Article pubs.acs.org/est
Structural Change of Aerosol Particle Aggregates with Exposure to Elevated Relative Humidity James F. Montgomery,*,† Steven N. Rogak,† Sheldon I. Green,† Yuan You,‡ and Allan K. Bertram‡ †
Department of Mechanical Engineering, The University of British Columbia, Vancouver, Canada V6T 1Z4 Department of Chemistry, The University of British Columbia, Vancouver, Canada V6T 1Z1
‡
S Supporting Information *
ABSTRACT: Structural changes of aggregates composed of inorganic salts exposed to relative humidity (RH) between 0 and 80% after formation at selected RH between 0 and 60% were investigated using a tandem differential mobility analyzer (TDMA) and fluorescence microscopy. The TDMA was used to measure a shift in peak mobility diameter for 100−700 nm aggregates of hygroscopic aerosol particles composed of NaCl, Na2SO4, (NH4)2SO4, and nonhygroscopic Al2O3 as the RH was increased. Aggregates of hygroscopic particles were found to shrink when exposed to RH greater than that during the aggregation process. The degree of aggregate restructuring is greater for larger aggregates and greater increases in RH. Growth factors (GF) calculated from mobility diameter measurements as low as 0.77 were seen for NaCl before deliquescence. The GF subsequently increased to 1.23 at 80% RH, indicating growth after deliquescence. Exposure to RH lower than that experienced during aggregation did not result in structural changes. Fluorescent microscopy confirmed that aggregates formed on wire surfaces undergo an irreversible change in structure when exposed to elevated RH. Analysis of 2D movement of aggregates shows a displacement of 5−13% compared to projected length of initial aggregate from a wire surface. Surface tension due to water adsorption within the aggregate structure is a potential cause of the structural changes.
1. INTRODUCTION The morphology of an aerosol aggregate impacts its behavior in the atmosphere and is an important consideration for its deposition in the human respiratory tract1,2 and for particle control technologies, such as HVAC air filters.3−5 Agglomerates of particles from combustion in engines1,6−8 and biomass burning3−5,9−11 have been extensively studied. Aggregates of other particles can also form in the atmosphere or within the media of air filters during particle removal.12,13 These aggregates can be formed from organic and inorganic substances, that may be hygroscopic or nonhygroscopic. Relative humidity (RH) plays a key role in the morphology and state of hygroscopic particles. A detailed review of H2O− NaCl interactions has been provided by Ewing.14 Numerous studies have investigated the interactions of water molecules with salt crystal structures. Atomic force microscopy and scanning polarization force microscopy have shown that water adsorbs preferentially at steps and nonuniformities in the crystal surface at relative humidity as low as 30%.15−17 Infrared spectroscopy has shown that at room temperature, H2O adsorbs on to surface defects at low RH and forms water adlayers and thin films on the crystal surface.18,19 The water interaction with salt crystals leads to changes in microscopic morphology such as rounding of edges and restructuring of steps that can have implications for particle−particle interaction.20 © XXXX American Chemical Society
The tandem differential mobility analyzer (TDMA) has previously been used to study growth of hygroscopic particles over a range of relative humidity from 0 to 100%. Hygroscopic particles show significant growth at their deliquescence relative humidity (DRH).21,22 Experiments have indicated restructuring of single hygroscopic particles due to water uptake through measurements of reduced mobility diameter with increased relative humidity below deliquescence.20,23−25 Weingartner et al.26,27 utilized the TDMA setup to study the morphological change of soot agglomerates generated from diesel combustion and spark electrode generators. They found that the mobility diameter of spark-generated agglomerates decreased with exposure to increasing relative humidity even at RH as low as ∼30%. Agglomerates from exhaust of a diesel engine under load showed a constant mobility diameter until an RH of approximately 90% was reached, at which point the mobility diameter increased by up to 5%. Similar measurements from the same engine under idling conditions showed decreases in mobility diameter of up to 1% for fuel with no additives and 5% with sulfur additives. Humidifying up to 100% RH and subsequent drying of soot samples from the engine under Received: June 29, 2015 Revised: September 22, 2015 Accepted: September 24, 2015
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DOI: 10.1021/acs.est.5b03157 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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aggregates studied was selected by controlling the voltage on the DMA column. Nominal aggregate mobility diameters of 115, 180, 280, 470, 550, and 650 nm were chosen for this study. The airstream containing size-selected aggregates was then mixed with air of a controlled relative humidity and the size distribution measured using a TSI 3936 Scanning Mobility Particle Spectrometer (SMPS). The particle size distributions before and after the coagulation loop are shown in Figure 1 for 0% RH conditions.
