS) field mission of 1996 several investigators (Chen et al. 1998; Twohy and Gandrud 1998; Talbot et al. 1998) found a strong sulfur component of the aerosol and detected ammonia in both bulk and individual aerosol samples. High mass fractions of water-insoluble material from surface sources were also found, consistent with the measurements of Hagen et al. (1994). Murphy et al. (1998) recently also noted the ubiquitous presence of organic components in upper-tropospheric aerosols. The role of aerosols in nucleating cirrus cloud particles is uncertain. Several investigators propose that the formation of cirrus clouds involves ice formation in ammoniated sulfate aerosols and suggest that homogeneous freezing is one possible nucleation process (Sassen and Dodd 1988; Heymsfield and Sabin 1989; DeMott et al. 1994; Jensen et al. 1994; Tabazadeh and Toon 1998). Knowledge of the chemical state of sulfate and sulfuric acid aerosol particles is required to understand the physical process of cirrus cloud formation. In relatively large bulk samples of aqueous ammonium sulfate solution, supercooled metastable states do not occur, and a phase diagram of the (NH 4 ) 2 SO 4 H 2 O system can be constructed from the thermodynamic model of Clegg et al. (1998), as shown in Fig. 1. The 2000 American Meteorological Society

2 15 NOVEMBER 2000 CHEN ET AL FIG. 1. Temperature-dependent phase diagram of (NH 4 ) 2 SO 4 H 2 O binary system. Line a represents the solubility of (NH 4 ) 2 SO 4 in water, and b represents the ice line where the ice phase forms in solution. These lines are shown extrapolated for metastable solution droplets below the eutectic point (marked as E ). Examples of water saturation ratio and temperature trajectories of aerosol particles, as described in the text, are also shown. The thick dotted line represents the experimental trajectory followed by solution drops for CFD sampling at 50 C. Similarly, the thin dotted line is for dry crystals. x axis may be viewed as the composition of the aqueous solution; it has been expressed in terms of the water saturation ratio in equilibrium with that composition. In this plot, the deliquescence curve (a) and the ice curve (b) represent the temperatures required for crystalline dissolution and solution freezing, respectively. A liquid solution exists between (a) and (b), and their intersection at the eutectic point (E) denotes the point of equilibrium between the three phases (crystalline, solution, ice). Within the solution regime, a decrease in temperature at a constant relative humidity will result in the appearance of the ice phase at the ice line, if there are nuclei available for ice formation. An increase in relative humidity at a constant temperature within the solid crystal regime will lead to deliquescence of solute at the deliquescence line. Below the eutectic point, the liquid phase cannot exist. For microscopic size samples, metastable liquid can occur, and the phase diagram is more complicated. Metastable liquid can exist well below curve (b) in Fig. 1 because solution droplets supercool. Pure water droplets 4 m in size can remain as liquid as cold as 235 K (e.g., Pruppacher 1995). The energy barrier to ice embryo formation prevents water from freezing homogeneously until this degree of supercooling is achieved. The same effect happens in solution droplets, but the exact position of the ice line for the metastable phase depends on the coupled effects of solution properties and surface tension on particle composition and freezing. Soluble substances decrease the freezing/melting point of a binary system as in curve (b). For submicroscopic size particles, curvature elevates the vapor pressure over solution droplets through the surface tension and thus affects the equilibrium composition and freezing temperature of small solution droplets. The net effect of all three factors is that small aerosol droplets require lower temperatures to freeze than are predicted from bulk solution at equilibrium. To model cirrus clouds, Sassen and Dodd (1988) noted that in many laboratory studies of solution droplets, the depression of the homogeneous freezing point ( T hf ) scaled in proportion with the melting point depression ( T m ), or T hf T m. (1) Here T hf is the change in freezing temperature of a solution particle, relative to the freezing point of a pure water droplet of the same size, which in turn is depressed below the freezing point of bulk water. In Eq. (1), T m assumes a positive value that increases with the solute concentration in solution droplets. Sassen and Dodd (1988) proposed that the proportionality constant was also valid at temperatures other than the temperatures at which drops freeze instantaneously, such that T* T T m, (2) where T* is the effective freezing temperature and T is the temperature of solution droplets. The effective freezing temperature may be used to calculate the homogeneous freezing rate of solution droplets by substitution into either a classical or simplified expression for the homogeneous freezing rate of pure water droplets [J hf (T), cm 3 s 1 ]. The probability of freezing, or the fraction of solution droplets of volume V d (cm 3 ) freezing in some increment of time t is given by F 1 exp[ J hf (T*)V d t]. (3) This expression permits calculation of the freezing fraction of a droplet population from a freezing rate that is empirically determined, rather than from classical theory. Classical theory does not readily explain the existence of different than 1. If T* were affected by the equilibrium melting point depression alone ( 1), then the temperature at which a given fraction of solution drops freezes is depressed from the temperature required to freeze the same fraction of pure water drops (of the same size size) by the interval T m.if 1, then

