Immersion and Contact Efflorescence Induced by Mineral Dust

Subscriber access provided by READING UNIV

Article

Immersion and Contact Efflorescence Induced by Mineral Dust Particles Shuichi B. Ushijima, Ryan D. Davis, and Margaret A. Tolbert J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b12075 • Publication Date (Web): 13 Jan 2018 Downloaded from http://pubs.acs.org on January 13, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry A is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Immersion and Contact Efflorescence Induced by Mineral Dust Particles

Shuichi B. Ushijima1,2, Ryan D. Davis3, and Margaret A. Tolbert1,2*. 1 Department of Chemistry University of Colorado Boulder, 216 UCB Boulder, CO, 80309 2 Cooperative Institute for Research in Environmental Science, 216 UCB Boulder, CO, 80309 3 Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, Berkeley, CA, 94720 *Corresponding Author e-mail: [email protected] phone: (303) 492-3179

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 33

Abstract The phase state of inorganic salt aerosols impacts their properties including the ability to undergo hygroscopic growth, catalyze heterogeneous reactions, and act as cloud condensation nuclei. Here, we report the first observation of contact efflorescence by mineral dust aerosol. The efflorescence of aqueous ammonium sulfate ((NH4)2SO4) and sodium chloride (NaCl) droplets by contact with three types of mineral dust particles (illite, montmorillonite, and NX Illite), were examined using an optical levitation chamber. Immersion mode efflorescence was also studied for comparison. We find that in the presence of mineral dust particles, crystallization occurred at a higher relative humidity (RH) when compared to the homogeneous phase transition. Additionally, crystallization by contact mode efflorescence occurred at a higher RH than the corresponding immersion mode. Crystallization efficiencies in the contact mode exhibited an ion-specific trend consistent with the Hoffmeister series. Estimates for lifetimes of a salt droplet to collide with dust particles suggests that collisions between the two aerosol types are likely to occur before the salt aerosol is removed by other atmospheric processes. Such collisions could then lead to the crystallization of salt droplets that would otherwise have remained liquid, changing the overall impact that salt aerosols have on atmospheric chemistry and climate.


Page 3 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Introduction Aerosols in the atmosphere directly affect global and regional climate by interacting with solar and terrestrial radiation and indirectly impact climate by influencing cloud formation1-4. In addition to composition, size, and abundance in the atmosphere, the particle’s liquid water content is an important factor in its climate impact1-3,5. To accurately model the influence of aerosols on climate, the phase states and the conditions under which phase transitions occur must be established. Soluble inorganic aerosols are found ubiquitously in the atmosphere and undergo transitions from a solid to aqueous phase (deliquescence) at a characteristic relative humidity, the deliquescence relative humidity (DRH). Transitioning from an aqueous droplet to solid crystalline phase (efflorescence) requires nucleation and is less predictable. Typically, the DRH is higher than the efflorescence relative humidity (ERH) resulting in a region between the two RH values where an aqueous solution is metastable. While the DRH of a salt can be predicted thermodynamically, the ERH must be determined experimentally because it is a process involving nucleation1,6. The homogeneous ERH has been well characterized for many atmospheric salts6,7, but it is known that the presence of a heterogeneous nucleus, such as a mineral dust particle, can increase the ERH8-13. Since aerosols are commonly found internally and externally mixed in the atmosphere, it is important to study the impact of solid particles on salt efflorescence. Heterogeneous efflorescence can occur via two distinct pathways: contact or immersion. Contact efflorescence occurs when a heterogeneous nucleus comes into contact with the exterior surface of a liquid aerosol and induces crystallization. In contrast, when an insoluble particle induces nucleation from within an aerosol, immersion efflorescence results. Relative humidity at the time when the heterogeneous nucleus and the liquid aerosol come together will drive the



Page 4 of 33

pathway of efflorescence. If the relative humidity is lower than the contact ERH, contact efflorescence will occur. However, if the relative humidity is higher than the contact ERH, a collision between the liquid aerosol and insoluble particle will not induce effloresce and the heterogeneous nucleus will become immersed. The droplet with an immersed particle will remain liquid until the relative humidity becomes low enough for immersion efflorescence to occur. While heterogeneous nucleation of ice in both contact and immersion mode has been studied in detail14-21, much less is known about heterogeneous efflorescence. A recent study from Davis et al.11 was the first to report observations of contact efflorescence. That study examined the ERH of ammonium sulfate ((NH4)2SO4), sodium chloride (NaCl), and ammonium nitrate (NH4NO3) by contact with other soluble inorganic crystals demonstrating that soluble salt crystals can induce efflorescence upon contact at an RH that is significantly higher than the homogeneous ERH. In another study, Davis and Tolbert12 demonstrated that contact-mode efflorescence can be induced by collisions with charged amorphous organic aerosols that do not induce immersion-mode efflorescence. That study provided evidence for a hydration-mediated ion-specific nucleation pathway that is relevant to the non-equilibrium conditions following a collision, and thus unique to contact-mode efflorescence. Other studies have shown that immersed particles of metal oxides and mineral dust initiate efflorescence of (NH4)2SO4 and NH4NO3 at a higher RH than homogeneous ERH8-10,13. However, a comparison between contact and immersion efflorescence has yet to be established for heterogeneous nucleus composed of mineral dust. In the present study, efflorescence of salt droplets interacting with mineral aerosols are probed in both immersion and contact mode nucleation. Mineral dust particles are of interest as heterogeneous nuclei due to their abundance in Earth’s atmosphere22 and their ability to act as effective ice nuclei14,15. Field studies indicate