load showed decreases from the original dry diameter of up to 1%. The change in agglomerate mobility diameter was attributed to a restructuring of the particles due to surface tension of localized condensation. The magnitude of the decrease in mobility diameter with change in RH depends on particle size and engine operating conditions. Visualization methods have been developed to allow for the study of particle and aggregate structures, as well as phase change characteristics in high relative humidity conditions. Optical microscopy has shown that aggregates form from consecutive deposition of particles on fiber structures12,28,29 in an airflow similar to the conditions in fibrous air filters. The deliquescence and efflorescence of aerosol particles have been studied using a combination of optical (or fluorescence or Raman) microscopy and environmentally controlled flow cells.30−33 The flow cell allows for control of temperature and relative humidity, while particle phase change is observed and recorded. The impact of relative humidity changes on aggregates of hygroscopic particles has not previously been studied using these techniques. Experiments of filter performance have shown that the relative humidity of the airstream can impact flow resistance and filtration efficiency of loaded filter media. Higher relative humidity during loading can result in a slower growth in flow resistance.34−36 Changes to the relative humidity of the airstream incident on the loaded filter can cause substantial changes in performance.37,38 These changes result only when the filters are exposed to increases in relative humidity and are found to be irreversible with reductions in incident RH. The purpose of this work is to investigate the impact of relative humidity on the structure of particle aggregates formed by coagulation while airborne or after deposition on a wire mesh. The results may provide insight in to the potential role of humidity changes below deliquescence (0−80% RH) on aggregate structures that may be present in some locations of the atmosphere. The structural properties of aggregates can also potentially change the performance characteristics of measurement devices used to study properties of atmospheric particles.
Figure 1. Size distribution of hygroscopic (NaCl, Na 2 SO4 , (NH4)2SO4) and nonhygroscopic (Al2O3) particles before (solid symbols) and after (open symbols) coagulation.
The distributions are normalized by the peak concentration, which is naturally much reduced after coagulation. The exact number of particles contained in the final aggregates is expected to be highly variable due to the range of particle sizes in the original and final distributions. It is expected to range from tens to several hundreds. The distribution before coagulation is similar for all inlet RH tested in this work (0−60%). Three types of atmospherically relevant hygroscopic substances were chosen: NaCl, Na2SO4, and (NH4)2SO4, with deliquescence relative humidity of 75.3, 84.2, and 80.2%, respectively, at 298 K.39 The solution concentrations for NaCl, Na2SO4, and (NH4)2SO4 were 5, 6, and 4.1 g/L, respectively. The NaCl solution was made from commercially available salt, and the level of impurity is unknown. The Na2SO4, and (NH4)2SO4 solutions were prepared from scientific grade powder (Fisher Scientific). Tests were also performed with aggregates of nonhygroscopic Al2O3 particles atomized from suspension (20 g/L). Alpha alumina powder (LECO Corporation) was suspended in distilled H2O and continuously mixed to maintain a suspension during atomization. The alumina particle distribution generated from suspension could contain both single and multi particle aggregates due to the potential for aggregation within solution. 2.2. Microscopy Analysis. The impact of RH on aerosol aggregate structures was analyzed using fluorescence microscopy. The visualization technique is a modified version of that used to determine phase change characteristics of single particles.31,32 The aggregates analyzed in this work are formed from particles deposited on wire mesh from bulk aerosol flow using techniques similar to those of filter loading experiments.38 Schematics of the loading and visualization apparatus are shown in Figure S2 (Supporting Information).