3 3754 JOURNAL OF THE ATMOSPHERIC SCIENCES additional supercooling of solution drops, over that due to the presence of solute, is required for freezing. Values of 1 suggest that some solution or surface effects cause freezing to occur more readily in the solute drops, since the temperature required for freezing a fraction F is warmer than the temperature expected from the melting point depression. Values of range from 1.4 to 2.3 for various solutes, based on laboratory studies of supermicron droplets with fixed composition, usually suspended in a fluid other than air (DeMott 2000). Few studies have determined for solutes of importance to cirrus cloud formation. Furthermore, the validity of such results for describing the freezing behavior of small haze droplets in a vapor field is unknown. DeMott (2000) has noted the equivalence of the approach given in (1) (3) versus a modified classical approach. Expression (3) has been used in modeling cirrus, where F seldom approaches a value of 1. This equation is used in the present study to interpret laboratory measurements on the freezing fraction of liquid aerosol particles in terms of coefficient. These laboratory experiments determined the freezing conditions of solution droplets in a size range of haze particles appropriate to the atmosphere. Freezing occurred while particles were freely suspended in air and interacted with a surrounding vapor field. The size of the liquid solution droplet, and thereby V d, is determined by the relative humidity and water activity (e.g., via the Köhler equation). Droplet size also determines droplet composition, from which T m may be calculated based on tabulated data or aerosol thermodynamic models (e.g., Clegg et al. 1998). In metastable systems, crystallization of solute from solution (efflorescence) does not happen at the same humidity as deliquescence. Depending on the chemical properties, supersaturation is required to form a solid embryo without the presence of nuclei. Therefore the efflorescence effect broadens the region of metastable states for solution droplets (see, e.g., Imre et al. 1997). Several investigators have recently reported results from experiments examining the low temperature phase transition of ammonium sulfate and sulfuric acid by different techniques. Bertram et al. (1996) used Fourier transform infrared (FTIR) extinction spectroscopy to measure the freezing temperatures of liquid sulfuric acid droplets in a cooled flow tube. They found polydisperse sulfuric acid droplets (average diameter of 0.4 m) supercooled by about 35 K below the temperature at which the corresponding bulk solution freezes ( 1). Cziczo and Abbatt (1999) used a similar apparatus to measure the temperature-dependent deliquescence, efflorescence, and supercooling behaviors of polydisperse ammonium sulfate aerosol particles (average diameter of 0.35 m). They found pure water drops to supercool by 39 K before freezing. In contrast, ammonium sulfate droplets exhibited less than 39 K of supercooling, and the supercooling was inversely proportional to the solution concentration ( 1). Koop et al. (1998) observed the freezing temperatures of m sulfuric acid particles supported on a hydrophobic surface using an optical microscope technique. In contrast to the flow tube results of Bertram et al. (1996), Koop et al. found that the degree of supercooling increased to a greater degree as the weight percent of solute in droplets increased ( 1.9). It is not clear if differences between experiments on supermicron droplets of fixed composition and smaller liquid aerosols in the size range of upper-tropospheric particles ( 1 m) are real or are artifacts of experimental methods. For example, the determination of composition is not a trivial matter in most studies. Also, whereas the droplet freezing studies of Koop et al. (1998) permit some resolution of the proportion of particles in which ice embryos have formed, the FTIR flow tube studies indicate only the point where ice is present at detectable levels, not where it is the dominant component. Consequently, it is difficult to compare the conditions for the onset of freezing by different methods and difficult to interpret the relevance for upper-tropospheric ice formation. The use of polydisperse aerosol particles also complicates interpretation of results, since nucleation and freezing are strongly dependent on particle size. A complication in understanding the role of soluble aerosol particles in forming ice in cirrus clouds is the already mentioned fact that they may exist as liquid or dry aerosol particles, depending on the environmental conditions and their life history. Besides efflorescence, the possibility exists that chemically different crystallites may form in solution droplets as they dry or cool. These changes in phase state can impact the conditions of redeliquescence and freezing of particles. Also, there is the possibility that crystallized particles may form ice directly by deposition nucleation. Tabazadeh and Toon (1998) and Martin (1998) have described these various scenarios. In order to be most relevant to actual nucleation by soluble aerosol particles in cirrus, the studies reported in this paper used monodisperse aerosol particles of sizes and compositions expected to be present in the upper troposphere. The aerosol particles were exposed to welldefined ice supersaturations and temperatures in a continuous flow thermal gradient diffusion chamber (CFD). Since the particles were suspended in the sample flow, there was no influence of supporting surfaces. The capability to expose either liquid or crystallized aerosol particles to various conditions also allowed for the examination of the effect of particle phase state on the nucleation process. Conditions in the chamber were selected to be relevant to temperatures and humidities in the UT. The fraction of particles that formed ice crystals could be determined as a function of the CFD sample conditions. 2. Experiment description a. Continuous flow diffusion chamber All ice formation experiments were conducted using a new laboratory version of a CFD built at Colorado