Page 5 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


mineral dust particles are the dominant ice residual under many atmospheric conditions23,24. Mineral dust is one of the most abundant species in the atmosphere by mass, with a size distribution that spans from submicron to supermicron particles. Compositionally, the particles mainly contain feldspar, quartz, and clay mineral species22. A study by Broadley et al.15 determined that illite, a clay mineral, was a major component of mineral dust particles and that NX Illite, a mixture of minerals including illite, feldspar, quartz, and kaolinite, is a good analogue of atmospheric dust. Furthermore, there is evidence that as a dust plume travels and undergoes atmospheric aging, clay minerals become more prevalent22. For example, comparisons between mineralogy of aerosols collected at Sal Island off the coast of Africa and Miami, FL show that the clay mineral’s weight percent increase over time25. Thus, for the present study, NX illite and clay minerals were chosen as possible heterogeneous nuclei. Heterogeneous efflorescence was examined by optically levitating single droplets of aqueous salts and exposing them to mineral dust particles. Three different mineral dust particles were used as heterogeneous nuclei for (NH4)2SO4 and NaCl: illite, montmorillonite, and NX Illite. To study ion-specific effects, montmorillonite was further used as contact nuclei for a range of chloride salts (NaCl, NH4Cl, and MgCl2). Although similar in structure, illite is a nonswelling clay while montmorillonite is a swelling clay, and responds differently to humidity15. For each salt aerosol and mineral dust pair, contact and immersion ERH was determined. Additionally, the immersion ERH of (NH4)2SO4 by multiple particles of immersed illite was also probed.

Experimental Preparation of Aqueous Solution Droplet



Page 6 of 33

Aqueous droplets were prepared using a 5 wt% solutions of (NH4)2SO4 (SigmaAldrich), NaCl (Fisher Scientific), NH4Cl (SigmaAldrich), and MgCl2 (Mallinckrodt) in HPLC grade water (SigmaAldrich). The aqueous salt solution was filtered through a 0.22 µm pore nylon filter and used to fill a droplet generator. The droplet generator (Microfab MJ-APB-20) ejected droplets of the filtered solution from a piezo-driven tip with a 20 µm orifice. By applying a controlled alternating positive and negative voltage to the piezo device, droplets were produced with a reproducible size range of 10 – 15 µm in diameter. While aqueous aerosols in the atmosphere are smaller than the droplets used for this study, the experiments should still be relevant for atmospheric particles large enough so that the Kelvin effect is minimal (>100nm)26.

Preparation of Mineral Dust Particles Illite (IMt-2) and montmorillonite (SWy-2b) were obtained from the Clay Mineral Society. The mixture NX Illite was obtained from Arginotec. Montmorillonite and NX Illite were used as received. Because the illite was a mixture with non-uniform grain sizes, the sample was first coarsely ground in a ceramic mortar and pestle. The coarsely ground illite was then ground further with a stainless-steel capsule and ball pestle (Wig-L-Bug) for 5 minutes to produce a fine powder. Each mineral was then mixed into HPLC grade water to make a slurry that was aerosolized with a nebulizer (Omron NE-U22). It has been shown that generating mineral dust aerosols from a slurry can change the surface properties of the particle due to a redistribution of the soluble species27. The effect that the change may have on heterogeneous efflorescence has not been studied and is not known. To prevent the nebulizer from clogging, supermicron particles from the slurry were allowed to settle out before nebulization. Settling


Page 7 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


velocities (w) used to calculate settling times for each mineral dust were determined based on Stokes’ law: 2 − = 9 where ρp and ρf are the particle and fluid densities respectively, g is the acceleration due to gravity, r is the particle radius, and µ is the fluid’s dynamic viscosity. Densities of 2.2 g/cm3 and 2.7 g/cm3 were used for illite and montmorillonite, and NX Illite respectively. A density of 0.998 g/cm3 and a viscosity of 1.002 mPa·s was used for water at 20 °C. For a particle with a diameter of 1 µm, the settling time to fall 3 cm was 12.7 and 9 hours for illite and montmorillonite, and NX Illite respectively. The size distributions of the aerosolized mineral dust particles using the method described above and measured with a scanning mobility particle sizer (SMPS TSI model 3010) are shown in Figure S1 of supporting information.

Experimental Arrangement The optical levitation apparatus has been described in detail previously28 and is shown schematically in Figure 1. Two counter propagating 532 nm ND:YaG lasers are used to optically trap droplets of aqueous solutions inside a closed flow cell with windows. The laser directed from below has a Gaussian profile while the laser directed from above has a Bessel profile. While it is possible to trap multiple droplets at different positions in the trap, this study probed single levitated droplets. A nebulizer creates a mist from the mixture of mineral dust in water. Pure nitrogen gas carries the mist through a diffusion drier where residual water is removed leaving dry dispersed mineral dust particles. The dry gas flow is then mixed with a humidified nitrogen gas flow. The ratio of dry to humidified nitrogen controls the RH inside the system. The mixed gas flow enters



Page 8 of 33

the cell from an inlet below the trapped droplet and exits above the droplet. At each inlet and outlet, the RH and temperature is measured using individual probes (Vaisala HMP60). The RH at the trapping region is calculated as the average of the two probes (±1 S.D.) The entire flow cell is attached to a sliding stage that moves in both horizontal directions. The droplet’s position is determined by the lasers and controlled by optics placed outside of the flow cell, and thus is independent of the flow cell’s position. In contrast, the mineral dust particles flow through the center of the cell. Thus, by moving the flow cell, the particle stream can be moved into and out of the droplet’s projected area.

