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Studies of Single Aerosol Particles Containing Malonic Acid, Glutaric Acid, and Their Mixtures with Sodium Chloride. I. Hygroscopic Growth Francis D. Pope,*,† Ben J. Dennis-Smither,†,| Paul T. Griffiths,‡ Simon L. Clegg,§ and R. Anthony Cox† Department of Chemistry, UniVersity of Cambridge, Lensfield Road, Cambridge, CB2 1EW, U.K., Department of Geography, UniVersity of Cambridge, Downing Place, Cambridge, CB2 3EN, U.K., and School of EnVironmental Sciences, UniVersity of East Anglia, Norwich, NR34 7TJ, U.K. ReceiVed: January 4, 2010; ReVised Manuscript ReceiVed: March 18, 2010
We describe a newly constructed electrodynamic balance with which to measure the relative mass of single aerosol particles at varying relative humidity. Measurements of changing mass with respect to the relative humidity allow mass (m) growth factors (maqueous/mdry) and diameter (d) growth factors (daqueous/ddry) of the aerosol to be determined. Four aerosol types were investigated: malonic acid, glutaric acid, mixtures of malonic acid and sodium chloride, and mixtures of glutaric acid and sodium chloride. The mass growth factors of the malonic acid and glutaric acid aqueous phase aerosols, at 85% relative humidity, were 2.11 ( 0.08 and 1.73 ( 0.19, respectively. The mass growth factors of the mixed organic/inorganic aerosols are dependent upon the molar fraction of the individual components. Results are compared with previous laboratory determinations and theoretical predictions. 1. Introduction Aerosols are fundamental components of the earth’s climate system because of their interaction with solar and terrestrial radiation. They can directly scatter and absorb radiation, and indirectly they influence the microphysics of clouds. The total radiative forcing due to aerosols is large and negative, leading to a net atmospheric cooling, which offsets a significant fraction of the global warming due to greenhouse gases. The most recent intergovernmental panel on climate change (IPCC) report indicates that aerosols dominate the uncertainty in the total radiative forcing budget of the earth.1 Both direct and indirect aerosol radiative effects are influenced by aerosol particle size. For the direct aerosol effect, the propensity for a particle to scatter light is a function of the particle size. For the indirect effect, the ability of a particle to act as a cloud condensation nucleus (CCN) is also particle size dependent.2 In addition to their radiative effects, aerosols also influence atmospheric chemistry by providing heterogeneous surfaces, and volumes, for reactions to occur upon and within.2 Hence aerosol size, surface area, and volume are crucial parameters for the determination of the rates of heterogeneous processes. Furthermore, aerosols have been linked with human health effects; the ability of aerosols to interact with pulmonary function is linked to the penetration depth within the lungs. Aerosol size determines the penetration depth.3 The total mass of tropospheric aerosol can have up to 90% of its composition contributed by organic matter.4 The watersoluble component of this material can contain ∼20-50% polycarboxylic acids, which are similar in weight and functionality to humic-like substances (HULIS).5,6 To understand the * Corresponding author. Phone: (+44) 01233 331791; e-mail: fdp21@ cam.ac.uk. † Department of Chemistry, University of Cambridge. ‡ Department of Geography, University of Cambridge. § University of East Anglia. | Current address: School of Chemistry, University of Bristol, Cantock’s Close, BS8 1TS, U.K.