4 15 NOVEMBER 2000 CHEN ET AL State University. The prototype CFD was first described in the work of Rogers (1988, 1994). Tests conducted on this CFD with man-made and natural aerosols (Kreidenweis et al. 1998) confirmed the key capabilities of the CFD to detect, separate and collect ice nuclei from the total aerosol population. An aircraft CFD was developed later for measurements in the upper troposphere and lower stratosphere during the NASA SUCCESS experiment (Rogers et al. 1998; Rogers et al. 2000, hereafter RDKC). The laboratory CFD instrument is designed as described in RDKC. Its principles of operation are briefly described here. The CFD consists of two vertical concentric copper cylinders coated by ice. The ice surfaces are kept at different temperatures to produce both temperature and humidity gradients in the annular space between them. Two low temperature bath circulators (Neslab ULT-80) were used to distribute a coolant (Syltherm XLT, Dow Chemical) through copper coils in thermal contact with the CFD walls. By varying the temperatures of the walls, the humidity in the space between them can be controlled over the range from ice saturation (well below water saturation) to 20% supersaturation relative to water at average temperatures down to about 60 C. Into this annular space, a laminar flow of aerosol particles is injected between two layers of dry, cold, and particlefree sheath flows. The sheath flows stabilize the position of the sample stream into a region with well-defined temperature and humidity. The temperature and vapor concentrations (water and ice supersaturations) are calculated from the wall temperatures, geometry and flow rates. The lamina with particles is 10% of the total flow. The maximum ranges of relative humidity with respect to water (RH w ) and temperature across this lamina under conditions used for this study were about 3% and 1.5 C, respectively. Measurement precision was 0.6% RH w and 0.7 C. This 150-cm-long chamber gives the sample a residence time 12 s for operation at room pressure (84 kpa) in the 40 to 60 C temperature range. Calculations showed this residence time to be sufficient to achieve Köhler equilibrium between the particles used and the ambient water vapor concentration and also potentially to nucleate and grow ice crystals. Starting from a few tenths of a micron, ice crystals formed in the sample stream grow to a few micrometers inside the CFD, due to the several tens of percent of supersaturation relative to ice in the sample stream. In contrast, small liquid aerosols in equilibrium with the same conditions will remain at submicrometer sizes. Although earlier versions of CFD chambers had a low humidity (ice saturated) section near the outlet to exaggerate the size difference between water droplets and ice crystals, no evaporation section was used in the new CFD. The number concentration and size distribution of aerosol particles was observed at the bottom of the CFD chamber with an optical counter (Climet model 7350A). The optical counter has three outputs, digital, high gain, and low gain. In our experiments, only high gain and low gain data were passed to a multichannel analyzer, to observe the size distributions of small and large particles. The size resolution is a function of particle velocity. In our flow conditions, the typical limits for high gain and low gain were m and m, respectively. The size gap between the two gains limited our ability to obtain the full range of the size distribution for all sets of conditions. In this work, all initial sizes of particles used were from to 0.2 m in diameter, below the detection limit of the optical counter. By comparing the difference of size distributions, the activation fraction of ice crystals could be calculated. All conditions of the CFD, including sample location, temperatures, airflow rates, and other thermodynamic conditions were calculated in real time and recorded by the data acquisition system. The data system also monitored and recorded pressures inside/outside the system, as well as the particle size spectra measured by the optical counter over preset time intervals (12 s in this work). b. Aerosol generation and conditioning Two generation systems were used to produce sulfate and sulfuric acid aerosol particles. Figure 2 shows a schematic representation of the system used to generate ammoniated sulfate particles. Clean dry air passed through an atomizer (TSI 3076) to disperse a sulfate solution into small droplets. Solutions were made from reagent grade salts and double deionized water at different concentrations, depending on the size of aerosol required. After the atomizer, the flow was temporarily heated to 60 C, and then moisture was removed from the airstream by a diffusion dryer (TSI 3062). To further reduce the humidity of the aerosol flow and to stabilize aerosol concentrations, the exiting flow from the diffusion dryer entered a 5-L glass jar and was mixed with another dry airflow. The flow was then split, with one fraction discarded as excess and the other used as a polydisperse aerosol source for a differential mobility analyzer (DMA; TSI 3071A). The DMA was used to select monodisperse aerosol particles. The number concentration and dewpoint of the aerosol sample stream entering the CFD chamber were monitored, respectively, by an optical condensation nucleus counter (TSI model 3010) and a chilled mirror hygrometer (General Eastern 1211 optical dewpoint sensor). The dewpoint temperatures achieved in this system could be as low as 32 C, less than 2% RH w at typical room temperature. Low humidity in the sample flow was important for preventing spurious relative humidity increases and for preserving the initial phase state of aerosol particles when they entered the cold inlet manifold of the CFD. Concerns during the latter stages of this research about the consistency of the initial phase state of the ammoniated sulfates as generated, and changes in phase that might occur as the particles entered the CFD at

