Water uptake characteristics of individual atmospheric particles having coatings

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1 Atmospheric Environment 41 (2007) Water uptake characteristics of individual atmospheric particles having coatings Trudi A. Semeniuk a,b, Matthew E. Wise a,b, Scot T. Martin c, Lynn M. Russell d, Peter R. Buseck a,b, a School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA b Department of Chemistry & Biochemistry, Arizona State University, Tempe, AZ 85287, USA c Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA d Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093, USA Received 12 September 2006; received in revised form 30 March 2007; accepted 3 April 2007 Abstract We used an environmental transmission electron microscope to observe deliquescence and hygroscopic growth of atmospheric particles with hygroscopic coatings over the range 0 100% relative humidity (RH). The particles were collected from polluted and clean environments. Types included a sulfate-coated NaCl/silicate aggregate particle, a sulfatecoated sea-salt particle, and a Mg-rich, chloride-coated sea-salt particle. They all exhibited initial water uptake between 50% and 60% RH, although the first major morphological changes occurred at 70% RH. A deliquescence sphere, adjacent to the core particle, formed between 70% and 76% RH when deliquescence occurred or when the liquid phase was able to break out of the solid exterior coating. The deliquescence sphere grew to engulf the particle with increasing RH. Some particles developed a splatter zone associated with a particle coating. Efflorescence occurred over the range 49 44% RH. Our results indicate that some coated particles undergo a multi-step deliquescence process and that composition of the different phases within the coating affects deliquescence and hygroscopic growth below 76% RH. Above 76% RH, the dominant hygroscopic growth was due to water uptake by NaCl. Efflorescence of these particles also was strongly linked to NaCl, although the presence of other phases inhibited formation of a single NaCl crystal. Our results show that the observed coatings can both enhance particle solubility and lower the effective deliquescence RH of the particle. Thus, these coatings cause important phase and size changes for aerosol particles that could feed back into many other chemical and physical processes that contribute to radiative forcing within the atmosphere. r 2007 Elsevier Ltd. All rights reserved. Keywords: Hygroscopic coatings; Deliquescence; Efflorescence; Individual particles; ETEM 1. Introduction Corresponding author. Tel.: ; fax: address: pbuseck@asu.edu (P.R. Buseck). Hygroscopic coatings on aerosol particles can alter the ways in which they influence atmospheric chemistry and Earth s radiative budget. In the dry state, they can affect particle surface interactions with light and other atmospheric constituents, e.g., /$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi: /j.atmosenv