hygroscropic properties of these complicated organic systems, it is desirable to understand the more simple dicarboxylic acids, e.g., glutaric acid (GA) and malonic acid (MA), the C5 and C3 straight chain dicarboxylic acids. Aerosols can contain both organic and inorganic components in mixed-component mixtures. Production of mixed-component aerosols can arise from several different routes, for example, by the agglomeration of different aerosol types. Dicarboxylic acids have a propensity to form hydrogen bonds, and thus are generally hydrophilic and water-soluble. Knowledge of aerosol growth characteristics under different environmental parameters is required to successfully model the direct radiative effect of aerosols, and their behavior as CCNs. The most important parameters, with respect to aerosol growth, are the temperature and the relative humidity. At any particular constant temperature, the relative humidity of the gaseous environment, and more precisely the water activity in air determines the thermodynamic state in which the aerosol will be found. Typically a threshold relative humidity exists where a solid to aqueous phase change occurs: the deliquescence point. Once aqueous, aerosols will grow, by taking up more water, with increasing relative humidity. It is possible to relate this growth factor to the CCN ability of the aerosol by several analytical methods.7 When the relative humidity is decreased, the aerosol will decrease in water content, and typically a phase change back from solid to aqueous solution will occur at a lower relative humidity than the thermodynamic limit, due to kinetic limitations in the formation of the crystalline solid. This efflorescence phase change is a statistical process, described by crystallization kinetics, and typically occurs over a range of relative humidity. The phase change and growth characteristics of aerosol can be temperature dependent.8 A detailed review of previous laboratory determinations of the solubility and hygroscopicity of MA, GA, and dicarboxylic acids in general has been published recently.9 Noticeably, a large disagreement exists in the observed hygroscopicity of GA, with the study of Peng and Chan showing significantly greater growth
10.1021/jp100059k 2010 American Chemical Society Published on Web 04/02/2010
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factors than the study of Cruz and Pandis who used, respectively, an electrodynamic balance (EDB) and tandem differential mobility analysis (TDMA) instrument.10,11 The only previous EDB experiments on the systems studied in this paper were by Chan and co-workers who investigated MA, GA, and their mixtures with sodium chloride in the temperature range 293-298 K.10,12,13 At high relative humidities (>97%), the hygroscopicity of mixtures of GA and NaCl have been measured using the aerosol optical tweezers technique.14 In this work, mixtures of MA and GA with NaCl will be referred to as MA-NaCl and GA-NaCl, respectively. This study investigates the temperature-dependent hygroscopic growth of MA and GA, and also their mixtures with sodium chloride (NaCl) were investigated. In Part II of this work, the effect of the nonvolatile inorganic component on the vapor pressures of the organic acids are studied.15 2. Experimental Methods The EDB technique for levitating single particles has been used in numerous laboratories, in a configuration similar to these studies, for approximately 40 years. A particle, containing multiple charges, is levitated by placement in an electrodynamic potential well. Once the particle is levitated, the surrounding gas phase environment is perturbed in a controlled manner, and the resulting mass and size change of the particle is observed. The environmental perturbation can then be correlated to the resulting mass and size changes. Stable levitation is achieved by subjecting the charged particle to a superposed electrostatic DC field and electodynamic AC field. The force generated on the charged particle within the electrostatic field balances the weight of the particle, in addition to any other vertical forces present, which could include thermophoretic, photophoretic, and Stokesian drag forces.16 Within the AC field, the charged particle is focused toward the central minimum potential referred to as the null point. The movement of the particle away from the null point results in the generation of a restorative force upon the charged particle, therefore stabilizing the particle within the horizontal plane. In the absence of forces other than the particle weight and the balancing electrostatic force, the mass (m) of the particle can be equated to the DC voltage (VDC) using eq E2, where g is the gravitational force at the earth’s surface, n is the number of elementary charges present on the particle, q is the elementary charge, z is the distance between the ring electrodes, and C is a geometric factor that is dependent on the shape of the EDB arrangement.16 Importantly, the mass of the particle is proportional to the balancing DC voltage.
mg )
VDC nqC z
(E2)
Since the charge on a levitated particle remains constant, the relative change in particle mass can be followed by observation of the balancing DC voltage. The hygroscopic mass growth factor is related to the dry particle mass by mwet/mdry ) VDC,wet/ VDC,dry. The mass growth factor can be transformed into a diameter growth factor (dwet/ddry) using eq E3, where x refers to the nonaqueous component of the particle and F is the density. The equation assumes particle sphericity, which is true for aqueous particles, but only approximately true for nonaqueous crystalline or amorphous particles.