2. EXPERIMENTAL METHODS Structural changes of aggregates were investigated using two methods. Airborne aggregates formed by Brownian coagulation were investigated using a TDMA setup with humidity control. Aggregates formed by deposition on a wire mesh were investigated using fluorescence microscopy and a humidity controlled flow cell. 2.1. TDMA Analysis. In the TDMA setup (Figure S1, Supporting Information), aerosol samples were generated using a TSI 3076 constant output atomizer connected to cleaned, dry compressed air. An atomizer pressure of 275 kPa (40 psi) was used for all experiments in this work. The aerosol passed through a diffusion drier containing CaSO4 to reduce the RH to 500 nm) and decreasing GF with increasing aggregate size. The 20% → 60% experiments show the lowest GF and the greatest reduction in GF with increasing aggregate size as would be expected from the large change in relative humidity. The 20 → 40% experiments also show a GF statistically smaller than 1 for aggregates greater than ∼180 nm. The 20 → 40% experiments show smaller GF than do the 0 → 40% experiments presented in Figure 3a. These results show that significant changes in aggregate structure can occur for changes in RH other than that from 0% which are relevant to conditions associated with local weather changes in the atmosphere. Figure 3c shows the GF for aggregates formed at relative humidity of 20, 40, or 60% that are then mixed with a 0% RH flow stream (reducing RH to less than 6% in each case). There is no statistically significant change in the diameter of the peak size. This indicates that the change in structure seen in Figure 3a,b is not a result of a change in RH but is specific to an increase in relative humidity. These results are in line with changes to the properties of filters loaded with hygroscopic particles from previous work.38 3.1.2. Alumina Aggregates. TDMA experiments were also performed using nonhygroscopic Al2O3 particles atomized from a liquid suspension and dried in a manner similar to that described for NaCl. Figure 4 shows the Growth Factor versus original aggregate size for Al2O3 aggregates formed in the coagulation chamber at 0% RH and then mixed with air to a final elevated RH of 20, 40, or 60%. The results show no statistically significant change in aggregate size for the range of experiments performed. This is consistent with experiments of filters loaded with Al2O3 which showed no change in flow resistance with exposure to high RH34,36,38 and contrary to those loaded with hygroscopic particles. Weingartner et al.,26,27 however, showed that agglomerates of insoluble spark generated aerosols or diesel engine exhaust show a small restructuring when exposed to elevated RH. Image analysis of single particles have shown water uptake on hygroscopic particles below deliquescence but not on nonhygroscopic particles.43 The discrepancy may be a result of differences in primary particle size between experiments. The soot agglomerates of Weingertner et al.26,27 consisted of ∼10 nm primary particles compared to the ∼100 nm Al2O3 particles studied here. This would result in a significant difference in the
Figure 3. Impact of relative humidity changes on peak mobility diameter of NaCl aggregates for (a) increases from 0% RH, (b) increases from intermediate RH, and (c) decreases to 0% RH. The dashed lines represent one standard deviation of the 0 → 0% to indicate variability of the measurement method. D
DOI: 10.1021/acs.est.5b03157 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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with increasing RH similar to the previous work studying agglomerates of carbon particles.26 At DRH of NaCl (∼75%) and (NH4)2SO4 (∼80%) the trend reverses and the Growth Factor increases drastically to a GF > 1. The results show that restructuring of aerosol aggregates occurs for a number of aggregates of atmospherically relevant substances when exposed to elevations in relative humidity prior to deliquescence. The single particle control tests show no significant change in GF until the DRH at which point there is a significant increase as expected from deliquescence of single particles.21,22 Both NaCl and (NH4)2SO4 show similar trends of reduced GF prior to deliquescence. The aggregates of Na2SO4 particles also show a continuous decline in GF to the final measurement at 80% RH. No increase in GF was seen for Na2SO4 as the apparatus was limited to a RH less than the DRH of ∼84%. The Na2SO4 aggregates show less of a reduction in GF with increases in RH than the aggregates of other salts in this work. Potential reasons for this difference include the lower solubility of Na2SO4 and the potential formation of a thermodynamically stable hydrate (Na2SO4·10H2O) at higher RH. Experiments with additional salt types and combinations of salts can further enhance our understanding of the impact of particle properties on aggregate restructuring. The size of a collapsed aggregate can be estimated using a fractal model.46 If the primary particles are assumed to have a diameter equal to the peak concentration diameter before coagulation (75 nm) and form aggregates of 532 nm (to match NaCl in Figure 5) with a fractal dimension of 1.8 (representing diffusion limited aggregation)46 and mass mobility exponent of 2.1, complete collapse to a sphere would result in a diameter of 296 nm. This represents a GF of 0.56, which is substantially lower than the minimum 0.77 shown in Figure 5. The real aggregate will not collapse to form a perfect sphere as the particles have a physical limit on packing fraction and the rearrangement is likely to maintain certain aspects of the geometry before collapse. This theoretical approach nonetheless provides a suitable lower bound for the potential collapse of aggregates. The reduction in particle mobility diameter resulting from exposure to increased RH is consistent with a contraction of dendrite structures. Single hygroscopic aerosol particles show a restructuring or reformation of surface defects with increasing RH.20,22−24 This is a result of the interaction of H2O molecules with the salt surface, which results in adlayer formation of water at relative humidity far below deliquescence.43,47,48 The formation of water layers on the surface of salt particles forming an aggregate provides an explanation for contraction of the aggregate structure. As water adsorbs at the contact point between primary particles in the aggregates, it will impose surface tension forces.44,45 Force imbalances due to variations in geometry and surface properties could result in aggregate restructuring. 