5 3756 JOURNAL OF THE ATMOSPHERIC SCIENCES FIG. 2. Schematic of monodispersed ammoniated sulfate aerosol particle generation system. Salt solution is atomized and dried. Sizes of particles are selected by a DMA. Before entering the CFD chamber, RH w and number concentration of aerosol sample flow is measured. See text for a description of instrumentation. warm temperatures, motivated the use of a special preconditioning apparatus. This device consisted of two parts, a saturator and a precooler. The saturator was operated at room temperature. It consisted of a stainless steel Dewar bottle with double deionized water in the bottom, stainless steel entry and exit tubes, and wicking material to distribute vapor inside the bottle. Inside the saturator, the aerosol particles experienced a humidity around 90% RH w, much higher than the deliquescence point of either sulfate aerosol. Such a high humidity ensured the existence of ammoniated sulfate (either ammonium sulfate or bisulfate) as solution drops. In the absence of the precooler, a humidified sample would cause a transient high sample relative humidity and heavy frosting problems if introduced directly into the CFD. To remove the excess water vapor from the sample stream, the precooler was operated from 25 to 30 C before the CFD. Frost was thus deposited on the walls of the precooler instead of on the inside of the CFD, and the aerosol sample equilibrated to ice saturation at the precooler temperature. Frost that accumulated in the joint between the precooler and the saturator had to be removed at regular intervals when using the saturator precooler system. In the absence of the saturator, the precooler served to alter the thermodynamic trajectory of aerosol particles entering the CFD, with potential effects on freezing activity as described in section 2d. The sulfuric acid aerosol generator was constructed based on the design of Hanson and Lovejoy (1995, 1996) and Middlebrook et al. (1997). A small drop of analytical reagent grade sulfuric acid (Mallinckrodt Company) is placed in the middle of a V-shaped glass tubing in a heated aluminum block. The temperature of the block is controlled within 0.5 C. The sulfuric acid vapor saturates the warmed airstream and becomes supersaturated when cooled down to room temperature, at which point sulfuric acid droplets are nucleated homogeneously. To avoid the possibility of contamination from ammonia or other trace contaminants, high purity compressed nitrogen (4.8 grade, General Air Company) was used as the carrier gas. The sulfuric acid was heated to between 85 and 105 C, to generate sufficient concentrations of , 0.05-, and 0.1- m diameter sulfuric acid particles. Similar to the system used for ammoniated sulfate particles, a DMA was used to extract a nearly monodisperse particle sample stream from the sulfuric acid generator. High purity nitrogen was also used as the sheath flow for the DMA. The preconditioner was not necessary with sulfuric acid particles since these are known to be liquid down to very low relative humidity. c. Procedures The procedures used for experiments were as follows. The CFD chamber was flushed with clean, dry air before rapid uniform cooling to 25 C. Thin layers of ice ( 0.01 cm) were then applied to the CFD walls by pumping room temperature deionized water into the chamber and allowing the water to drain. After further cooling to the operational temperature regime, the sheath flows were recirculated for min while the wall temperatures stabilized. Sample flow and freezing measurements were initiated at this point. The relative humidity was raised in steps of 2% 3% RH w,upto conditions of supersaturation relative to water. As the humidity was changed, sample temperatures were kept constant within 0.3 C. In this way, the dependence of ice formation on both relative humidity and temperature was measured. Although daily variations in aerosol concentrations input to the CFD occurred, ice formation results are reported as freezing fractions of the number concentration, so that the measurements are comparable. Number concentrations on the order of a few hundred to 2000 cm 3 were used, to avoid vapor competition among the growing crystals and to minimize coincidence errors in the optical particle counter. An example of the observed changes of ice crystal size distribution with humidity is illustrated in Fig. 3. The freezing fraction at each RH was calculated by integrating particle counts above a size considered too

6 15 NOVEMBER 2000 CHEN ET AL FIG. 3. The variation of size distribution of ice crystals (measured at the CFD outlet) nucleated from initial m sulfuric acid particles as a function of sample relative humidity at 60 C sample temperature. The y axis is the normalized fraction of particles in each size bin. The threshold size for counting ice crystals at 1.6 m is shown. This was clearly a conservative threshold for small (0.05- m dry size) sulfuric acid particles. large for haze particles. All of the measurable changes in activated fraction and particle size were observable in the low gain optical particle counter size range (lower size limit 2.7 m) at temperatures 55 C and warmer. It was necessary to use high gain optical counter data at 60 C and under those conditions, a cutoff size to differentiate haze particles from ice crystals was set at 1.6 m. This cutoff size was necessary to prevent false counting of ammoniated sulfate haze drops (see section 3) and may have led to some undercounting of nucleated crystals in sulfuric acid experiments. It is also important to note that the bin sizes are approximate, as they are based on scattering from spheres whereas the actual measurement is made on an irregular, crystalline particle. After several hours of operation, ice crystals were being produced by frost evaporation, growth, and splintering of ice on the walls. A filter was inserted into the inlet flow between experiments to monitor potential leaking and to detect when this ice crystal production became significant. At that time, the chamber was warmed, and fresh layers of ice were applied. d. Particle phase state considerations To preface discussion of the experimental results, it is useful to consider the thermodynamic and compositional history of the various aerosol particles in experiments and the potential effects on the results. 