2 6226 T.A. Semeniuk et al. / Atmospheric Environment 41 (2007) coatings will enhance or retard the reactivity of aerosol particles in the atmosphere (Folkers et al., 2003; Usher et al., 2003). In addition, they can affect deliquescence and hygroscopic growth of the core particle at elevated relative humidity (RH). Change of phase from a dry particle to a solution droplet is associated with changes in size, shape (from nonspherical to spherical), and chemistry, e.g., a heterogeneously mixed solid particle with separate phases in the core and coating, may become a homogeneously mixed solution droplet in which all constituents are water soluble. The solid liquid phase change of atmospheric particles can also affect their ability to scatter light and their impact on the catalysis of liquid-phase reactions (Martin, 2000). Coatings on individual particles can be observed using electron-beam techniques in samples from a range of marine and continental environments (e.g., Buseck and Po sfai, 1999; Martin and Han, 2000). Coatings commonly result during mixing of different air masses, e.g., Asian pollution outflows into the Pacific marine air or oil-pollution plumes in desert environments (Levin et al., 1996; Parungo et al., 1992). Coated particles in these polluted environments range from ammonium sulfate on soot to sulfates on mineral dust. Although coatings commonly result from anthropogenic components, natural processes also form coatings. For example, organic molecules on the surface of the ocean can coat the surface of sea-salt particles during wavebreaking action. In addition, aging, oxidation, and reactions with other soluble atmospheric components all contribute to the formation of coatings. Thus, the proportion of coated aerosol particles within a population is likely to increase with time, typically resulting in a greater fraction of hydrophilic particles in the atmosphere. In multi-component coated particles, both the coating and core compounds can impact the hygroscopic behavior of the aerosol particle. Compounds in coatings can impact the hygroscopic properties of the core phases in two ways: by lowering the deliquescence RH (DRH) (Tang and Munkelwitz, 1993) and by reducing or enhancing overall water uptake of the core phases. For example, some organic compounds are hypothesized to make the surface of an aerosol particle hydrophobic, thus potentially impacting water uptake (Ellison et al., 1999). The insoluble or weakly soluble core phases can also affect the hygroscopic behavior of the aerosol particle, e.g., mineral dust particles can induce heterogeneous nucleation and efflorescence of aqueous coatings (Han et al., 2002; Han and Martin, 1999; Martin et al., 2001). Sulfate coatings, in particular, are common on mineral dust and sea-salt aerosol particles. Sulfate coatings can arise from H 2 SO 4 deposition after gasphase oxidation of SO 2 or dimethylsulfide (DMS). Furthermore, they can be created by cloud processing, in which particles scavenge anthropogenic sulfates (Parungo et al., 1992; Wurzler et al., 2000). Coatings on mineral dust have been reported from the Mediterranean and Asian regions, where air masses rich in anthropogenic SO 2 or sulfates interact with desert dust (Formenti et al., 2003; Levin et al., 2005). The heterogeneous reaction of SO 2 with mineral dust is hypothesized to reduce the load of anthropogenic sulfate in the atmosphere because of the increased deposition rate of coarsemode particles (Bauer and Koch, 2005; Levin et al., 1996; Parungo et al., 1992). Sulfate coatings have also been reported on sea-salt particles in most marine air masses (Li et al., 2003; Po sfai et al., 1995; Sievering et al., 1999). The coatings develop during initial formation of sea-salt particles through interaction of emissions of DMS from phytoplankton or through reaction of anthropogenic sulfates with sea salt (Buseck and Po sfai, 1999; Li et al., 2003; Po sfai et al., 1995; Zhao et al., 2006). We investigated the physical and chemical behavior of coated mineral and sea-salt aerosol particles over a range of atmospheric RH values using an environmental transmission electron microscope (ETEM). We present results for a sulfate-coated NaCl/silicate particle, a sulfate-coated sea-salt particle, and a Mg-rich chloride-coated sea-salt particle as representative of certain reactions in the atmosphere. Changes in size, shape, and phase of these individual particles from 0% to 100% RH are documented, providing information on the hygroscopic behavior of coated particles. 2. Methods 2.1. Sampling We studied aerosol particles from the pollution plume of an oil-refinery in the desert near Habshan, United Arab Emirates (UAE, 23/6/2002); marine air at Cape Grim, Australia (ACE-1, 11/12/1995); and polluted marine air at the Scripps Institution of Oceanography in San Diego, USA (SIO, 21/1/2006). Aerosol particles were directly impacted onto

