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Environmental scanning electron microscopy as a new technique to determine the hygroscopic behaviour of individual aerosol particles
Atmospheric Environment 36 (2002) 5909–5916
Environmental scanning electron microscopy as a new technique to determine the hygroscopic behaviour of individual aerosol particles Martin Ebert*, Marion Inerle-Hof, Stephan Weinbruch Fachgebiet Umweltmineralogie, Institut fur . Angewandte Geowissenschaften, Technische Universitat . Darmstadt, Schnittspahnstr. 9, D-64287 Darmstadt, Germany Received 14 June 2002; received in revised form 3 September 2002; accepted 16 September 2002
Abstract The hygroscopic behaviour of NaCl, (NH4)2SO4, Na2SO4 and NH4NO3 particles in the size range of 0.1–20 mm was studied by environmental scanning electron microscopy (ESEM). This technique allows the in-situ observation of individual aerosol particles while changing the temperature and/or relative humidity (RH) in the sample chamber. The hygroscopic behaviour of these particles (e.g., deliquescence, adsorption of water on the particle surface) becomes directly observable with a lateral resolution of the order of 8–15 nm. The deliquescence relative humidities (DRH) of the different salts, the temperature dependence of the DRH for NH4NO3, and the growth factors (at increasing relative humidities) for NaCl were determined. Generally, a good agreement between the values obtained by ESEM and those found in literature was achieved. However, the DRH of NaCl determined by ESEM is systematically higher (approximately 2%, absolute) than the values obtained by other techniques, which can be explained by the observed strong absorption of water onto the crystal surface prior to droplet formation. The efflorescence behaviour of individual particles can be studied only qualitatively due to influences of the sample substrate. Furthermore, it is demonstrated that the activation of soot can be studied at high resolution by ESEM. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Environmental scanning electron microscopy; Deliquescence; Relative humidity; Individual particle analysis; Hygroscopy
1. Introduction The growth of aerosol particles caused by the uptake of water during increasing relative humidities alters the physical and chemical properties of the aerosol strongly (see, e.g., Seinfeld and Pandis, 1998, and references therein). The hygroscopic growth influences light scattering (e.g., H.anel, 1984; Pilinis et al., 1995; Schwartz, 1996; Braun and Krieger, 2001), cloud formation and precipitation (Pruppacher and Klett, 1997; Martin, 2000), the atmospheric lifetime, and the chemical reactivity of the individual particles (Charlson et al., 1992). Due to its great importance, the hygroscopic *Corresponding author. Fax: +49-6151-164021. E-mail address: [email protected] (M. Ebert).
behaviour of aerosol particles has been studied for a long time (e.g., Junge, 1952; Orr et al., 1980; Tang, 1976). The majority of papers deals with the most common inorganic salt components of the ambient aerosol like NaCl, (NH4)2SO4, Na2SO4 and NH4NO3. The deliquescence relative humidities (DRH) of these pure salts are well known (e.g., Tang and Munkelwitz, 1993). More difficult to predict is the behaviour of salt mixtures (e.g., Tang and Munkelwitz, 1993; Potokuchi and Wexler, 1995; Ansari and Pandis, 1999) and internal mixtures of salts and insoluble components as, for example, soot (Dougle et al., 1998). To increase the complexity, organic components can alter the hygroscopic behaviour of salt particles severely (Saxena et al., 1995; Cruz and Pandis, 2000; Peng and Chan, 2001).
1352-2310/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 2 ) 0 0 7 7 4 - 4
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Information on the hygroscopic growth of the particles can be obtained by a variety of techniques including nephelometry (e.g., Dougle et al., 1998), tandem differential mobility analysis (e.g., Cruz and Pandis, 2000; H.ameri et al., 2000, 2001; Joutsensaari et al., 2001) and an electrodynamic balance (Cohen et al., 1987; Richardson and Snyder, 1994; Tang and Munkelwitz, 1993). However, these techniques do not provide information on morphological changes of the particles which accompany the growth process. Recently, atomic force microscopy (AFM) and transmission electron microscopy (TEM) were applied by Posfai et al. (1998) to study the morphology of wet and dry individual particles. Data acquisition in AFM, however, is too slow to image the deliquescence of aerosol particles. Environmental scanning electron microscopy (ESEM) enables an in-situ observation of the interaction between liquid water and aerosol particles with diameters in the sub-micrometer range. Thus, drop formation and particle growth (deliquescence), water adsorption on particle surfaces and activation of aerosol particles can be imaged. In addition, the chemical composition of individual particles can be studied in the same instrument by energy-dispersive X-ray microanalysis. Another advantage of ESEM is the fact that insulating samples need no conductive coating for charge compensation. Therefore, aerosol particles and their hygroscopic behaviour can be studied without any sample preparation and the surface morphology is not obscured by the coating. Hence, it is possible to relate the hygroscopic behaviour of individual particles to their chemical composition and mixing state which is a great advantage for the study of ambient aerosol samples. In the present paper, we want to demonstrate that the hygroscopic behaviour of individual aerosol particles can be determined accurately by ESEM. For this purpose, synthetic salt particles of simple chemical composition and known hygroscopic behaviour are investigated.