(
Fx mH2O d ) 1+ do FH2O mx
)
1/3
(E3)
The EDB used in this study is based upon the double ring electrode design of Davis et al., where the DC bias voltage is applied in superposition with the AC potential to two electrode rings.16 The chamber and initial design was supplied by Laucks Consulting Company. The EDB chamber, containing the electrodes, is made of stainless steel. It has an approximate cylindrical shape, with a diameter and length of 10 and 12 cm, respectively. The chamber contains eight ports with which to optically probe the trapped particle. The top of the system contains a sealable entrance from which the sample particle can be introduced. Typical AC voltages and AC frequencies required for stable levitation were typically 2-3 kV and 50-100 Hz. The DC voltage can be varied between 0-100 V. The configuration of the EDB allows the levitation of particles in the approximate size range of ∼20-50 µm. Small temperature gradients present within the EDB chamber lead to the existence of convection currents within the levitation region, and hence small pressure gradients. This complicates the conversion of balancing voltage to the relative mass, via eq E2, and hence the determination of the hygroscopic growth of the levitated particles. The force associated with the pressure gradient is proportional to the particle area, while the weight of the particle is proportional to the volume. Therefore, as the particle increases in size, the importance of the temperature effect should decrease. To retain a linear relationship between VDC and aerosol mass, the convection force is normalized. It was found that the perturbation is approximately linear with respect to the average cell temperature. Therefore, for each run, the small variation in balancing voltage with average cell temperature was recorded under low relative humidity conditions (1 h). Once dried out, the particle started to grow at approximately 68% relative humidity, and exhibited complete deliquescence at 75 ( 1%.
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Figure 4. Mass growth factors of MA, at 280.5 K, as a function of relative humidity. Symbols: solid gray square - deliquescence mode run starting from a fully dried solid particle; white open square deliquescence mode run starting from a partially dried solid particle; open circle and the solid gray circle - the resulting efflorescence mode runs from the two respective deliquescence mode runs. Lines: black and gray - deliquescence and efflorescence curves calculated using the fitted activity equation of Clegg and Seinfeld9 (solid) and UNIFAC (dotted) with the parameter set of Peng et al.10 At 280.5 K there was insufficient data to allow an estimate of the uncertainty, it is expected that the uncertainty will be similar to that recorded at higher temperatures.
The solubility in water of MA at 280.5 K is 11.65 mol kg-1,23 which is significantly less than that at 298.15 K (15.3 mol kg-1).9 Using this solubility in combination with the activity equation of Clegg and Seinfeld yields a calculated deliquescence point of 79% relative humidity. The experimental results in Figure 4 therefore suggest that MA is able to take up water significantly below its deliquescence point even from an initial dry state. We also show in Figure 4 a deliquescence curve obtained by exposing an MA particle to increasing humidity, following incomplete drying. These results demonstrate water uptake beginning at an even lower relative humidity than for the dry particle. In contrast to MA, the GA particles displayed much more clearly defined deliquescence behavior in their humidograms if care was taken to first produce dry particles (see Figure 3). The initially dried particles retain their dry mass, with increasing humidity until ∼85% relative humidity, at which point rapid growth is observed with the transition to a fully liquid particle occurring at 93 ( 1%. This agrees quite closely with the value of 87.8% at 298.15 K obtained using a solubility in water of 10.68 mol kg-1 at that temperature and the fitted activity equation of Clegg and Seinfeld.9 Efflorescence of the GA particles occurred within the range of 20-30% relative humidity. The results of Peng et al. for the efflorescence mode experiments,10 which are also shown in Figure 3, agree quite well with those obtained in this study. The agreement is much better than that between the results of Peng et al. and those of Cruz and Pandis, who used a TDMA instrument. Reasons for the lower hygroscopicity observed by Cruz and Pandis are suggested by Peng et al.11 Finally, we observed that an aerosol particle generated from a sample of aged GA solution that was kept in a closed flask, containing laboratory air, for 10 months underwent effluorescent phase changes at greater relative humidities than freshly made samples. This may indicate chemical processing of the GA