3.2. Microscopy Analysis. Image analysis of aggregate structure on wire mesh samples can provide further insight into the physical changes that occur as aggregates formed at low relative humidity are exposed to airstreams with elevated RH. A sample optical image of a loaded mesh is shown in Figure 6. The long white cylindrical features are the steel wires of the woven mesh. Deposited on these wires are aggregates of NaCl nanoparticles. The aggregates range considerably in size but are often in the range of 10−30 μm, 2 orders of magnitude larger than the constituent primary particles from the original flow.
Figure 4. Impact of relative humidity changes on peak mobility diameter of Al2O3 aggregates for increases from 0% RH.
wettability of the surface structure due to different contact angles between particles.44,45 3.1.3. Comparison of Hygroscopic Particles. Figure 5 compares the GF of aggregates formed at 0% RH and then
Figure 5. Growth factor of hygroscopic aggregates and primary particles formed at 0% RH and exposed to increasing relative humidity. The solid symbols represent data for aggregates. The open symbols represent data for single particles. The error bars are the standard deviation from multiple measurements during the same experiment.
exposed to air streams with elevated relative humidity up to 82%. The experiments have been conducted for aggregates of NaCl, Na2SO4, or (NH4)2SO4. Similar control experiments with primary particles (no coagulation) are shown for comparison. A direct comparison of the impact of salt type is not possible from this work due to the inability to ensure that aggregates of the same mobility diameter for different salts are comprised of the same primary particle size and structure. All aggregates tested show a statistically significant GF < 1 when relative humidity is increased above 30%−40% RH. The Growth Factor decreases (indicating shrinking aggregate size) E
DOI: 10.1021/acs.est.5b03157 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Figure 6. Sample optical image of NaCl aggregates formed on a wire mesh.
They are also substantially larger than the aggregates measured in the TDMA experiments which were limited by the measurement technique. Figure 7 shows two samples of image sequences obtained from the fluorescent imaging experiments. The fluorescent images are white where a Rhodamine particle is present in the field of view and black otherwise. The Rhodamine particles are deposited predominantly on the existing NaCl particles and provide an indication of the location and structure of the aggregates. The imaging area has been cropped to focus on a smaller region of interest resulting in different scales for each test. The images have been processed using ImageJ 1.46r to automatically adjust the brightness and contrast and to add labels and indicators. The two columns each represent a separate experiment. The left column focuses on aggregates that have formed within the wire mesh bounded by four wire strands represented by the black bounds in the corners of the image. The right column focuses on the tips of aggregate structures formed on top of a wire strand running vertically through the image. The five images in each column are spaced 2.5 min apart within the experimental procedure as described above. The second image represents time t = 5 min immediately before the flow was changed from 0% RH to the elevated value. The fourth image occurred immediately preceding the change back to 0% RH flow and the last image was 2.5 min after reverting back to 0% RH flow. The relative humidity at the time of each image is shown in the top left corner of the image. The complete sequence of images captured for these experiments and others are provided as compressed 30s videos in the Supporting Information and are used to inform the results in this section. The particle structure was found to be stable when exposed to 0% RH flow for the 5 min duration of these experiments and longer for other test cases (results not provided). After changing the valve positions to humidify the air (to 52% in the left column and 33% in the right) the aggregate structure starts to change within 20−30 s. The time delay is accounted for partially through the length of tubing between the control valves and the test cell. The aggregates in the left images separate to increase the open area between the wire mesh. The aggregate structures tend to collapse to more compact geometries, as shown in comparison to the fixed lines overlaid on both images. There is considerable movement of aggregates between panels while exposed to elevated RH. The aggregate motion ceases when the flow is reverted back to 0% RH. The changes shown in Figure 7 were repeatable for all other experiments with a variety of NaCl loading levels. Similar
Figure 7. Sample fluorescent images used for analysis. The left column shows changes for exposure to 52% RH and the right for exposure to 33% RH. The gray bars highlight specific areas of change.