3 T.A. Semeniuk et al. / Atmospheric Environment 41 (2007) carbon-film TEM grids using an MPS-3 microanalysis particle sampler (California Instruments, Inc.) for the UAE and SIO sets and a Casella cascade impactor for the ACE-1 field campaign. The samplers collected size-fractionated samples, of which we investigated the intermediate and fine size fractions, mm and o0.3 mm, respectively. Further information about the ACE-1 field campaign is given in Po sfai et al. (1999) and Sievering et al. (1999). Sampling methods similar to ACE-1 were employed at the SIO site Conventional TEM analyses Bright-field images and energy-dispersive X-ray spectrometry (EDS) measurements were recorded for 30 individual aerosol particles using a Philips CM200 TEM prior to ETEM analyses. The EDS measurements were carried out on each distinct phase of a particle. Measurements were restricted to 10-s exposure times to reduce particle damage; however, it is likely that volatile material evaporated (Po sfai et al., 2003). Selected-area electron diffraction confirmed the presence of NaCl and CaSO 4 in a number of particles. The morphological and chemical information obtained from conventional TEM analysis of these multi-component aerosol particles was critical to our interpretation of water uptake during ETEM experiments ETEM analyses Incremental study of water uptake on the particles was carried out using a modified Tecnai F20 TEM with an environmental cell. Temperature within this cell was maintained at 18 1C by liquid nitrogen cooling and resistive heating, while water vapor pressure was incrementally increased up to 15.5 Torr and then decreased back to vacuum conditions, exposing particles to RH values ranging from 0% to 100%. A sequence of images was recorded for increasing RH for each particle, with images taken at 15%, 40%, 50%, 55%, 60%, 65%, 70%, 75%, 82%, 89% and 100% RH. Similarly, a sequence of images for decreasing RH was recorded for each particle, with images taken at 60%, 49%, 44%, 40%, 30% and 0% RH. The electron beam was turned off after each set of images was taken to limit radiation exposure to the particles and thus reduce beam damage and particle heating (Wise et al., 2007). Deliquescence and efflorescence were observed for each of the particles examined. Prior to each ETEM experiment, the accuracy of RH measurements was verified by measuring the DRH of laboratory-generated NaCl particles. The DRH for these NaCl particles was 7572%, in agreement with the known DRH of NaCl. The ETEM experimental setup and procedure used in this study was slightly modified from that described in Wise et al. (2005) and identical to that described more fully in Wise et al. (2007). 3. Results and discussion We studied 30 coated particles; several examples and types of particles with natural coatings are illustrated at 0% RH in Fig. 1 to show their diversity. We describe the deliquescence behavior of three particles from this set in detail: (1) a NaCl/silicate aggregate particle coated by mixed-cation sulfates (UAE), (2) a sea-salt particle coated by mixed-cation sulfates (SIO), and (3) a sea-salt particle coated by mixed-cation chlorides (ACE-1). These particles comprise the intermediate-size fraction of our samples and, because of their larger sizes, best illustrate the changes we observed in replicate experiments carried out on the fine-size fraction. Coated particles were typically more abundant in the intermediate-size range at all locations. Bright-field TEM images and the EDS analyses of these three particles are given in Figs. 1 and 2. The X-ray intensities comprising the signals in Fig. 2 are a function of many factors including atomic mass, concentration, thickness, etc. Therefore, the low S signals in the spectra corresponding to the coatings were probably the result of low concentration. To obtain quantitative information, a calibration for each element with a known mass standard of approximately the same concentration and thickness is needed. Although this type of quantitative analysis is not included in this study, we believe each coating had a sulfate component. Sulfur peaks were associated with most of the coatings we measured. Likewise, we were able to use the presence or absence of peaks to infer a number of inorganic phases: NaCl, CaSO 4, MgSO 4, and MgCl 2. For example, the EDS spectra with only Mg and Cl peaks present suggest that MgCl 2 occurred as a distinct phase. The stoichiometry of other inorganic phases, especially hydrates, in the coatings could not be unambiguously determined by our measurements.

4 6228 T.A. Semeniuk et al. / Atmospheric Environment 41 (2007) Fig. 1. Bright-field images of eight-coated particles from UAE, ACE-1, and SIO field campaigns. Images 1 and 2 show particles with a partial sulfate coating; the other images show particles completely coated by a mixed sulfate or chloride phase or phases. Three particles were selected for water-uptake studies: (1) sulfate-coated NaCl/silicate aggregate (image 1); (2) mixed-cation, sulfate-coated sea-salt particle (image 4); and (3) Mg-rich chloride-coated sea-salt particle (images 8 and 9). The letters denote areas where the EDS spectra were obtained from these particles (Fig. 2) Hygroscopic properties of coated particles Deliquescence and hygroscopic growth of coated particles occurred over the range % RH. In contrast, efflorescence of these particles occurred over the narrower RH range of 44 49%. The complexity of water uptake phenomena in coated particles necessitated the use of efflorescence observations to interpret former wet regions of particles. For example, regions that showed clear recrystallization or skeletal outlines on return to 0% RH were interpreted as formerly wet. The first indication of water uptake on the coated NaCl/silicate aggregate particle occurred at 60% RH (Fig. 3), with the formation of a zone of droplets on the carbon substrate around the silicate grain. We infer that the droplets formed when the particle impacted the TEM grid and then became evident upon water uptake during the ETEM experiment. Their hygroscopic growth suggests a soluble phase at 60% RH on the surface of the aerosol particle (assuming that the splattered droplets arose from the surface regions of the particle when impacted). Water uptake, however, was minor at this RH value, which may explain why visible morphological changes did not occur on the core particle. At 70% RH, the droplets in the splatter zone surrounding the particle grew, and the first apparent change in phase (deliquescence) occurred on the surface of the particle (circled in Fig. 3 for clarity). The growth of this deliquescence sphere was much more pronounced than the growth