Imaging Efflorescence and Collisions Efflorescence is monitored using light scattering focused onto two charge coupled device (CCD) cameras and recorded using a LabView program, allowing both far field and near field scattering to be imaged at the same time. The scattering pattern of a liquid droplet is distinctly different from a crystalline solid, allowing for a visual identification of efflorescence. Upon efflorescence, the droplet loses water, resulting in a significant change in mass and upward movement in the trap. The sudden change in position is used as a second determination of efflorescence. Finally, the scattered laser light is used to observe collisions between the droplet and mineral dust particles. Scattering from mineral dust particles can be imaged onto the same CCD camera that is imaging the droplet. Methods to monitor collisions between a heterogeneous nucleus and aqueous droplet are discussed in detail in Davis et al.28. Figure 2 illustrates how a collision with a heterogeneous nucleus can result in an effloresced salt aerosol or an aqueous droplet with a particle immersed inside. Far field images are shown for an experiment with (NH4)2SO4 contacted by illite. When the collision induces crystallization, the


Page 9 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


light scattering from (NH4)2SO4 changes distinctly from linear fringes to a mosaic pattern. The particle also moves up in the field of view of the image as it loses mass due to loss of water. When the collision does not cause efflorescence but instead becomes immersed inside the droplet, the linear fringes remain and the scattering does not move in the field of view. However, the immersed particle will occasionally disrupt the light and causes ripple-like features to appear in the image. In the figure, to highlight the disruptions the far field light scattering images, in green, were self-correlated. The far field image is filtered by a Gaussian smoothing function to remove noise. Then the absolute difference between the image and the image shifted by 20 pixels to the right is calculated. The resulting image of the defect is further enhanced by converting the result into a binary image by choosing a threshold intensity, where only the pixels that are brighter than the threshold will be indicated by red. As seen in the figure, the pure droplet exhibits very little defects whereas the droplet with an immersed particle shows a cluster of defects in the area where the ripples appear. With this process, we are able to determine whether a collision occurred, and whether that collision resulted in efflorescence or the mineral dust particle became internally mixed with the aqueous droplet.

Determining Heterogeneous ERH Contact and immersion ERH values were determined through different experimental methods. For contact efflorescence, an aqueous particle was trapped at a RH significantly higher than the homogeneous ERH but lower than the DRH. For each trial, the trapped droplet was introduced to a stream of mineral dust particles for 60 seconds. During the period of exposure to dust particles, the droplet was exposed to an average of a collision every 9.5±1.6 seconds. The droplet was monitored for whether it effloresced or not during the trial. After 60 seconds, if the



Page 10 of 33

droplet had not yet effloresced it was ejected from the trap and a new droplet was caught. The process was repeated to calculate a probability of efflorescence (PEff) as the ratio of observed contact efflorescence events to the number of trials done at that RH. Thus, a PEff of 1.0 implies that efflorescence was observed during every trial of 60 seconds of dust exposure conducted at that RH. Each trial had on average ≤6.3 collisions, and thus over 16% of collisions led to efflorescence. Contact ERH was determined as the RH where PEff is equal to 0.5. To determine the immersion ERH an aqueous droplet was exposed to mineral dust particles at a RH significantly higher than both homogeneous ERH and contact ERH. Once a mineral dust particle became immersed in the droplet, the droplet with the immersed mineral dust was then isolated from the particle stream, using the sliding stage, to prevent any additional collisions. The recorded video of the droplet being exposed to the mineral dust was reviewed to ensure that collisions had occurred and verify the number of immersed mineral dust particles. Droplets that had been exposed to more than three collisions were discarded. The RH inside the cell was then lowered at a rate of ~1% RH/min until efflorescence occurred. The immersion ERH was determined as the average RH (±1 S.D.) for all immersion efflorescence events observed. To further explore the immersion ERH as a function of total surface area of immersed mineral dust, multiple particles of illite were immersed into aqueous droplets and the immersion ERH was determined in a similar manner as above. However, in contrast to the immersion experiments described above, the droplet was exposed to the mineral dust particle stream for an extended period of time. By varying the exposure time, the number of immersed particles was changed. The number of particles immersed was determined by recording and analyzing videos taken during dust exposure and counting the collisions. To calculate the total surface area of the


Page 11 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


immersed illite, the particle size distribution shown in Figure S1 of supporting information was used. Based on the measured size distribution of three samples, the geometric mean was 380 nm with a geometric standard deviation of 1.63.

Results Heterogeneous Efflorescence by Mineral Dust Experimental results for contact heterogeneous efflorescence of (NH4)2SO4 and NaCl are shown in Figure 3. The data was fit with a sigmoid curve to determine the RH at PEff = 0.5 and thus contact ERH. The sigmoid curve was constrained to have a maximum value of 1 and a minimum value of 0 when the fit was performed. For both (NH4)2SO4 and NaCl the contact ERH for all three heterogeneous nuclei was well above the homogeneous ERH. Furthermore, the probability of contact efflorescence decreased relatively rapidly with RH above the contact ERH. For (NH4)2SO4, the transition was steepest for montmorillonite while the most gradual was for illite. For NaCl the steepest transition was for NX Illite and the most gradual was for montmorillonite. Results for the complementary experiments of MgCl2 and NH4Cl contact efflorescence with montmorillonite contact nuclei are shown in Figure S2 of supporting information. The contact ERH for MgCl2 was 10.9±0.6% and for NH4Cl was 54.0±0.3%. Homogeneous ERH and DRH values for MgCl2 were determined as 3.7±0.4% and 13.7±0.5% using the optical levitator. For NH4Cl, literature values for homogeneous ERH and DRH values were used as 45% and 77% respectively6. Results from both contact and immersion efflorescence experiments for (NH4)2SO4 and NaCl are summarized in Figure 4. It can be seen that the heterogeneous ERH for each salt and dust combination in both immersion and contact mode is higher than the homogeneous ERH.