2. Experimental All measurements were carried out in a PHILIPS XL30 ESEM LaB6 instrument equipped with an energydispersive X-ray (EDX) detector. The special design of the vacuum system of the ESEM allows working pressures up to approximately 30 mbar in the sample chamber during imaging with secondary and backscattered electrons. Details of ESEM can be found elsewhere (e.g., Danilatos, 1988; Goldstein et al., 1994). The working pressure can be set by any non-flammable and non-corrosive gas including H2O. In our experiments, the total pressure in the sample chamber was always equal to the partial pressure of H2O. The pressure in the sample chamber can be varied in steps of E0.13 mbar (0.1 Torr) in the range of 0.13–26.6 mbar. In the range of 51C to
601C, the temperature of the sample can be varied in steps of 0.11C by use of a Peltier element. Particles of NaCl, (NH4)2SO4, Na2SO4 and NH4NO3 were prepared from p.a. grade substances (Merck, Darmstadt, Germany) by grinding. The phase composition of the material was checked by X-ray diffraction analysis. The particles were spread on stainless steel crucibles of 6 mm diameter. A different crucible was used for each salt sample. The DRH of the different salt samples, where drop formation starts, was determined at a temperature of 51C. At this temperature, the vapour pressure of liquid water corresponds to a RH of 100% is 8.75 mbar. It is not possible to determine the DRH at much higher temperatures by ESEM, as in this case the pressure in the sample chamber necessary to stabilize liquid water prevents high quality imaging. Initially the samples were held for 30 min at RH of 15%. Then, the RH was raised to the DRH. After each pressure increase, the sample was inspected for 1–3 min before the pressure was raised again. During the whole experiment, the secondary electron signal was stored as a digital video file. Thus, all morphological changes of the particle as a function of the water pressure are documented. After drop formation was observed at the DRH, the RH was reduced again in E0.13 mbar steps down to the efflorescence point where crystallisation occurs. In some experimental runs, the RH was raised above the DRH in order to determine droplet growth. The growth factors of the droplets and the efflorescence behaviour were studied only for NaCl. Growth factors were determined at relative humidities up to 90%. For NH4NO3, the temperature dependence of the DRH was measured in the temperature range from 51C to 151C in steps of 2.51C. All experiments with salt particles were carried out at magnifications less than approximately 10 000 (scan area of 12.8 mm 8.6 mm). Higher magnifications (i.e. higher current densities), which would be favourable for the detection of small morphological details, led to partial evaporation of the particles, especially of (NH4)2SO4 and NH4NO3. The evaporation is most likely caused by a temperature rise induced by the electron beam. At lower magnifications, most particles were stable during investigation. Activation of ambient soot agglomerates was studied at a temperature of 51C and slight supersaturations (RHp102%). The soot agglomerates were sampled on Formvar foils supported on Cu grids (generally used in transmission electron microscopy). These thin foils show only a small carbon peak in the EDX spectrum. 3. Results and discussion 3.1. Deliquescence relative humidity The DRH of more than 200 individual particles of NaCl, (NH4)2SO4, Na2SO4 and NH4NO3 with sizes
M. Ebert et al. / Atmospheric Environment 36 (2002) 5909–5916
between 0.1 and 20 mm was determined by ESEM at a temperature of 51C. The results are summarized in Table 1. The morphological changes observed during the deliquescence process are shown in Fig. 1 for a Na2SO4 particle. At water pressures p7 mbar (RHp83.7%), the Na2SO4 grain is solid (Fig. 1a). No change in the particle morphology was observed when the RH was raised up to this value. Increasing the water pressure by another E0.13 mbar leads to immediate droplet formation (Fig. 1b). The RH of this point (85% for this particle of Na2SO4) was taken as the DRH. A sharp transition within Dp ¼ 0:13 mbar between the solid state and the saturated liquid was observed for particles of (NH4)2SO4, Na2SO4, and NH4NO3. Hence, the DRH of these salts could be determined easily and quite accurately. For NaCl, the situation was more complex, as substantial adsorption of water on the crystal surface was observed at relative humidities below the DRH (Fig. 2). This process became first visible by secondary electron imaging at p ¼ 6:426:5 mbar (RH=75–76%). The water film on the surface of the NaCl crystals seems to be stable, as droplet formation (deliquescence) was not observed within 30 min. When p was raised to Table 1 Deliquescence relative humidity (DRH) determined by ESEM Substance
Temperature (1C)
DRH (%)a
nb
NaCl (NH4)2SO4 Na2SO4 NH4NO3 NH4NO3 NH4NO3 NH4NO3 NH4NO3
5 5 5 5 7.5 10 12.5 15
78.071.3 81.371.5 82.871.2 74.070.8 72.771.1 71.871.2 69.771.0 67.870.9
58 37 90 34 15 12 15 10
a b
Mean value 71 S.D. Number of particles studied.