1) SOLUTION DROPS If sulfate or sulfuric acid particles enter the CFD in the liquid state, such as is assured using the preconditioner system, the processes occurring in the CFD will be as illustrated in Fig. 4. The sample air adjusts to the temperature, humidity, and flow conditions during the FIG. 4. Schematic evolution of solution drops and ice crystals passing through CFD. Haze droplets (circles) grow and dilute for a short distance. Then some droplets freeze (stars). Some ice crystals freeze later than the others, therefore they are smaller at the CFD outlet. first several tenths of seconds of residence time in the CFD. At the end of this transition region, liquid aerosol particles within the sample lamina are exposed to the CFD temperature and humidity, and the particles grow toward their equilibrium sizes (at water subsaturation) as described by the Köhler equation. This adjustment can take up to a few seconds, depending on particle size and the humidity condition. If the temperature is too warm to form ice homogeneously, these drops will remain unchanged at their equilibrium sizes through the CFD. When the temperature in the CFD is below about 40 C, some ice embryos can be formed inside the metastable solution droplets. As more liquid water freezes to ice, the concentration of the remnant solution will be limited by the drop temperature, which goes through a transient warming during freezing due to the release of latent heat. The rate of crystallization is probably not related directly to the vapor diffusion, although it will be affected by the concentration of salt ions. Nevertheless, these ice embryos should grow quickly into large ice crystals because the environment is highly supersaturated with respect to ice.

7 3758 JOURNAL OF THE ATMOSPHERIC SCIENCES The above scenario assumes that the humidity and temperature profiles across the sample lamina are uniform. Since these profiles vary slightly with radial and axial position, some aerosol particles may experience stronger thermodynamic forcing and initiate the ice phase ahead of the others, and therefore have longer growth times. Those that form ice later, due to thermodynamics and the stochastic nature of freezing, will have less time to grow. The consequence of different growth times is a broadened size distribution of ice crystals. This has been observed in the optical particle counter (OPC) data (Fig. 3). It is possible that some of the particles begin freezing near the end of the chamber and are therefore not identified as ice crystals when the OPC data are processed. To avoid underestimating the number of ice crystals, a longer residence time can be obtained by reducing flow rate. However, a critical flow rate is required at each temperature. Flow rates below the critical value lead to thermal buoyancy effects and resultant uncertainty in the calculated position of the lamina inside the CFD. Operating the CFD at lower pressure lowers the critical flow rate, so this is a potential means for increasing residence times in future studies. 2) DRY CRYSTALS: (NH 4 ) 2 SO 4 Very little is known about the mechanisms of ice formation by dry soluble (or insoluble) particles at temperatures below 40 C. Therefore the following discussions are mainly hypothetical. In our experimental studies, ammonium sulfate aerosol particles were the only ones that could be completely dried in the generation system. Therefore this discussion describes the evolution of the dry ammonium sulfate particles inside the CFD. Dry ammonium sulfate particles were passed to the CFD whenever the preconditioner unit was not used or whenever the saturator was bypassed during use of the preconditioner. In either case, the sample flow had very low water vapor content ( 1% 5% RH w at 20 C). Thus, the only difference in the two dry aerosol cases was that the precooler lowered the temperature of the aerosol stream by up to 50 C in advance of the CFD transition region. This precooling would extract any excess water vapor above ice saturation at 25 to 30 C, and it would assure a temperature trajectory below the eutectic temperature ( 19 C) in Fig. 1 (thin dotted line). In the absence of precooling, the temperature trajectory would also pass beneath the eutectic, except perhaps when the sample dewpoint temperature exceeded the eutectic temperature. When the dry aerosols entered the low temperature CFD transition region and adjusted to the CFD humidity, there were two possible ice nucleation pathways they could take: direct formation of ice by deposition, or deliquescence followed by homogeneous freezing. At the point that ice saturation is exceeded, deposition nucleation is possible, although not likely. For higher CFD relative humidity set points, the dry particles cross the extrapolation of the liquid phase line. The extrapolation of this line below the eutectic point on the phase diagram of Fig. 1 is assumed to be valid for metastable aerosol particles. In other words, there seems no reason to expect the eutectic temperature to have any meaning for the aerosol particles if the ice phase is not already present. This is an issue of some debate, as evidenced by statements in the literature that deliquescence will not occur below the eutectic temperature (Imre et al. 1997; Tabazadeh and Toon 1998; Martin 1998). If dry particles deliquesce, they should freeze to ice in the same manner as for solution drops [section 2d(1)]. If the particles remain crystallized, supersaturation with respect to ice could ultimately drive ice formation by deposition (Huffmann 1973). 