Page 12 of 33

Compared to the homogeneous ERH of 35% for (NH4)2SO4 the heterogeneous ERH was 42 – 57%. For NaCl, which has a homogeneous ERH of 45%, the heterogeneous ERH ranged from 49 – 63%. The observed increases in ERH shows that the hysteresis effect can be dampened in the presence of mineral dust particles. The narrowing of the meta-stable region for an aqueous particle could thus increase the length of time that a salt aerosol stays in the crystalline phase. In addition to raising the ERH of the salts, a difference between the two modes of heterogeneous efflorescence was observed. As seen in Figure 4 the contact ERH for all pairs of dust and salt were higher than its immersion ERH. The difference between the ERH values of the two modes varied depending on the salt and dust pair and was most pronounced for NaCl with montmorillonite as the contact nucleus. The higher ERH for contact mode than immersion mode is consistent with prior work in our laboratory that found amorphous organic aerosol induced salt efflorescence via contact but not when immersed12. The higher heterogeneous efflorescence efficiency in contact mode compared to immersion mode is also consistent with findings from studies on heterogeneous ice freezing16-19. A study by Shaw et al.17 showed that when the heterogeneous nucleus was in contact with the surface of the droplet, freezing temperatures were significantly higher than when the nucleus was inside the droplet. In another study by Niehaus and Cantrell18, contact ice nucleation by soluble salts was examined. That study showed that soluble species, which are thought to be ineffective immersion mode ice nuclei, can induce freezing at significantly warmer temperatures than homogeneous freezing.

Immersion ERH as a Function of Surface Area of Immersed Illite The above results show that contact efflorescence can often be more effective than immersion efflorescence where there are less than or equal to three particles immersed inside.


Page 13 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


However, previous work has shown that the heterogeneous ERH of atmospheric salts by an immersed metal oxide particle could be enhanced by increasing the surface area of the heterogeneous nucleus9,10. Thus, immersion ERH of (NH4)2SO4 was examined as a function of the total surface area of the immersed illite by increasing the number of immersed particles and the results are shown in Figure 5. To calculate total surface area of the immersed illite each particle was assumed to be spherical with a diameter of 380nm. The surface area of one particle was scaled by the number of collisions and thus the number of particles in the droplet. By varying the number of immersed illite particles the total surface area was changed. It can be seen that the immersion ERH increases with surface area and that it takes many immersed illite aerosols for the immersion ERH to approach the contact ERH.

Discussion Crystal Lattice Match A factor that is often used to compare a particle’s effectiveness at causing heterogeneous nucleation is the crystal lattice match. It is suggested that when a heterogeneous nucleus has a similar lattice structure as the target crystal, it will be more efficient at initiating nucleation. Thus, when a heterogeneous nucleus has a high lattice match to the salt, the heterogeneous ERH should be higher11,29,30. A study by Davis et al.11 showed that when the calculated lattice mismatch was lower than 0.12, a strong trend between lattice mismatch and contact ERH was observed. To numerically compare lattice structures of the heterogeneous nucleus to the salt crystal, lattice mismatches (δ) between their crystal faces were calculated, − , − , , + , , , = 2



Page 14 of 33

where a1 and a2 are the two lattice constants used to define a crystal face for the heterogeneous nucleus (HN) and salt aerosol (s). A lower mismatch value indicates that the two crystal lattice structures are more similar. Each of the three faces (100, 010, 001) of the salt was compared to the three faces of the heterogeneous nucleus, resulting in nine calculated lattice mismatches. The lowest mismatch value of the nine was used for analysis. Since inorganic salts have distinct homogeneous ERH and DRH values, and thus different hysteresis ranges, when comparing heterogeneous ERH values between multiple salts σERH is used11. The parameter σERH is calculated as: (!"# − ℎ%&'"#) (!"# − '"#) where ERH and DRH are the homogeneous transitions and hetERH is the heterogeneous ERH σERH =

value for either immersion or contact mode efflorescence. An efficient heterogeneous nucleus would be indicated by σERH that is closer to zero. Heterogeneous σERH is plotted against the calculated lattice mismatch values for illite and montmorillonite in Figure 6. NX Illite was excluded from the analysis because it is a mixture of minerals and has no singular lattice structure. A table with the lattice constants of all species analyzed in the study is shown in Table S1 of supporting information. In addition, heterogeneous ERH of (NH4)2SO4 by immersed particles of three metal oxides are included8. The trend between contact σERH and lattice mismatch observed by Davis et al.11 is also shown as a dashed line. For combinations with a lattice mismatch less than 0.12 ((NH4)2SO4 with illite and montmorillonite), they seem to follow the trend between lower lattice mismatch and lower σERH. For all other combinations analyzed in the present study, the mismatch is larger than 0.12 and does not seem to have a strong correlation between lattice mismatch and heterogeneous σERH. It is important to note that the calculation for lattice mismatch used here does not fully address the complexities of crystal


Page 15 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


growth on substrates. A study done with a more detailed model on epitaxial growth of (NH4)2SO4 on metal oxide particles have been done by Martin et al.30. However, regardless of how crystal lattice mismatch is determined, crystal lattice structure does not explain why contact ERH is higher than immersion ERH for the same salt and dust pair. The crystal lattice of the nucleus and the salt is unchanged whether the particle comes into contact with the outer surface or is immersed inside the droplet. However, surface properties could be different thus factors other than crystal lattice should be considered when considering the difference between contact and immersion mode efflorescence.