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6.6 mbar (RH=78%), droplet formation occurs as fast as for the other salts investigated (within a second or so). Adsorption of water on the particle surface was also observed for a few NH4SO4 and NH4NO3 grains. However, the thickness of the water film was much smaller compared to NaCl. The DRH determined by ESEM (this study) is generally in good agreement with the results of various other techniques (Table 2). For comparison of the different results, the DRH determined in this study (at 51C) was calculated for a temperature of 251C using Eq. (1) of Tang and Munkelwitz (1993). DHs 1 1 DRHðTÞ ¼ DRHð298Þ exp A R T 298 T CðT 298Þ ð1Þ B ln 298 with DRH(T) the deliquescence relative humidity at temperature T (in Kelvin), DRH(298) the deliquescence relative humidity at 298 K, DHs the enthalpy of solution, R the gas constant, and A; B; and C empirical constants. The empirical constants A; B and C are taken from Tang and Munkelwitz (1993). The enthalpy of solution was assumed to be independent of temperature. It should be emphasized here that the pressure in the sample chamber can be changed only in steps of E0.13 mbar (0.1 Torr). Therefore, the precision of our measurements is limited by this instrumental parameter. As can be seen from Table 2, the DRH determined by ESEM is in excellent agreement with results of previous studies which were conducted with a variety of techniques including an electrodynamic balance (EDB), tandem differential mobility analysis (TDMA), nephelometry and Fourier transform infrared spectroscopy (FTIR). For (NH4)2SO4, Na2SO4 and NH4NO3, the value obtained by ESEM is well in the range of data reported in the literature. For NaCl, however, the DRH determined by ESEM (77.571.3% at 251C) is
Fig. 1. Secondary electron images of a Na2SO4 particle at a temperature of 51C, before (a) and after (b) droplet formation.
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systematically higher than the literature data (Table 2). The higher value obtained by ESEM can be explained as follows. We have defined the DRH as the RH where droplet formation is first observed. However, for NaCl substantial adsorption of water on the particle surface prior to droplet formation (at lower RHs) became visible by secondary electron imaging (Fig. 2). For NaCl, the RH where water adsorption is first visible by ESEM is approximately 75%. This value is very close to the DRH reported by the other techniques (Table 2). It is, thus, likely that the DRH obtained by EDB, TDMA, and FTIR does reflect substantial adsorption of water on the crystal surface rather than droplet formation. It should be emphasized here that the water film on the NaCl surface is stable for at least 30 min, i.e., droplet formation is not observed at this RH. In addition, the exact DRH value obtained by EDB, TDMA, and FTIR depends on a number of parameters (including particle geometry, surface tension and density) which cannot be determined by these techniques and which are often not known accurately (Weis and Ewing, 1999; Kr.amer et al., 2000). In contrast, the deliquescence
process is directly observed for individual particles by our technique. 3.2. Temperature dependence of the deliquescence relative humidity In order to test the reliability of ESEM for the determination of the hygroscopic behaviour of individual aerosol particles, the temperature dependence of the DRH was determined. We have chosen ammonium nitrate, as the temperature dependence of the DRH is quite pronounced for this substance. The DRH was measured at the five different temperatures of 51C, 7.51C, 101C, 12.51C and 151C (Table 1). To ensure thermal equilibrium with the cooled substrate, the temperature was held constant for 1 h before the pressure was raised. The temperature dependence obtained by ESEM and the theoretical values, calculated from Eq. (1) (using the empirical constants published by Tang and Munkelwitz, 1993) are shown in Fig. 3. An excellent agreement between the results of ESEM and the theoretical predictions is
Fig. 2. Secondary electron images of the deliquescence behaviour of NaCl particles at a temperature of 51C. (a) Solid particles at an RH of 70%; (b) first just detectable morphological changes (arrows) due to water adsorption at an RH of 75%; (c) a thin film of water (arrows) is visible on all particles at an RH of 76.7%; (d) droplet formation at an RH of 78.3%.
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Table 2 Deliquescence relative humidity (DRH) at 251C Substance
DRH (%) a
Technique
Reference
NaCl NaCl NaCl NaCl NaCl NaCl NaCl NaCl NaCl
77.571.3 75.370.1 7571 75.3 7571 7572 76.070.8 75.071.0 75
ESEM EDB FTIR EDB EDB FTIR TDMA TDMA TDMA
This study Tang and Munkelwitz (1993) Cziczo et al. (1997) Richardson and Snyder (1994) Cohen et al. (1987) Cziczo and Abbat (2000) H.ameri et al. (2001) Cruz and Pandis (2000) Joutsensaari et al. (2001)
(NH4)2SO4 (NH4)2SO4 (NH4)2SO4 (NH4)2SO4 (NH4)2SO4 (NH4)2SO4 (NH4)2SO4 (NH4)2SO4 (NH4)2SO4 (NH4)2SO4
79.871.5a 79.970.5 7971 7972 80 80.071.2 81 80 79.071.0 79
ESEM EDB FTIR FTIR EDB EDB EDB Nephelometry TDMA TDMA
This study Tang and Munkelwitz (1993) Cziczo et al. (1997) Han and Martin (1999) Tang and Munkelwitz (1994) Richardson and Spann (1984) Cohen et al. (1987) Dougle et al. (1998) Cruz and Pandis (2000) Joutsensaari et al. (2001)
NH4NO3 NH4NO3 NH4NO3 NH4NO3
61.970.8a 61.8 60 62.270.8
ESEM EDB Nephelometry EDB
This study Tang and Munkelwitz (1993) Dougle et al. (1998) Richardson and Hightower (1987)
Na2SO4 Na2SO4 Na2SO4 Na2SO4
84.471.2a 84.270.3 84 85
ESEM EDB EDB EDB
This study Tang and Munkelwitz (1993) Tang and Munkelwitz (1994) Cohen et al. (1987)
a
Measured at 51C, calculated for 251C, according to Tang and Munkelwitz (1993). ESEM: environmental scanning electron microscopy; EDB: electrodynamic balance. FTIR: Fourier transform infrared spectroscopy; TDMA: Tandem differential mobility analyser.
obtained. Therefore, it can be safely concluded that the temperature regulation in the instrument works accurately and that the particles were in thermal equilibrium with the cooled substrate.