3) PARTIAL CRYSTALLIZATION OF NH 4 HSO 4 AEROSOLS There is now strong evidence, based on recent thermodynamic models (Clegg et al. 1998) and laboratory vapor pressure and bulk phase measurements (Chelf and Martin 1999; Yao et al. 1999), that letovicite [(NH 4 ) 3 H(SO 4 ) 2 ] crystallizes within liquid ammonium bisulfate when it is dried sufficiently at temperatures below 310 K. A liquid phase remains in the form of acidified bisulfate. These results were not known at the start of this study but were discovered in the CSU laboratory at about the same time as the above-noted studies. Using a humidified tandem differential mobility analyzer (HTDMA), letovicite was identified as the major compound in the dried bisulfate aerosol from the generation system used in the present study, on the basis of the observed deliquescence RH w at room temperature (Brechtel and Kreidenweis 2000). Hence, the liquid/anhydrous solid phase line relevant to our studies of the bisulfate system, and plotted with our results in the figure, is that for letovicite. The eutectic temperature of the bisulfate system is 31 C (Chelf and Martin 1999). As a consequence of the multiphase nature of the generated bisulfate aerosols and the lower eutectic temperature, it was more likely that the bisulfate aerosols that were not exposed to precooling had a thermodynamic trajectory upon entering the CFD that crossed the deliquescence line at a temperature warmer than the eutectic. This should have led to the dissolution of letovicite in most cases. Use of the saturator and precooler should have led to the same situation of a pure liquid NH 4 HSO 4 particle entering the CFD. The expected behavior is then as described in section 2d(1). Precooling ( 25 to 30 C) alone should only have made it more likely that the particles did not encounter a deliquescence condition until the temperature was less than the eutectic temperature. Then the question of the feasibility

8 15 NOVEMBER 2000 CHEN ET AL of deliquescence is the same as discussed in section 2d(2). 3. Results and discussion Three series of experiments were conducted with monodisperse aerosol particles: 0.2- m ammonium sulfate, 0.2- m ammonium bisulfate, and m sulfuric acid particles. A single DMA produces not just one particle size, but also some percentage of larger particles that contain multiple charges (i.e., doublets and triplets). Thus, the 0.2- m monodisperse aerosol particles contained 51% 0.2- m, 33% m, and 16% m particles. For m sulfuric acid drops, the doublets and triplets were and m, respectively, and the percentages of singlets, doublets, and triplets were 90%, 9%, and 1%. The existence of multiply charged particles complicated the interpretation of ice formation results to some degree. The likely effect of multiplets was that the onset conditions for ice formation were probably dominated by the largest particles, especially in the cases of the ammoniated sulfates. All multiplet sizes probably contributed to the total ice formation at higher activation fractions. Compensation for this factor in analyzing and interpreting results is discussed in sections 3a and 3d. To eliminate the impacts of doublets and triplets on interpreting the difference between haze and ice particles, the lowest ice crystal size threshold used to process the particle size spectra was 1.6 m, which is four times larger than the m particles. Under the conditions (e.g., 60 C, 90% RH w ) of ice formation, the haze particles will be at most 2.5 times larger than their dry size, based on the Köhler equation. Under ordinary laboratory conditions, sulfuric acid drops cannot be crystallized. Therefore the preconditioner was not needed for H 2 SO 4 experiments. It was not feasible to generate 0.2- m sulfuric acid particles with the sulfuric acid generation system used. The m sulfuric acid size is consistent with the observations of young aircraft exhausts (Schumann et al. 1996) and are representative of tropospheric sulfuric acid drops generated via gas-to-particle conversion. Therefore the experimental results are directly relevant to formation conditions of aircraft contrails, as well as formation of cirrus clouds. Other sizes of sulfuric acid particles, from to 0.1 m, were also tested in a few cases, for the purpose of investigating the size effect on our results. As previously stated, the activated fraction is the ratio of the ice crystal number concentration counted by the OPC divided by the input aerosol number concentration. Conditions for activated fractions of 0.1%, 1%, and 10% were determined in all experiments. We define 0.1% activated fraction as the onset conditions for ice formation. At this level, it may not be possible to rule out a contribution from heterogeneous ice nucleation by contaminants within the reagent grade sulfate salts and double deionized water used for making atomized solutions. However, based on the low level of heterogeneous contamination noted by DeMott and Rogers (1990) for cloud droplets formed in similar aerosols and the fact that solutions were remade on a regular basis, we believe that heterogeneous effects were not significant even at the 0.1% activation level. A total of 96 tests were conducted at temperatures between 40 and 60 C: 35 tests with 0.2- m ammonium sulfate, 48 tests with 0.2- m ammonium bisulfate, and 13 tests with m sulfuric acid. These laboratory ice formation results are related to the formation of cirrus cloud