Active Site Another model often used to explain heterogeneous nucleation is the active site model9,10,31. It is believed that active sites are be the result of defects to the minerals structure and their specific nature would be dependent upon the particle type and preparation method. Active sites are thought to be where crystallization is most likely to begin9. A study of ice nucleation onto feldspar particles using an environmental SEM showed that freezing initiated exclusively at defects on the mineral surface21. Past work on immersion efflorescence by metal oxides suggested that by increasing the surface area of the immersed particle, the number of active sites increase, and thus has a higher immersion ERH9,10. As shown in the results, immersion ERH for (NH4)2SO4 seems to increase with increased surface area of illite and thus the total number of active sites. While the active site model is consistent with the observed trend in efflorescence efficiency with surface area, it does not explain the difference between contact and immersion ERH. Unless the active sites are altered by becoming immersed into solution, the



Page 16 of 33

heterogeneous nucleus should have the same number of active sites regardless of whether it causes efflorescence in immersion or contact mode.

Ion-Specific Effects Ion-specific interactions between the mineral dust and the ions in solution may influence efflorescence and thus heterogeneous ERH. A recent study by Davis and Tolbert12 demonstrated that contact efflorescence initiated by charged amorphous organic aerosols (polystyrene latex spheres) exhibits an ion-specific trend, consistent with the Hoffmeister series, which is a set of ions ordered based on how well they precipitated out proteins from a solution32. In addition to following the Hoffmeister series, the trend also correlated with hydration strengths of the aqueous ions attracted to the charged surface of the heterogeneous nucleus. Studies have shown that aggregation of montmorillonite, which has a negative surface charge, was influenced by the cation in solution, demonstrating that aqueous cations are attracted to the surface of Namontmorillonite33. To observe the ion specific effect, contact efflorescence by montmorillonite of two additional chlorides, ammonium chloride (NH4Cl) and magnesium chloride (MgCl2), were studied. Since montmorillonite has a negatively charged surface the cations were altered while keeping the chloride anion constant. Additionally, NH4+ and Mg2+ were specifically chosen as the two lie on opposite sides of Na+ in the Hofmeister series. Heterogeneous σERH were plotted against the three cations in order of decreasing hydration strength in Figure 7. As shown in the figure, the negatively charged surface of montmorillonite was more efficient at causing efflorescence of the cation with higher hydration strength. The observed trend is consistent with the study by Davis and Tolbert12 where the contact nucleus was a polystyrene latex sphere with a charged surface. The results suggest that


Page 17 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the surface charge of the colliding heterogeneous nucleus influences the counter ions in solution, affecting heterogeneous efflorescence efficiencies via a destabilizing ion-specific hydrationmediated pathway, as proposed by Davis and Tolbert12. Ion-specific effects could also explain the observed difference between contact and immersion ERH. It has been suggested by Fukuta34 that the spreading of water onto the surface of the heterogeneous nucleus causes the difference between immersion and contact freezing. The forced movement of water creates a region with higher energy that then increases the chances for a critical ice germ to form. However, when the heterogeneous nucleus is already immersed, the liquid is already spread onto the particle surface and no region of higher energy is formed. For efflorescence, a similar effect could occur where upon collision, due to the supersaturation of the aqueous phase, ions in the droplet are forced to share water molecules with the incoming contact nucleus to create a hydration shell. The forced sharing destabilizes the aqueous phase which is stabilized by uptake of additional water from the surrounding to fully hydrate the contact nucleus or by crystallization12. The amount of energy increase could depend on the present ion’s hydration strength as well as the surface charge of the heterogeneous nucleus, thus the ion-specific trend in which contact efflorescence efficiency is correlated to hydration strengths of the counterions in solution.

Collision Lifetimes of Salt Aerosol in a Dust Plume Samples of atmospheric particles have shown that salt and dust aerosols are found internally mixed in many cases35,36. The mixed aerosols, when wet, could undergo immersion efflorescence. As discussed, immersion efflorescence is not as efficient as contact efflorescence. However, contact efflorescence requires a collision between the aqueous salt droplet and a mineral dust particle.



Page 18 of 33

Previous work has shown that the lifetime of a single aqueous salt particle with respect to collisions with a heterogeneous nucleus could be comparable to its atmospheric lifetime11. The lifetime (τ) of a single salt particle to collide with a heterogeneous nucleus is calculated as ) = (* ∗ ,)- where K is the coagulation coefficient in units of cm3/s and N is the heterogeneous nucleus (HN) concentration in units of cm-3. The coagulation coefficient depends strongly on the particles sizes K = 2 π (2"012 + 2" ) (!012 + ! ) where R is the radius of the salt and HN and D is their estimated diffusion coefficient in units of cm2/s. Diffusion coefficients are estimated by interpolating data from table 9.5 in Seinfeld and Pandis1. Coagulation coefficients were calculated by using a mineral dust particle size distribution based on in-situ measurements taken at Peking University in Beijing, China an hour after a dust storm event had begun37. The particle size distribution was broken up into equally spaced intervals on a log scale to estimate the dust aerosol number concentrations at each dust particle radius. For each size and corresponding number concentration (N1, N2, N3, …), coagulation coefficients (K1, K2, K3, …) and lifetimes (τ1, τ2, τ3, …) were determined. The final collision lifetime was then calculated: - 1 1 1 1 + + + ⋯ = (* ∗ , + * ∗ , + *5 ∗ ,5 + ⋯ ) ) ) )5 To determine lifetimes for various number concentrations of dust, the original size distribution

τ=

was scaled by 0.002 – 2. Number concentrations were also converted to mass concentrations by assuming spherical particles with a density of 2.6 g/cm3. Figure 8 shows the results of the calculation as a color plot, where the red colors represent shorter collision lifetimes and the blue colors represent longer lifetimes. Salt particle diameters ranging from 50 nm to 2.5 µm were assumed to represent both submicron and