The growth factors determined by ESEM (this work) and by TDMA (H.ameri et al., 2001; Joutsensaari et al., 2001) are compared in Fig. 4. The deviation between the different approaches is generally smaller than 0.2 (absolute). All measurements shown in Fig. 4 (ESEM and TDMA) were performed with grains large enough (X50 nm diameter) to neglect the Kelvin effect. For decreasing RH, the deviation between ESEM and TDMA is also p0.2 (absolute). However, the exact value of the efflorescence RH cannot be determined accurately by ESEM due to heterogeneous nucleation
ESEM (this study)
72 DRH [%]
3.3. Growth factors and efflorescence
theory (Tang and Munkelwitz, 1993)
75
69 66 63
NH4NO3 5
10
15
20
25
temperature [˚C] Fig. 3. DRH of NH4NO3 as function of temperature determined by ESEM (open circles) and calculated by Eq. (1) using the data of Tang and Munkelwitz (1993). Error bars are 71 S.D.
M. Ebert et al. / Atmospheric Environment 36 (2002) 5909–5916
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on the substrate. Therefore, crystallisation occurs at systematically higher RH (e.g., 65% for NaCl) compared to particles in the airborne state, and it must be concluded that the hysteresis can be studied only qualitatively by ESEM. 3.4. Activation of ambient soot agglomerates Besides the determination of the deliquescence behaviour, we have also studied activation of insoluble material. We have chosen soot agglomerates for our experiments, as this component is an important constituent of the atmospheric aerosol. The hydration behaviour of soot influences the optical properties, the lifetime and the chemical reactivity of the agglomerates (e.g., Chughtai et al., 1996; Mikhailov et al., 2001). Soot was sampled next to a main traffic road in the city of Darmstadt (Germany) and approximately 20 agglomerates were studied by ESEM at a temperature of 51C. In the present paper, all particles predominantly
growth factor
2.5
2.0
RH ↑, this study RH ↓, this study RH ↑, Joutsensaari. et al.,2001 RH ↓, Joutsensaari. et al.,2001 RH ↑, Hämeri et al., 2001 RH ↓, Hämeri et al., 2001
NaCl
1.5
composed of the element carbon (EDX analysis) and consisting of primary particles with 5–80 nm diameter are defined as soot. The agglomerates investigated are partly aged (Fig. 5a), i.e., they did not exhibit the chainlike structure described for fresh combustion soot . u. et al., 1995). (e.g., Forrest and Witten, 1979; Koyl Besides carbon, no other element could be detected by energy-dispersive X-ray microanalysis. Up to an RH of 99–100%, uptake of water was not observed (Fig. 5a). At an RH of 100% (or at a small supersaturation), the agglomerates became activated and a water film was observed (Fig. 5b). When the RH was reduced again, evaporation of water starts at an RH of 100%. After evaporation (Fig. 5c), the agglomerates were somewhat compacted, as could be seen from the reduced projection area of the agglomerates. We did not attempt to determine the compaction quantitatively. Compaction of soot agglomerates after exposure to high water pressures was already described by e.g. Huang et al. . (1994) and Kollensberger et al. (1999). Activation of combustion particles at relative humidities of 100% or at slight supersaturation has been reported in several papers (e.g., Hallett et al., 1989; Hagen et al., 1989; Hudson et al., 1991; Pitchford et al., 1991; Huang et al., 1994). In detail, however, activation of soot depends on several factors including the surface chemistry, the presence of additional phases and the mixing state (Chughtai et al., 1996; Mikhailov et al., 2001). As demonstrated, the activation of insoluble material can be imaged by ESEM. We will expand our future studies to investigate the influence of additional phases on the hygroscopic behaviour of ambient soot.
1.0 20
40
60
80
100
relative humitidy [%] Fig. 4. Growth factors of NaCl particles determined by ESEM (this study) and by TDMA (H.ameri et al., 2001; Joutsensaari et al., 2001) for increasing (m) and decreasing (k) RH.
4. Conclusions Environmental scanning electron microscopy (ESEM) has been shown to be a powerful tool for the investigation of the hygroscopic behaviour of individual atmospheric aerosol particles with diameters down to
Fig. 5. Secondary electron images of the activation of ambient soot agglomerates. (a) Inactivated at an RH of 100%; (b) activated at slight supersaturation; (c) compacted agglomerates after the RH was reduced again.
M. Ebert et al. / Atmospheric Environment 36 (2002) 5909–5916
approximately 100 nm. In contrast to all other techniques, the hygroscopic behaviour can be visualized and a number of parameters (including chemical composition and particle morphology) can be determined simultaneously. We have shown that the DRH of NH4NO3, NaCl, (NH4)2SO4 and Na2SO4 and the temperature dependence of the DRH of NH4NO3 can be measured with an uncertainty of typically 1% (absolute). For NaCl, substantial adsorption of water on the crystal surface prior to drop formation was observed. This process may explain the slightly higher value of the DRH obtained by ESEM in comparison to the literature data. Growth factors can also be determined quite reliably at increasing RH. At decreasing RH however, growth factors can be only obtained well above the efflorescence relative humidity. The exact value of the efflorescence relative humidity cannot be determined by ESEM, due to the influence of the substrate. It was further demonstrated that activation of insoluble aerosol particles (e.g., soot agglomerates) can be imaged with high lateral resolution.