5 T.A. Semeniuk et al. / Atmospheric Environment 41 (2007) Fig. 2. EDS spectra obtained with a Philips CM200 TEM at 40,000 times magnification and beam size 5 (6 nm) for intervals of 10 s. (A, B): Spectra from a NaCl grain and a coated silicate grain within an aggregate particle; (C, D): Spectra from a NaCl grain and its mixed-cation sulfate coating; and (E, F): Spectra from a NaCl grain and its Mg- and Cl-rich coating. A carbon peak from the carbon substrate is present in all spectra. 0 % RH NaCl 60 % 70 % 2 µm silicate droplets 76 % NaCl 82 % 89 % del. Fig. 3. Images of a sulfate-coated NaCl/silicate aggregate as RH was increased from 0% to 89% (denoted by the up arrows). The white circle in the 70% RH panel highlights the first apparent change on the surface of the particle. No significant changes were observed from 89% to 100% RH; hence the image at 100% RH is not shown.

6 6230 T.A. Semeniuk et al. / Atmospheric Environment 41 (2007) of the particles in the splatter zone, and we surmise that this area of the particle contained a different phase. At 76% RH, deliquescence of NaCl occurred, forming a large sphere attached to the silicate grain. Droplets around the silicate and the smaller deliquescence sphere that formed at 70% RH (highlighted with the dashed arrow) also grew markedly at 76% RH. The droplets in the splatter zone and the two separate deliquescence spheres both grew over the range % RH, although this mostly occurred outside the viewing region. The main difference between coated and uncoated NaCl/mineral aggregate particles (cf. Fig. 6, Wise et al., 2007) appears to reside in the formation of the splatter zone around the particle and the formation of a protruding deliquescence sphere for RHo76%. The first signs of deliquescence on the mixed cation, sulfate-coated sea-salt particle occurred on the exterior of the particle at ca. 61% RH (circled for clarity in Fig. 4). Although no splatter zone was present around this particle, initial water uptake occurred at the same RH as the initial water uptake of the splatter zone around the sulfate-coated silicate grain in Fig. 3. We surmise that the phase taking up water in both particles was the same: on the silicate grain it formed a splatter zone, whereas on the sea-salt particle it formed an exterior zone to the main particle. We suggest that because the silicate grain was insoluble, the mixed-cation sulfates were present only on the surface of the silicate grain. If the sulfates were deliquesced under ambient conditions prior to sampling, then this liquid on the surface of the particle would have been ejected when it impacted the TEM substrate, forming a splatter zone. In contrast, if the mixedcation sulfates were present with a soluble phase like NaCl and the whole particle was deliquesced under ambient conditions prior to sampling, then it would have impacted the TEM substrate as a droplet and not ejected the mixed-cation sulfate-phase. As the RH was increased to 70%, the deliquescence sphere on the first phase grew, and a second area began to take up water (circled for clarity in Fig. 4). These droplets grew to form a sphere that 0 % RH 16 % 40 % 2 µm 61 % 65 % 70 % 75 % 82 % 89 % Fig. 4. Images of a sulfate-coated sea-salt particle as RH was increased from 0% to 89% RH. No significant changes were observed from 89% to 100% RH; hence the image at 100% RH is not shown.