Page 19 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


supermicron sized particles. Newly formed particles of (NH4)2SO4 and the subsequent growth of the aerosol into the accumulation mode are represented in the lower end of the size range1. Sea spray aerosol has a wide range of particles sizes produced through bubble bursting actions. Each bubble that bursts creates a few jet drops of a larger size and many more film drops that are less than a micrometer in radius38. Dust concentrations range from ~2 – 2,000 µg/m-3 to represent regions that are far from the source, such as marine environments and near the source such as East Asia and Northern Africa34,39-41. In Figure 8, it can be seen that salt aerosols will have a shorter lifetime as the dust number increases. At the lower end representing a typical dust concentration far from the source39, below 10 µg/m3, the salt lifetimes with respect to a collision are on the order of a few days to a few weeks. During low dust periods, aqueous salt particles smaller than 1 µm most likely do not collide with dust particles. At concentrations between 10 – 100 µg/m3 of dust, which are observed occasionally in regions far from the source and is a typical concentration for regions near the source39.40, the salt lifetimes are a few hours to a few days long. Collisions between salt and dust particle are likely to occur and have some impact on the aqueous stability. During severely dusty and dust storm events, greater than 100 µg/m3, collisions occur within a few seconds to a few hours37,40. During such an event, collisions between salt and dust will be a dominant atmospheric process for salt aerosols of all sizes and could provide a major pathway for contact efflorescence. Figure 8 also shows that the size of the salt aerosol plays a significant role in the collision lifetime. The calculated lifetimes are shorter for larger salt aerosols due to the sensitivity of the coagulation coefficients to particle size. The coagulation coefficient increases when the size difference between the salt and dust particle increases. Due to the high number concentration of



Page 20 of 33

the small dust particles in the assumed distribution, the larger salt aerosols have a much shorter collision lifetime and have a higher probability of undergoing efflorescence.

Conclusion Mineral dust particles in the atmosphere can have a significant impact on the phase state of soluble inorganic aerosol. Salt particles could exist as a solid more often than currently estimated due to collisions with dust particles. The total effect will depend on whether the two types of particulate are internally mixed or externally mixed. When the salt and dust aerosols are internally mixed, immersion efflorescence will slightly decrease the hysteresis effect. However, if they are externally mixed, collisions between aerosols could cause the more efficient contact efflorescence to occur. As shown in the calculation of coagulation lifetimes, collisions are strongly dependent on the dust concentration. In regions where dust concentrations are typically high, contact efflorescence could play a significant role in shaping the phase state of soluble inorganic aerosols. Crystal lattice and active site models have been useful when comparing heterogeneous efflorescence efficiencies of different particles, but struggle to explain the difference between the two modes of nucleation. When several chlorides with various cations were heterogeneously effloresced by montmorillonite, efflorescence efficiencies were higher for the cations with higher hydration strength. The observed trend validates the mechanism proposed in our proposed in our previous publication12 demonstrating that hydration-mediated ion-specific interactions between the ion in the aqueous droplet and surface charge of the heterogeneous nucleus affects contact mode efflorescence. The influence that the heterogeneous nucleus has on the ions in solution could also explain observed differences between immersion and contact mode efflorescence.


Page 21 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Supporting Information Detailed results for contact efflorescence experiments of MgCl2 and NH4Cl by montmorillonite (Figure S1). A table of lattice constants for all salts and heterogeneous nucleus species used to calculate lattice mismatch (Table S1).

Acknowledgments We would like to acknowledge the National Science Foundation (AGS1506691) and the Cooperative Institute for Research in Environmental Science Graduate Student Research Award for funding the presented work.

References 1. Seinfeld, J. H.; Pandis, S. N. Atmoshperic Chemistry and Physics 2nd Ed.; WileyInterscience: Hoboken, NJ, 2006. 2. Andreae, M. O.; Rosenfeld, D. Aerosol-Cloud-Precipitation Interactions. Part 1. The Nature and Sources of Cloud-Active Aerosols. Earth-Sci. Rev. 2008, 89, 13-41. 3. Boucher, O.; D. Randall, P. A.; Bretherton, C.; Feingold, G.; Forster, P.; Kerminen, V. M.; Kondo, Y.; Liao, H.; Lohmann, U.; Rasch, P.; Satheesh, S. K.; Sherwood, S.; Stevens, B.; Zhang, X. Y. Clouds and Aerosols. In Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Stocker, T. F.; Qin, D.; Plattner, G. K.; Tignor, M.; Allen, S. K.; Boschung, J.; Nauels, A.; Xia, Y.; Bex, V.; Midgley, P. M., Eds.;



Page 22 of 33

Cambridge University Press: Cambridge, United Kingdom and New York, NY, USA, 2013. 4. Ramanathan, V.; Crutzen, P. J.; Kiehl, J. T. Rosenfeld, D. Aerosols, Climate, and the Hydrological Cycle. Science. 2001, 294, 2119-2124. 5. Kaufman, Y. J.; Tanre, D.; Boucher, O. A Satellite View of Aerosols in the Climate System. Nature. 2001, 419, 215-223. 6. Martin, S. T. Phase Transitions of Aqueous Atmospheric Particles. Chem. Rev. 2000, 100, 3403-3454. 7. Czizco, D. J.; Abbatt, J. P. D. Deliquescence, Efflorescence, and Supercooling of Ammonium Sulfate Aerosols at Low Temperatures: Implications for Cirrus Cloud Formation and Aerosol Phase in the Atmosphere. J. Geophys. Res. 1999, 104, 1378113790. 8. Han, J.; Martin, S. T. Heterogeneous Nucleation of the Efflorescence of (NH4)2SO4 Particles Internally Mixed with Al2O3, TiO2, and ZrO2. J. Geophys. Res. 1999, 104, 3543-3553. 9. Martin, S. T.; Han, J.; Hung, H. The Size Effect of Hematite and Corundum Inclusions on the Efflorescence Relative Humidities of Aqueous Ammonium Sulfate Particles. Geophys. Res. Lett. 2001, 28, 2601-2604. 10. Han, J.; Hung, H; Martin, S. T. Size Effect of Hematite and Corundum Inclusions on the Efflorescence Relative Humitities of Aqueous Ammonium Nitrate Particles. J. Geophys. Res. 2002, 107, AAC 3-1 – 3-9.