Acknowledgements We would like to thank R. Apfelbach for X-ray diffraction analysis and T. Dirsch for assistance at the environmental scanning electron microscope.
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Environmental scanning electron microscopy as a new technique to determine the hygroscopic behaviour of individual aerosol particles Martin Ebert*, Marion Inerle-Hof, Stephan Weinbruch Fachgebiet Umweltmineralogie, Institut fur . Angewandte Geowissenschaften, Technische Universitat . Darmstadt, Schnittspahnstr. 9, D-64287 Darmstadt, Germany Received 14 June 2002; received in revised form 3 September 2002; accepted 16 September 2002
Abstract The hygroscopic behaviour of NaCl, (NH4)2SO4, Na2SO4 and NH4NO3 particles in the size range of 0.1–20 mm was studied by environmental scanning electron microscopy (ESEM). This technique allows the in-situ observation of individual aerosol particles while changing the temperature and/or relative humidity (RH) in the sample chamber. The hygroscopic behaviour of these particles (e.g., deliquescence, adsorption of water on the particle surface) becomes directly observable with a lateral resolution of the order of 8–15 nm. The deliquescence relative humidities (DRH) of the different salts, the temperature dependence of the DRH for NH4NO3, and the growth factors (at increasing relative humidities) for NaCl were determined. Generally, a good agreement between the values obtained by ESEM and those found in literature was achieved. However, the DRH of NaCl determined by ESEM is systematically higher (approximately 2%, absolute) than the values obtained by other techniques, which can be explained by the observed strong absorption of water onto the crystal surface prior to droplet formation. The efflorescence behaviour of individual particles can be studied only qualitatively due to influences of the sample substrate. Furthermore, it is demonstrated that the activation of soot can be studied at high resolution by ESEM. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Environmental scanning electron microscopy; Deliquescence; Relative humidity; Individual particle analysis; Hygroscopy
1. Introduction The growth of aerosol particles caused by the uptake of water during increasing relative humidities alters the physical and chemical properties of the aerosol strongly (see, e.g., Seinfeld and Pandis, 1998, and references therein). The hygroscopic growth influences light scattering (e.g., H.anel, 1984; Pilinis et al., 1995; Schwartz, 1996; Braun and Krieger, 2001), cloud formation and precipitation (Pruppacher and Klett, 1997; Martin, 2000), the atmospheric lifetime, and the chemical reactivity of the individual particles (Charlson et al., 1992). Due to its great importance, the hygroscopic *Corresponding author. Fax: +49-6151-164021. E-mail address: [email protected] (M. Ebert).
behaviour of aerosol particles has been studied for a long time (e.g., Junge, 1952; Orr et al., 1980; Tang, 1976). The majority of papers deals with the most common inorganic salt components of the ambient aerosol like NaCl, (NH4)2SO4, Na2SO4 and NH4NO3. The deliquescence relative humidities (DRH) of these pure salts are well known (e.g., Tang and Munkelwitz, 1993). More difficult to predict is the behaviour of salt mixtures (e.g., Tang and Munkelwitz, 1993; Potokuchi and Wexler, 1995; Ansari and Pandis, 1999) and internal mixtures of salts and insoluble components as, for example, soot (Dougle et al., 1998). To increase the complexity, organic components can alter the hygroscopic behaviour of salt particles severely (Saxena et al., 1995; Cruz and Pandis, 2000; Peng and Chan, 2001).
1352-2310/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 2 ) 0 0 7 7 4 - 4
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Information on the hygroscopic growth of the particles can be obtained by a variety of techniques including nephelometry (e.g., Dougle et al., 1998), tandem differential mobility analysis (e.g., Cruz and Pandis, 2000; H.ameri et al., 2000, 2001; Joutsensaari et al., 2001) and an electrodynamic balance (Cohen et al., 1987; Richardson and Snyder, 1994; Tang and Munkelwitz, 1993). However, these techniques do not provide information on morphological changes of the particles which accompany the growth process. Recently, atomic force microscopy (AFM) and transmission electron microscopy (TEM) were applied by Posfai et al. (1998) to study the morphology of wet and dry individual particles. Data acquisition in AFM, however, is too slow to image the deliquescence of aerosol particles. Environmental scanning electron microscopy (ESEM) enables an in-situ observation of the interaction between liquid water and aerosol particles with diameters in the sub-micrometer range. Thus, drop formation and particle growth (deliquescence), water adsorption on particle surfaces and activation of aerosol particles can be imaged. In addition, the chemical composition of individual particles can be studied in the same instrument by energy-dispersive X-ray microanalysis. Another advantage of ESEM is the fact that insulating samples need no conductive coating for charge compensation. Therefore, aerosol particles and their hygroscopic behaviour can be studied without any sample preparation and the surface morphology is not obscured by the coating. Hence, it is possible to relate the hygroscopic behaviour of individual particles to their chemical composition and mixing state which is a great advantage for the study of ambient aerosol samples. In the present paper, we want to demonstrate that the hygroscopic behaviour of individual aerosol particles can be determined accurately by ESEM. For this purpose, synthetic salt particles of simple chemical composition and known hygroscopic behaviour are investigated.