7 T.A. Semeniuk et al. / Atmospheric Environment 41 (2007) partially engulfs the sea-salt particle at 70% RH. The main deliquescence sphere and a splatter zone around the main particle formed at 75% RH, which we infer was due to water uptake by NaCl. The coated exterior appears to form a barrier to growth of the main sphere, which grew out adjacent to the particle. Both the sphere and droplet zone grew over the range 82 89% RH. The main difference between coated and uncoated NaCl particles (cf. Fig. 5, Wise et al., 2007) is that the formation of the main deliquescence sphere in the latter occurs adjacent to the main particle. Upon decreasing RH, slight changes in the sulfatecoated sea-salt particle occurred at 60% RH, but full efflorescence occurred at ca. 49% RH (i.e., the morphology does not change between 49% and 0% RH in Fig. 5). EDS measurements from different regions of the effloresced particle indicate that the main deliquescence sphere crystallized back into NaCl (Fig. 6A, B), for which the thinner portion of the effloresced sphere was marked by a sulfurbearing residue (Fig. 6D). The left-most portion of the particle was also enriched in sulfates (Fig. 6C). The recrystallized splatter zone consisted of NaCl (Fig. 6E). Both the morphology and the chemistry of the effloresced-coated particle were markedly different from pure NaCl particles (Fig. 4; Wise et al., 2007). The mixed solute formed on deliquescence appears to increase the efflorescence RH (ERH of NaCl is 45% RH, Biskos et al., 2006) and to inhibit formation of well-defined cubes of NaCl. Deliquescence of the Mg-rich coated exterior of the sea-salt particle occurred at 50% RH (circled for clarity in Fig. 7). At 60% RH, portions of the splatter zone began to pick up water, and the previously wet area of the particle (highlighted with a dashed arrow) grew slightly. The main deliquescence sphere developed more fully at 70% RH, associated with hygroscopic growth of the deliquesced phases. The deliquescence sphere grew adjacent to the main particle, located at the site of initial water uptake within the coated exterior. As RH was increased to 76%, we assume the whole particle became deliquesced due to the large growth of the main deliquescence sphere. Efflorescence of the particle (Fig. 8) occurred at 45% RH, coincident with the efflorescence of NaCl. The presence of the Mg-rich chloride coating did not appear to affect particle efflorescence. No other morphological changes occurred as the RH returned to 0% RH. As with the sulfate-coated NaCl particle in Fig. 5, the main difference between coated and uncoated NaCl is that the deliquescence sphere forms adjacent to the main particle. 60 % 49 % 2 µm 45 % 0 % Fig. 5. Images of the sulfate-coated sea-salt particle in Fig. 4 as RH was decreased from 60% to 0% (denoted by down arrows).

8 6232 T.A. Semeniuk et al. / Atmospheric Environment 41 (2007) Fig. 6. EDS spectra obtained from the sulfate-coated sea-salt particle after efflorescence (Fig. 5) using the Philips CM200 TEM at 40,000 times magnification and beam size 5 (6 nm) for intervals of 10 s. A carbon peak from the carbon substrate is present in all spectra. Our results indicate that soluble coatings on insoluble or slightly soluble core particles developed splatter zones on impact with the TEM grid that were visible on deliquescence during ETEM experiments. Splatter zones were observed on different core particles (e.g., gypsum, clay minerals, and sea salt) and had different compositions. We were unable to correlate the DRH of a number of splatter zones with specific compounds, and they may comprise hydrated inorganic or organic phases (e.g., the phase present in particle coatings having DRH values of 60%). The use of ETEM to enhance and identify splatter zones around particles has further potential for a more detailed study of coatings. The presence of relatively insoluble phases (e.g., CaSO 4 ) within particle coatings appeared to inhibit formation of the main deliquescence sphere around the core particle; the sphere typically formed adjacent to the core particle and only partially engulfed the original grain. It was round and was commonly found with a splatter zone around the core particle. The deliquescence sphere typically grew from the region of the particle that first underwent deliquescence and broke out from the non-deliquesced coating. We also observed formation of deliquescence spheres adjacent to the core particle in deliquescence experiments on carbonaceous particles having soluble inclusions or soluble grains with insoluble organic coatings (Semeniuk et al., 2007). Although observing particles on a flat surface may influence this phenomenon, we suggest that a shell comprising less-soluble material can occur as a coating around particles in the atmosphere. The site of initial water uptake in the three particles described here was close to or coincident with the locations where we made EDS measurements (Figs. 1, 3, 4, and 7). However, this apparent spatial correlation occurs in only eight of the 30 particles documented in this study. The correlation is likely circumstantial and does not appear to affect our observations on water uptake, but further work will be done to confirm this supposition. The sequence of deliquescence we observed partly reflects the nature of the inorganic components within the particle, e.g., DRH of MgCl 2 is 33% and DRH of MgSO 4 7H 2 O is 66% (Gmelin, 1992), but