Page 23 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


11. Davis, R. D.; Lance, S.; Gordon, J. A.; Ushijima, S. B.; Tolbert, M. A. Contact Efflorescence as a Pathway for Crystallization of Atmospherically Relevant Particles. Proc. Natl. Acad. Sci. 2015, 112, 15815-15820. 12. Davis, R. D.; Tolbert, M. A. Crystal Nucleation Initiated by Transient Ion-Surface Interactions at Aerosol Interfaces. Science Advances. 2017, 3, e1700425. 13. Pant, A.; Parsons, M. T.; Bertram, A. K. Crystallization of Aqueous Ammonium Sulfate Particles Internally Mixed with Soot and Kaolinite: Crystallization Relative Humidities and Nucleation Rates. J. Phys. Chem. A. 2006, 110, 8701-8709. 14. Murray, B. J.; O’Sullivan, D.; Atkinson, J. D.; Webb, M. E. Ice Nucleation by Particles Immersed in Supercooled Cloud Droplets. Chem. Soc. Rev. 2012, 41, 6519-54. 15. Broadley, S. L.; Murray, B. J.; Herbert, R. J.; Atkinson, J. D.; Dobbie, S.; Malkin, T. L.; Condiffe, E.; Neve, L. Immersion Mode Heterogeneous Ice Nucleation by an Illite Rich Powder Representative of Atmospheric Mineral Dust. Atmos. Chem. Phys. 2012, 12, 287-307. 16. Durant, A. J.; Shaw, R. A. Evaporation Freezing by Contact Nucleation Inside-out. GeoPhys. Res. Lett. 2005, 32, L20814. 17. Shaw, R. A.; Durant, A. J.; Mi, Y. Heterogeneous Surface Crystallization Observed in Undercooled Water. J. Phys. Chem. B. 2005, 109, 9865-9868. 18. Niehaus, J.; Cantrell, W. Contact Freezing of Water by Salts. J. Phys. Chem. Lett. 2015, 6, 3490-3495. 19. Niehaus, J.; Bunker, K. W.; China, S.; Kostinski, A.; Mazzoleni, C.; Cantrell, W. A Technique to Measure Ice Nuclei in the Contact Mode. J. Atmos. Oceanic Tech. 2014, 31, 913-922.



Page 24 of 33

20. Hoffmann, N.; Duft, D.; Kiselev, A.; Leisner, T. Contact Freezing Efficiency of Mineral Dust Aerosols Studied in an Electrodynamic Balance: Quantitative Size and Temperature Dependence for Illite Particles. Faraday Discuss. 2013, 165, 383-390. 21. Kiselev, A.; Bachmann, F.; Pedevilla, P.; Cox, S. J.; Michaelides, A.; Gerthsen, D.; Leisner, T. Active Sites in Heterogeneous Ice Nucleation – The Example of K-Rich Feldspars. Science. 2017, 355, 367-371. 22. Usher, C. R.; Michel, A. E.; Grassian, V. H. Reactions on Mineral Dust. Chem. Rev. 2003, 103, 4883-4940. 23. Richardson, M. S.; DeMott, P. J.; Kreidenweis, S. M.; Cziczo, D. J.; Dunlea, E. J.; Jimenez, J. L.; Thomson, D. S.; Ashbaugh, L. L.; Borys, R. D.; Westphal, D. L.; Casuccio, G. S.; Lersch, T. L. Measurements of Heterogeneous Ice Nuclei in the Western United States in Springtime and their Relation to Aerosol Characteristics. J. Geophys. Res. 2007, 112, D02209. 24. Cziczo, D. J.; Froyd, K. D.; Hoose, C.; Jensen, E. J.; Diao, M.; Zondlo, M. A.; Smith, J. B.; Twohy, C. H.; Murphy, D. M. Clarifying the Dominant Sources and Mechanisms of Cirrus Cloud Formation. Science. 2013, 340, 1320-4. 25. Glaccum, R. A.; Prospero, J. M. Saharan Aerosols Over the Tropical North Atlantic – Mineralogy. Marine Geology. 1980, 37, 295-321. 26. Biskos, G.; Malinowski, A.; Russell, L. M.; Buseck, P. R.; Martin, S. T. Nanosize Effect on the Deliquescence and the Efflorescence of Sodium Chloride Particles. Aerosol Sci. Technol. 2006, 40, 97-106.