2. Experimental All measurements were carried out in a PHILIPS XL30 ESEM LaB6 instrument equipped with an energydispersive X-ray (EDX) detector. The special design of the vacuum system of the ESEM allows working pressures up to approximately 30 mbar in the sample chamber during imaging with secondary and backscattered electrons. Details of ESEM can be found elsewhere (e.g., Danilatos, 1988; Goldstein et al., 1994). The working pressure can be set by any non-flammable and non-corrosive gas including H2O. In our experiments, the total pressure in the sample chamber was always equal to the partial pressure of H2O. The pressure in the sample chamber can be varied in steps of E0.13 mbar (0.1 Torr) in the range of 0.13–26.6 mbar. In the range of 51C to
601C, the temperature of the sample can be varied in steps of 0.11C by use of a Peltier element. Particles of NaCl, (NH4)2SO4, Na2SO4 and NH4NO3 were prepared from p.a. grade substances (Merck, Darmstadt, Germany) by grinding. The phase composition of the material was checked by X-ray diffraction analysis. The particles were spread on stainless steel crucibles of 6 mm diameter. A different crucible was used for each salt sample. The DRH of the different salt samples, where drop formation starts, was determined at a temperature of 51C. At this temperature, the vapour pressure of liquid water corresponds to a RH of 100% is 8.75 mbar. It is not possible to determine the DRH at much higher temperatures by ESEM, as in this case the pressure in the sample chamber necessary to stabilize liquid water prevents high quality imaging. Initially the samples were held for 30 min at RH of 15%. Then, the RH was raised to the DRH. After each pressure increase, the sample was inspected for 1–3 min before the pressure was raised again. During the whole experiment, the secondary electron signal was stored as a digital video file. Thus, all morphological changes of the particle as a function of the water pressure are documented. After drop formation was observed at the DRH, the RH was reduced again in E0.13 mbar steps down to the efflorescence point where crystallisation occurs. In some experimental runs, the RH was raised above the DRH in order to determine droplet growth. The growth factors of the droplets and the efflorescence behaviour were studied only for NaCl. Growth factors were determined at relative humidities up to 90%. For NH4NO3, the temperature dependence of the DRH was measured in the temperature range from 51C to 151C in steps of 2.51C. All experiments with salt particles were carried out at magnifications less than approximately 10 000 (scan area of 12.8 mm 8.6 mm). Higher magnifications (i.e. higher current densities), which would be favourable for the detection of small morphological details, led to partial evaporation of the particles, especially of (NH4)2SO4 and NH4NO3. The evaporation is most likely caused by a temperature rise induced by the electron beam. At lower magnifications, most particles were stable during investigation. Activation of ambient soot agglomerates was studied at a temperature of 51C and slight supersaturations (RHp102%). The soot agglomerates were sampled on Formvar foils supported on Cu grids (generally used in transmission electron microscopy). These thin foils show only a small carbon peak in the EDX spectrum. 3. Results and discussion 3.1. Deliquescence relative humidity The DRH of more than 200 individual particles of NaCl, (NH4)2SO4, Na2SO4 and NH4NO3 with sizes
M. Ebert et al. / Atmospheric Environment 36 (2002) 5909–5916
between 0.1 and 20 mm was determined by ESEM at a temperature of 51C. The results are summarized in Table 1. The morphological changes observed during the deliquescence process are shown in Fig. 1 for a Na2SO4 particle. At water pressures p7 mbar (RHp83.7%), the Na2SO4 grain is solid (Fig. 1a). No change in the particle morphology was observed when the RH was raised up to this value. Increasing the water pressure by another E0.13 mbar leads to immediate droplet formation (Fig. 1b). The RH of this point (85% for this particle of Na2SO4) was taken as the DRH. A sharp transition within Dp ¼ 0:13 mbar between the solid state and the saturated liquid was observed for particles of (NH4)2SO4, Na2SO4, and NH4NO3. Hence, the DRH of these salts could be determined easily and quite accurately. For NaCl, the situation was more complex, as substantial adsorption of water on the crystal surface was observed at relative humidities below the DRH (Fig. 2). This process became first visible by secondary electron imaging at p ¼ 6:426:5 mbar (RH=75–76%). The water film on the surface of the NaCl crystals seems to be stable, as droplet formation (deliquescence) was not observed within 30 min. When p was raised to Table 1 Deliquescence relative humidity (DRH) determined by ESEM Substance
Temperature (1C)
DRH (%)a
nb
NaCl (NH4)2SO4 Na2SO4 NH4NO3 NH4NO3 NH4NO3 NH4NO3 NH4NO3
5 5 5 5 7.5 10 12.5 15
78.071.3 81.371.5 82.871.2 74.070.8 72.771.1 71.871.2 69.771.0 67.870.9
58 37 90 34 15 12 15 10
a b
Mean value 71 S.D. Number of particles studied.
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6.6 mbar (RH=78%), droplet formation occurs as fast as for the other salts investigated (within a second or so). Adsorption of water on the particle surface was also observed for a few NH4SO4 and NH4NO3 grains. However, the thickness of the water film was much smaller compared to NaCl. The DRH determined by ESEM (this study) is generally in good agreement with the results of various other techniques (Table 2). For comparison of the different results, the DRH determined in this study (at 51C) was calculated for a temperature of 251C using Eq. (1) of Tang and Munkelwitz (1993). DHs 1 1 DRHðTÞ ¼ DRHð298Þ exp A R T 298 T CðT 298Þ ð1Þ B ln 298 with DRH(T) the deliquescence relative humidity at temperature T (in Kelvin), DRH(298) the deliquescence relative humidity at 298 K, DHs the enthalpy of solution, R the gas constant, and A; B; and C empirical constants. The empirical constants A; B and C are taken from Tang and Munkelwitz (1993). The enthalpy of solution was assumed to be independent of temperature. It should be emphasized here that the pressure in the sample chamber can be changed only in steps of E0.13 mbar (0.1 Torr). Therefore, the precision of our measurements is limited by this instrumental parameter. As can be seen from Table 2, the DRH determined by ESEM is in excellent agreement with results of previous studies which were conducted with a variety of techniques including an electrodynamic balance (EDB), tandem differential mobility analysis (TDMA), nephelometry and Fourier transform infrared spectroscopy (FTIR). For (NH4)2SO4, Na2SO4 and NH4NO3, the value obtained by ESEM is well in the range of data reported in the literature. For NaCl, however, the DRH determined by ESEM (77.571.3% at 251C) is
Fig. 1. Secondary electron images of a Na2SO4 particle at a temperature of 51C, before (a) and after (b) droplet formation.