microscopy experiments performed for loading with Al2O3 did not show a change in aggregate structure with exposure to elevated RH. These results are consistent with the TDMA measurements reported above and previous studies of the response of loaded filters, which show a reduction in the flow resistance (consistent with the opening of flow channels between the wire mesh) when exposed to an increase in RH above that during loading with hygroscopic particles.38 Figure 8 shows the magnitude of displacement of a sample of eight distinct Rhodamine particles on different aggregates during the same microscopy experiment shown in the right column of Figure 7. Displacement was similarly measured for the other experiments with NaCl particles as well as those with Al2O3 particles. This analysis is limited to 2D displacement of the structures, which is expected to be smaller than the actual three-dimensional change experienced. The average displacement occurring during the first 5 min of the experiments, while the RH is maintained at 0% was 0.20 and 0.22 μm, for NaCl F
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AUTHOR INFORMATION
Corresponding Author
*Tel.: 604 822 2781. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
J.M. received funding for this work through a Natural Sciences and Engineering Research Council of Canada Postgraduate Scholarship D (PGS D).
Figure 8. Sample displacement measurements of eight Rhodamine particles deposited on aggregates of NaCl during relative humidity exposure experiments.
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and Al2O3 experiments, respectively. During the 5 min of exposure to elevated RH (30−60% RH) the NaCl particles show significant displacement (av 1.33 μm) compared to the displacement of Al2O3 particles (0.40). During the final 5 min, after reverting back to 0% RH, there was little additional displacement of the particles (0.19 and 0.18 μm, for NaCl and Al2O3 experiments, respectively). If the aggregates are assumed to be 10−30 μm the measured displacement during the 5 min of exposure to elevated RH in NaCl experiments represents 5− 13% of the structure size. A more robust analysis of magnitude and characteristics of structural change can be determined by modified experiments to produce 3D reconstructions of the aggregates before and after exposure for comparison. The behavior of particle aggregates has implications for particle control and measurement technology. The results of the current work emphasize the importance of relative humidity in characterization and operation of particle removal devices such as HVAC air filters or personal respirator equipment. A restructuring of particle aggregates explains the reduction in flow resistance seen in filters loaded with hygroscopic particles when exposed to elevated relative humidity.38 A change in particle structure could also impact measurement technology that relies on flow37,38 (filters) or optical49,50 (aethalometer) properties of collected aerosol if hygroscopic particles are present and humidity variations are significant.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b03157. Video: Microscopy Experiment A − NaCl, Pf* = 30, 0% → 33% (AVI) Video: Microscopy Experiment B − NaCl, Pf* = 30, 0% → 33% (AVI) Video: Microscopy Experiment C − NaCl, Pf* = 8, 0% → 52% (AVI) Video: Microscopy Experiment D − NaCl, Pf* = 8, 0% → 52% (AVI) Schematic of TDMA experiment; schematic of the mesh loading and visualization flow cell apparatus; sample lognormal curve fitting procedure to determine peak aggregate size from measured TDMA data for a sample 0 → 60% experiment with NaCl (PDF) G
DOI: 10.1021/acs.est.5b03157 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.est.5b03157 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