Page 25 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


27. Garimella, S.; Huang, Y. W.; Seewald, J. S.; Cziczo, D. J. Cloud Condensation Nucleus Activity Comparison of Dry- and Wet-Generated Mineral Dust Aerosol: the Significance of Soluble Material. Atmos. Chem. Phys. 2014, 14, 6003-6019. 28. Davis, R. D.; Lance, S.; Gordon, J. A.; Tolbert, M. A.Long Working-Distance Optical Trap for in Situ Analysis of Contact-Induced Phase Transformations. Anal. Chem. 2015, 87, 6186-6194. 29. van Meel, J. A.; Sear, R. P.; Frenkel, D. Design Principles for Broad-Spectrum ProteinCrystal Nucleants with Nanoscale Pits. Phys. Rev. Lett. 2010, 105, 205501. 30. Martin, S. T.; Schlenker, J.; Chelf, J. H.; Duckworth, O. W. Structure-Activity Relationships of Mineral Dusts as Heterogeneous Nuclei for Ammonium Sulfate Crystallization from Supersaturated Aqueous Solutions. Environ. Sci. Technol. 2001, 35, 1624-1629. 31. Fletcher, N. H. Active Sites and Ice Crystal Nucleation. J.Atmos. Sci. 1969, 26, 12661271. 32. Baldwin, R. L. How Hofmeister Ion Interactions Affect Protein Stability. Biophys. J. 1996, 71, 2056-2063. 33. Tian, R.; Yang, G.; Li, H.; Gao, X.; Liu, X.; Zhu, H.; Tang, Y. Activation Energies of Colloidal Particle Aggregation: Towards a Quantitative Characterization of Specific Ion Effects. Phys. Chem. Chem. Phys. 2014, 16, 8828-8836. 34. Fukuta, N. A Study of the Mechanisms of Contact Ice Nucleation. J. Atmos Sci. 1975, 32, 1507-1603.



Page 26 of 33

35. Andreae, M. O.; Charlson, R. J.; Bruynseels, R.; Storms, H.; Gieken, R. V.; Maenhaut, W. Internal Mixture of Sea Salt, Silicates, and Excess Sulfate in Marine Aerosols. Science. 1986, 232, 1620-3. 36. Li, J.; Anderson, J. R.; Buseck, P. R. TEM Study of Aerosol Particles from Clean and Polluted Marine Boundary Layers over the North Atlantic. J. Geophys. Res. 2003, 108, 4189. 37. Wehner, B.; Wiedensohler, A.; Tuch, T. M.; Wu, Z. J.; Hu, M.; Slanina, J.; Kiang, C. S. Variability of the Aerosol Number Size Distribution in Beijing, China: New Particle Formation, Dust Storms, and High Continental Background. Geophys. Res. Lett. 2004, 31, L22108. 38. Fitzgerald, J. W. Marine Aerosols: A Review. Atmospheric Environment. 1991, 25, 533545. 39. Prospero, J. M. Long-term Measurement of the Transport of African Mineral Dust to the Southeastern United States: Implications for Regional Air Quality. J. Geophys. Res. 1999, 104, 15917-15927. 40. Chou, C.; Formenti, P.; Maille, M.; Ausset, P.; Helas, G.; Harrison, M.; Osborne, S. Size Distribution, Shape, and Composition of Mineral Dust Aerosols Collected During the African Monsoon Multidisciplinary Analysis Special Observation Period 0: Dust and Biomass-Burning Experiment Field Campaign in Niger, January 2006. J.Geophys. Res. 2008, 113, D00C10. 41. Tegen, I.; Fung, I. Modeling of Mineral Dust in the Atmosphere: Sources, Transport, and Optical Thickness. J. Geophys. Res. 1994, 99, 22897-22914.


Page 27 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figures

Figure 1. The experimental setup of the optical levitator. The flow system controls RH and flows particles into the optical cell. The optical system traps droplets and the light scattering is imaged onto CCD cameras. The two beam profiles, Bessel and Gaussian, are shown.



Page 28 of 33

Figure 2. Diagram outlining the difference between efflorescence of an aqueous droplet upon contact with a heterogeneous nucleus (top) and the immersion of the particle into the droplet (bottom). An example of (NH4)2SO4 droplet and illite particle are shown. The original light scattering is shown in green with the defects of the self-correlation shown in red


Page 29 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Contact efflorescence experiment results for (NH4)2SO4 (top) and NaCl (bottom) by different mineral dusts. The data for each dust (filled circles) was fitted with a sigmoid curve (dashed line) to calculate contact ERH.



Page 30 of 33

Figure 4. Summary of results for heterogeneous efflorescence of (NH4)2SO4 (top) and NaCl (bottom) by NX Illite, Na-Montmorillonite, and Illite. Solid bars represent immersion ERH, dashed bars represent contact ERH. Homogeneous ERH and DRH of (NH4)2SO4 and NaCl are marked by solid black lines.


Page 31 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Heterogeneous ERH of (NH4)2SO4 by the calculated total surface area of immersed illite. Solid black lines on the graph represent the homogeneous ERH and DRH values while the dashed black line is the contact ERH by illite.

Figure 6. Heterogeneous σERH of (NH4)2SO4 (Square) and NaCl (Diamond) as a function of lattice mismatch with different minerals (represented by different colors). Contact σERH are represented as unfilled markers and immersion σERH are represented as filled markers. The dashed line shows the trend seen by Davis et al.11. * Data retrieved from Han and Martin8.



Page 32 of 33

Figure 7. Comparing contact σERH of various chloride salts by Na-Montmorillonite with respect to the hydration strength of the cations of the salts.

Figure 8. Calculated collision lifetimes (days) for a single salt aerosol to collide with a mineral dust particle in a plume. The particle size distribution of mineral dust used was measured at 3:00AM during a dust storm event at Peking University, China from Wehner et al. (Figure 1d)37. To calculate lifetimes for a range of mineral dust particle concentrations, the distribution was scaled by factors between 0.002 and 2. A density of 2.6 g/cm3 was assumed to estimate the dust aerosols mass concentration.


Page 33 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Tables of Contents Image


Immersion and Contact Efflorescence Induced by Mineral Dust

Recommend Documents