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systematically higher than the literature data (Table 2). The higher value obtained by ESEM can be explained as follows. We have defined the DRH as the RH where droplet formation is first observed. However, for NaCl substantial adsorption of water on the particle surface prior to droplet formation (at lower RHs) became visible by secondary electron imaging (Fig. 2). For NaCl, the RH where water adsorption is first visible by ESEM is approximately 75%. This value is very close to the DRH reported by the other techniques (Table 2). It is, thus, likely that the DRH obtained by EDB, TDMA, and FTIR does reflect substantial adsorption of water on the crystal surface rather than droplet formation. It should be emphasized here that the water film on the NaCl surface is stable for at least 30 min, i.e., droplet formation is not observed at this RH. In addition, the exact DRH value obtained by EDB, TDMA, and FTIR depends on a number of parameters (including particle geometry, surface tension and density) which cannot be determined by these techniques and which are often not known accurately (Weis and Ewing, 1999; Kr.amer et al., 2000). In contrast, the deliquescence
process is directly observed for individual particles by our technique. 3.2. Temperature dependence of the deliquescence relative humidity In order to test the reliability of ESEM for the determination of the hygroscopic behaviour of individual aerosol particles, the temperature dependence of the DRH was determined. We have chosen ammonium nitrate, as the temperature dependence of the DRH is quite pronounced for this substance. The DRH was measured at the five different temperatures of 51C, 7.51C, 101C, 12.51C and 151C (Table 1). To ensure thermal equilibrium with the cooled substrate, the temperature was held constant for 1 h before the pressure was raised. The temperature dependence obtained by ESEM and the theoretical values, calculated from Eq. (1) (using the empirical constants published by Tang and Munkelwitz, 1993) are shown in Fig. 3. An excellent agreement between the results of ESEM and the theoretical predictions is
Fig. 2. Secondary electron images of the deliquescence behaviour of NaCl particles at a temperature of 51C. (a) Solid particles at an RH of 70%; (b) first just detectable morphological changes (arrows) due to water adsorption at an RH of 75%; (c) a thin film of water (arrows) is visible on all particles at an RH of 76.7%; (d) droplet formation at an RH of 78.3%.
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Table 2 Deliquescence relative humidity (DRH) at 251C Substance
DRH (%) a
Technique
Reference
NaCl NaCl NaCl NaCl NaCl NaCl NaCl NaCl NaCl
77.571.3 75.370.1 7571 75.3 7571 7572 76.070.8 75.071.0 75
ESEM EDB FTIR EDB EDB FTIR TDMA TDMA TDMA
This study Tang and Munkelwitz (1993) Cziczo et al. (1997) Richardson and Snyder (1994) Cohen et al. (1987) Cziczo and Abbat (2000) H.ameri et al. (2001) Cruz and Pandis (2000) Joutsensaari et al. (2001)
(NH4)2SO4 (NH4)2SO4 (NH4)2SO4 (NH4)2SO4 (NH4)2SO4 (NH4)2SO4 (NH4)2SO4 (NH4)2SO4 (NH4)2SO4 (NH4)2SO4
79.871.5a 79.970.5 7971 7972 80 80.071.2 81 80 79.071.0 79
ESEM EDB FTIR FTIR EDB EDB EDB Nephelometry TDMA TDMA
This study Tang and Munkelwitz (1993) Cziczo et al. (1997) Han and Martin (1999) Tang and Munkelwitz (1994) Richardson and Spann (1984) Cohen et al. (1987) Dougle et al. (1998) Cruz and Pandis (2000) Joutsensaari et al. (2001)
NH4NO3 NH4NO3 NH4NO3 NH4NO3
61.970.8a 61.8 60 62.270.8
ESEM EDB Nephelometry EDB
This study Tang and Munkelwitz (1993) Dougle et al. (1998) Richardson and Hightower (1987)
Na2SO4 Na2SO4 Na2SO4 Na2SO4
84.471.2a 84.270.3 84 85
ESEM EDB EDB EDB
This study Tang and Munkelwitz (1993) Tang and Munkelwitz (1994) Cohen et al. (1987)
a
Measured at 51C, calculated for 251C, according to Tang and Munkelwitz (1993). ESEM: environmental scanning electron microscopy; EDB: electrodynamic balance. FTIR: Fourier transform infrared spectroscopy; TDMA: Tandem differential mobility analyser.
obtained. Therefore, it can be safely concluded that the temperature regulation in the instrument works accurately and that the particles were in thermal equilibrium with the cooled substrate.