9 T.A. Semeniuk et al. / Atmospheric Environment 41 (2007) % RH 15 % 50 % 2 µm 60 % 65 % 70 % 76 % 82 % 89 % Fig. 7. Images of a Mg-rich chloride-coated sea-salt particle as RH was increased from 0% to 89%. No significant changes were observed from 89% to 100% RH; hence the image at 100% RH is not shown. it may also reflect chemical mixing of components. For example, changes in particle morphology associated with deliquescence of NaCl occurred at lower RH in the sea-salt particle from SIO (e.g., Fig. 4) than in laboratory-generated NaCl (Wise et al., 2007). Mixtures of NaCl and MgCl 2 in laboratory studies also took up water at lower RH (Chan et al., 2000). Highly soluble organic compounds mixed with salt also are predicted to decrease the DRH (Ming and Russell, 2002). This most likely explains our recurrent observation of water uptake associated with NaCl at around 70% RH in both sea-salt particles in this study. Main hygroscopic growth in our particles occurred above 76% RH, i.e., post-deliquescence of NaCl (Figs. 3, 4, and 7). We suggest that the dominant hygroscopic growth of these particles was associated with water uptake in the NaCl component, i.e., the phase having the highest growth factor in the particles we studied. The inorganic components that deliquesced below 76% RH, e.g., MgCl 2, only appeared to enhance this growth, whereas less hygroscopic species such as CaSO 4 did not appear to significantly affect this growth. Thus, chloride salts appeared to dominate the hygroscopic growth for the particles studied in this paper. In contrast, Ming and Russell (2001) modeled the effect of organic components on the hygroscopic growth of sea salt and found that the organic components significantly decreased hygroscopic growth of particles above 76% RH. Efflorescence occurred between 44% and 49% RH for all NaCl-bearing particles in our study, i.e., close to the ERH of NaCl, suggesting efflorescence of these multi-component particles also was dominated by NaCl. 4. Implications Many coated particles exhibited initial water uptake between 50% and 60% RH, which was significantly lower than expected for most ambient desert and marine aerosols particles. Moreover, we found that the deliquescence of coated particles

10 6234 T.A. Semeniuk et al. / Atmospheric Environment 41 (2007) % 49 % 2 µm 45 % 0 % Fig. 8. Images of the deliquesced Mg-rich chloride-coated sea-salt particle in Fig. 7 as RH was decreased from 60% to 0%. proceeds as a multi-step process involving stepwise changes in shape and size. The values and rates at which water was incorporated into a coated particle appeared to depend on the mixing state of its constituents. In contrast, efflorescence of these coated particles occurred as a single-step process linked to recrystalization of NaCl. Despite the complex water uptake behavior of coated particles, the main hygroscopic growth of particles in this study arose from water uptake by NaCl. Other organic or inorganic components in these internally mixed particles did not appear to significantly affect this water uptake. This behavior implies that droplet nucleation might also be dominated by NaCl water uptake for particles with similar surface properties. Such particles can occur in environments as diverse as continental marine and industrial pollution plumes. Acknowledgments This material is based upon work supported by the National Science Foundation under Grant no Any opinions, findings, and conclusions or recommendations expressed are those of the authors and do not necessarily reflect the views of the National Science Foundation. We acknowledge use of UAE field campaign sample material collected in summer 2002 during Phase 1 of the Rainfall Enhancement project funded by the Department of Water Resources Studies, Office of H.H. the President in the UAE and the National Center for Atmospheric Research through the sponsorship of Roelof Bruintjes. UAE TEM grids were prepared by Li Jia and Tomoko Kojima, and collected by Vidal Salazar and Tara Jensen. The aircraft was provided by Weather Modification Incorporated of Fargo, North Dakota. We also acknowledge use of TEM grids collected by Herman Sievering during the ACE-1 field campaign. We thank Miha ly Po sfai for help in selecting samples for study and for useful comments. We gratefully acknowledge the use of the electron microscope facilities within the Center for Solid State Science at Arizona State University. We thank Karl Weiss, John Wheatley, Renu Sharma, and Peter Crozier for their assistance with developing our ETEM technique. References Bauer, S.E., Koch, D., Impact of heterogeneous sulfate formation at mineral dust surfaces on aerosol loads and radiative forcing in the Goddard Institute for Space Studies general circulation model. Journal of Geophysical Research 110, D17202, doi: /2005jd

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