in the natural atmosphere and also compared to experimental observations from other published studies. a. (NH 4 ) 2 SO 4 From the previous discussion, there are two possible mechanisms for ice formation by (NH 4 ) 2 SO 4 particles in the CFD and in the atmosphere: homogeneous freezing of solution drops and deposition nucleation by anhydrous particles. As the experiments were conceived, it was expected that dry ammonium sulfate crystals would produce results consistent with either deliquesced or dry particles in response to CFD conditions. It became apparent that variability in the initial RH w of the air provided to the CFD might actually have altered the phase state of the particles during the rapid cooling at the entry to the CFD in some of the first experiments performed without preconditioning. There were a total of 18 tests of ammonium sulfate without any preconditioning. Ten of these gave results that were more representative of the known dry crystals experiments on the basis of later preconditioned results (see below). The measurements of ice formation conditions on nominal 0.2- m (NH 4 ) 2 SO 4 aerosols are shown in Fig. 5. Experiments conducted without the preconditioner are plotted as filled marks in Fig. 5. The freezing behavior of preconditioned wet aerosols agreed with data from several of the initial experiments conducted with no preconditioner, while the behavior of dry and precooled particles agreed with our other early experiments. For this reason, the experiments without the preconditioner have been objectively segregated by symbol in Fig. 5, to indicate dry versus wet particle results for F (0.1% activation). The onset of ice formation (0.1% activation, empty circles in Fig. 5) is close to the 1.0 line, defined for the monodisperse particles using Eq. (1) (3), the Köhler equation and J hf (T) from DeMott et al. (1997). Calculations were made for each of the multiplet particle sizes present in the DMA output in order to define the RH w for a cumulative fraction nucleated of F 0.01 at 1.0. Water activity was taken from the model of Clegg et al. (1998) in determining the equilibrium size and volume of particles at different RH w conditions. The 1% activation results (triangles in Fig. 5) agree fairly well with the degree of

9 3760 JOURNAL OF THE ATMOSPHERIC SCIENCES FIG. 5. Experimental ice formation results overlain on the phase diagram of the (NH 4 ) 2 SO 4 H 2 O system. Bulk solution (a) and supercooling (b) curves are given as thick solid curves, extrapolated below the eutectic temperature as thick dashed curves. Circles (F 0.001, wet), squares (F 0.001, dry), and triangles (F 0.01, wet) mark data for initial 0.2- m diameter particles in this work; crosses (Detwiler 1980; F 0.01; average dry diameter 0.2 m) and diamonds (Cziczo and Abbatt 1999; F undefined; average wet diameter 0.35 m) are from other studies. A line representing conditions for continental cirrus formation (Heymsfield and Milosevich 1995) is also identified. Thin and dotted lines of 1 and 1.7 are based on Eqs. (2) and (3), with F 0.01 and haze particles at diameters in equilibrium with the vapor (diameter 0.2 m atrh w 2%). Filled marks represent the experiments without the preconditioner, while open marks are preconditioned results. supercooling proposed for upper-tropospheric aerosol particles by Sassen and Dodd (1988), that is, the 1.7 line. Data for F 0.1 (not shown) also suggested 1.7. The degree of supercooling required for ice formation mostly exceeds the depression of the melting curve by about 38 C, even more so at lower RH w (higher solute concentration). Particles that were precooled only, and thus should have remained dry, required higher RH w than wet particles to form ice. The onset of ice formation for these particles (0.1% activation, squares) is also shown in Fig. 5. Ice formation at the 1% activation level was not observed for dry particles at RH w 100%, except at 60 C ( 96% RH w ). The humidities required for ice initiation by the dry particles are about 8% 10% RH w higher than those of droplets at the same temperature. At 60 C, the relative humidity with respect to ice (RH i ) is 167% for the onset of ice formation on dry particles. If this case represents deposition nucleation, these results indicate that dry 0.2- m ammonium sulfate particles are very poor heterogeneous ice nuclei. If the ice formation mechanism is homogeneous freezing nucleation, then deliquescence is impeded for some reason toward higher RH w from the condition expected based on the model of Clegg et al. (1998) at low temperatures. Otherwise, results would be expected to follow the wet case at all temperatures for which the onset RH w of wet particles exceeded the deliquescence RH w. These experiments demonstrate that the initial experimental series without the precooler had a wider variation of results. Apparently, the particles were deliquescing in some cases and not in others. The state of the particle was sensitive to the thermodynamic path in going from dry room temperature conditions to humid low temperature conditions. It is also interesting to note in Fig. 5, and in the similar figures that follow, that freezing at RH w s near 100% appears to occur at colder temperatures than expected; particles in equilibrium with such high humidities should be quite dilute and should freeze at temperatures closer to those for a pure water particle of the same size. A likely cause of this behavior is that the particles did not have sufficient time in our experimental apparatus to reach their equilibrium sizes when RH w 100%, with the result that the compositions of the particles that froze were more concentrated than theory would predict and thus required lower temperatures to freeze. The results of other laboratory studies of ice formation from (NH 4 ) 2 SO 4 particles are also shown in Fig. 5. In both studies, submicron