The growth factors determined by ESEM (this work) and by TDMA (H.ameri et al., 2001; Joutsensaari et al., 2001) are compared in Fig. 4. The deviation between the different approaches is generally smaller than 0.2 (absolute). All measurements shown in Fig. 4 (ESEM and TDMA) were performed with grains large enough (X50 nm diameter) to neglect the Kelvin effect. For decreasing RH, the deviation between ESEM and TDMA is also p0.2 (absolute). However, the exact value of the efflorescence RH cannot be determined accurately by ESEM due to heterogeneous nucleation
ESEM (this study)
72 DRH [%]
3.3. Growth factors and efflorescence
theory (Tang and Munkelwitz, 1993)
75
69 66 63
NH4NO3 5
10
15
20
25
temperature [˚C] Fig. 3. DRH of NH4NO3 as function of temperature determined by ESEM (open circles) and calculated by Eq. (1) using the data of Tang and Munkelwitz (1993). Error bars are 71 S.D.
M. Ebert et al. / Atmospheric Environment 36 (2002) 5909–5916
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on the substrate. Therefore, crystallisation occurs at systematically higher RH (e.g., 65% for NaCl) compared to particles in the airborne state, and it must be concluded that the hysteresis can be studied only qualitatively by ESEM. 3.4. Activation of ambient soot agglomerates Besides the determination of the deliquescence behaviour, we have also studied activation of insoluble material. We have chosen soot agglomerates for our experiments, as this component is an important constituent of the atmospheric aerosol. The hydration behaviour of soot influences the optical properties, the lifetime and the chemical reactivity of the agglomerates (e.g., Chughtai et al., 1996; Mikhailov et al., 2001). Soot was sampled next to a main traffic road in the city of Darmstadt (Germany) and approximately 20 agglomerates were studied by ESEM at a temperature of 51C. In the present paper, all particles predominantly
growth factor
2.5
2.0
RH ↑, this study RH ↓, this study RH ↑, Joutsensaari. et al.,2001 RH ↓, Joutsensaari. et al.,2001 RH ↑, Hämeri et al., 2001 RH ↓, Hämeri et al., 2001
NaCl
1.5
composed of the element carbon (EDX analysis) and consisting of primary particles with 5–80 nm diameter are defined as soot. The agglomerates investigated are partly aged (Fig. 5a), i.e., they did not exhibit the chainlike structure described for fresh combustion soot . u. et al., 1995). (e.g., Forrest and Witten, 1979; Koyl Besides carbon, no other element could be detected by energy-dispersive X-ray microanalysis. Up to an RH of 99–100%, uptake of water was not observed (Fig. 5a). At an RH of 100% (or at a small supersaturation), the agglomerates became activated and a water film was observed (Fig. 5b). When the RH was reduced again, evaporation of water starts at an RH of 100%. After evaporation (Fig. 5c), the agglomerates were somewhat compacted, as could be seen from the reduced projection area of the agglomerates. We did not attempt to determine the compaction quantitatively. Compaction of soot agglomerates after exposure to high water pressures was already described by e.g. Huang et al. . (1994) and Kollensberger et al. (1999). Activation of combustion particles at relative humidities of 100% or at slight supersaturation has been reported in several papers (e.g., Hallett et al., 1989; Hagen et al., 1989; Hudson et al., 1991; Pitchford et al., 1991; Huang et al., 1994). In detail, however, activation of soot depends on several factors including the surface chemistry, the presence of additional phases and the mixing state (Chughtai et al., 1996; Mikhailov et al., 2001). As demonstrated, the activation of insoluble material can be imaged by ESEM. We will expand our future studies to investigate the influence of additional phases on the hygroscopic behaviour of ambient soot.
1.0 20
40
60
80
100
relative humitidy [%] Fig. 4. Growth factors of NaCl particles determined by ESEM (this study) and by TDMA (H.ameri et al., 2001; Joutsensaari et al., 2001) for increasing (m) and decreasing (k) RH.
4. Conclusions Environmental scanning electron microscopy (ESEM) has been shown to be a powerful tool for the investigation of the hygroscopic behaviour of individual atmospheric aerosol particles with diameters down to
Fig. 5. Secondary electron images of the activation of ambient soot agglomerates. (a) Inactivated at an RH of 100%; (b) activated at slight supersaturation; (c) compacted agglomerates after the RH was reduced again.
M. Ebert et al. / Atmospheric Environment 36 (2002) 5909–5916
approximately 100 nm. In contrast to all other techniques, the hygroscopic behaviour can be visualized and a number of parameters (including chemical composition and particle morphology) can be determined simultaneously. We have shown that the DRH of NH4NO3, NaCl, (NH4)2SO4 and Na2SO4 and the temperature dependence of the DRH of NH4NO3 can be measured with an uncertainty of typically 1% (absolute). For NaCl, substantial adsorption of water on the crystal surface prior to drop formation was observed. This process may explain the slightly higher value of the DRH obtained by ESEM in comparison to the literature data. Growth factors can also be determined quite reliably at increasing RH. At decreasing RH however, growth factors can be only obtained well above the efflorescence relative humidity. The exact value of the efflorescence relative humidity cannot be determined by ESEM, due to the influence of the substrate. It was further demonstrated that activation of insoluble aerosol particles (e.g., soot agglomerates) can be imaged with high lateral resolution.
Acknowledgements We would like to thank R. Apfelbach for X-ray diffraction analysis and T. Dirsch for assistance at the environmental scanning electron microscope.
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