particles were used. The data from Detwiler (1980) were obtained in a static diffusion chamber using (NH 4 ) 2 SO 4 particles of unknown phase state. His results for 1% ice activation are consistent with the freezing conditions of wet sulfate aerosols within the uncertainty of measurements. In particular, Detwiler (1980) noted the deeper supercooling with respect to the bulk phase needed to nucleate particles below 50 C. In contrast, the flow tube experiments on wet (NH 4 ) 2 SO 4 from Cziczo and Abbatt (1999) indicate ice formation at much lower relative humidity and decreasing degrees of supercooling with respect to the bulk phase. The fact that their results fall above the 1 curve appears to indicate an ordering effect of the solute on homogeneous freezing of solution droplets. This is contrary to the expected solution effects on freezing. The current results for the onset of freezing wet ammonium sulfate particles agree with Cziczo and Abbatt (1999) down to about 88% RH w and then deviate to follow the 1 line. b. NH 4 HSO 4 Experiments with NH 4 HSO 4 aerosols could also be separated into two categories based on insights provided by later preconditioned experiments. The results of experiments are shown in Fig. 6. The approach and ex-

10 15 NOVEMBER 2000 CHEN ET AL FIG. 6. Experimental ice formation results overlain on the NH 4 HSO 4 H 2 O phase diagram. Bulk solution (a), supercooling (b), and cirrus onset curves are as indicated in Fig. 5. Circles (F 0.001, wet), squares (F 0.001, dry), and triangles (F 0.01, wet) mark data for initial 0.2- m diameter particles in this work. Lines of 1 and 1.7 are based on Eqs. (2) and (3), with F 0.01 and haze particles in equilibrium with vapor (diameter 0.2 m atrh w 2%). Filled and opened marks have the same meaning as in Fig. 5. perimental process were exactly the same as for the ammonium sulfate experiments, except we expected that the particles would be partially crystallized at low RH w in our generation system. In the 44 experiments with 0.2- m particles, there were 30 tests without any preconditioning (shown as filled marks); 14 tests were with controlled initial phase states. Among those without preconditioning, only six were determined to reflect the partially crystallized phase state of particles that were generated at low humidity (filled squares in Fig. 6). In the other 24 cases, liquid bisulfate was presumed to be present after particles adjusted to the set CFD conditions on the basis of preconditioned aerosol experiments. The experiments with the preconditioner identified different ice formation behaviors that were related to differences in the phase states of ammonium bisulfate particles. Without the saturator but with the precooler in place, the partially crystallized solution droplets of letovicite with acidic ammonium bisulfate required very high RH w for the onset of ice formation (open squares in Fig. 6). Ice formation did not occur until water supersaturation at temperatures higher than 50 C and required RH w above 95% between 50 and 60 C. With the information available, we cannot tell if the acidic solution ultimately freezes, or if the dissolution of letovicite did not occur, or is time dependent at relative humidities above the extrapolated deliquescence RH w. The latter case might indicate that the experimental residence time was not sufficient. It is possible that the amount of letovicite crystallized in the bisulfate aerosols may have had some impact on results. The degree of conversion to letovicite would be expected to be much greater upon drying to very low RH w (as in these experiments) in comparison to the degree of dissociation achieved in atmospheric situations. With the saturator and precooler in place, the aerosol particles should have been retained as fully liquid drops without partial crystallization before entering the CFD. These drops began to freeze (0.1% activated fraction) along a line defined by 1 in Eq. (1) at temperatures above 50 C (open circles in Fig. 6). No impact of the requirement for dissolution of letovicite is detected in this case, which is what is expected for solution drops. A condition of F 0.01 required higher RH w (and higher ) on average. Data collected for F 0.1 (not shown) required still higher RH w. Similar ice formation behaviors were noted between wet or deliquesced ammonium bisulfate droplets and ammonium sulfate droplets. Perhaps this is not surprising since the homogeneous freezing process is responsible for ice formation in each case and the ionic compositions inside solutions of NH 4 HSO 4 and (NH 4 ) 2 SO 4 are similar. c. H 2 SO 4 Applying the same procedure as for the ammoniated sulfate experiments, the conditions for 0.1%, 1%, and 10% nucleated fraction were measured at 40, 45, 50, 55, and 60 C. The results for m diameter sulfuric acid particles are shown in Fig. 7. At 60 C, 0.1% of the initially m sulfuric acid droplets were frozen at 88.5% RH w in the time available. This result compares to an RH w of about 84% for the onset of ice formation in the larger liquid (NH 4 ) 2 SO 4 and NH 4 HSO 4 particles. The degree of supercooling of H 2 SO 4 particles determined in the current study is greater than in Bertram et al. (1996), who used 0.4- m particles. From Fig. 7, our results for m sulfuric acid particles agree better with Koop et al. (1998). The comparison to Koop et al. (1998) is not straightforward in this figure because they studied much larger liquid particles, the composition of which were fixed and not determined by RH w. Consideration of size effects in homogeneous freezing and discussion of comparisons to other datasets are given in the next section. d. Composition and size effects on homogeneous freezing nucleation Statistical analyses for differences in ice formation conditions were conducted on the 0.1% liquid freezing

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