Figure 5. (a) Mass growth factors of GA-NaCl (molar ratio 1:2.4), at 295 K, as a function of relative humidity. Symbols: solid squares, and open circles - deliquescence mode (solid) and efflorescence mode (open) growth curves measured in this study (errors are 1 σ and discussed in text). Lines: gray and black - deliquescence (gray) and efflorescence (black) curves calculated using the fitted activity equation of Clegg and Seinfeld.9 (b) Calculated percentages of NaCl and MA in aqueous solution corresponding to the deliquescence curve in (a). Lines: black and gray - the percentages of GA and NaCl in aqueous solution as opposed to the solid phase calculated using the fitted activity equation of Clegg and Seinfeld.9
within the flask. Possible reasons for this processing include reaction with trace impurities present in the solution (purity of GA was 99%), surface oxidation by the presence of gaseous oxidants (unlikely, because flask was sealed), or photolytic reaction occurring within the visibly transparent flask. This behavior was not investigated further. 3.3. Mixtures of MA and GA with NaCl at 295 K. Two different molar ratios of dicarboxylic acid/NaCl were investigated for both MA-NaCl (2:1 and 1:1 molar ratio) and GA-NaCl (1:2.4 and 1.3:1 molar ratio). The humidogram of GA-NaCl, with molar ratio 1:2.4, is shown in Figure 5. For any particle containing two or more solutes, deliquescence will occur over a range of relative humidity unless the particle contains amounts of solutes corresponding to the eutonic composition (for which an aqueous solution would be simultaneously saturated with respect to both solid phases). Calculations for GA-NaCl mixtures at 298.15 K by Clegg and Seinfeld9 using the Zdanovskii-Stokes-Robinson approach to estimate activities in the mixture (see their Figure 21) suggest that, for the 1:2.4 ratio, water uptake should begin at about 60% relative humidity and be complete at about 73%. Our own calculations, plotted in Figure 5, using the Clegg et al.20 method
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Pope et al. There is good agreement. Because uptake of water was observed at all relative humidities in our studies of pure MA particles at 295 K, it was assumed in the model calculations shown in Figure 6 that all MA was present in the aqueous phase at all relative humidities. The deliquescence point of the NaCl component can be seen to occur at a lower relative humidity (67 ( 1%) when compared to a pure NaCl particle. Efflorescence was observed at ∼31% relative humidity. Conclusions
Figure 6. (a) Mass growth factors of MA-NaCl (molar ratio 1:1), at 295 K, as a function of relative humidity. Symbols: black squares deliquescence mode growth curves measured in this study (errors are 1 σ and discussed in text). Lines: black, dark gray and light gray deliquescence mode (black), deliquescence mode with the solid MA phase suppressed (dark gray), and the deliquescence mode with both NaCl and MA suppressed (light gray) calculated using the fitted activity equation of Clegg and Seinfeld.9 (b) Calculated percentages of NaCl and MA in aqueous solution, corresponding to the calculation with solid phase MA suppressed in panel a. Lines: black and gray lines - the percentages of MA and NaCl in aqueous solution as opposed to the solid phase calculated using the fitted activity equation of Clegg and Seinfeld.9 The solid phase of MA has been suppressed because this gives the best agreement between laboratory and modeled data as shown in panel a.
of estimating activities, predict a higher value relative humidity for the onset of water uptake of about 67%. This agrees quite well with the measurements that show that uptake of water begins at a relative humidity of ∼66%, and complete deliquescence occurs at greater than 75% relative humidity. The range of efflorescence points determined for the mixtures in the experiments is about 30% to 40%, similar to the efflorescence of pure NaCl. The growth curve of the liquid particles above the deliquescence point is predicted accurately by the model, and, because most of the water uptake can be attributed to the NaCl, it is also well-described by the pure NaCl growth curve weighted by the mole fraction of NaCl present in the GA-NaCl particles. The deliquescence mode humidogram of 1:1 molar ratio MA-NaCl is shown in Figure 6, together with predictions based upon an MA solubility of 15.3 mol kg-1, the fitted activity model of Clegg and Seinfeld,9 and water and solute activities in the aqueous particle estimated using the approach of Clegg et al.20
The newly constructed EDB described in this study has provided new data sets with which to evaluate the hygroscopicity of both MA and GA, and their mixtures with NaCl. Our results agree satisfactorily with previous determinations of the hygroscopicity of both MA and GA. We found that MA particles at 280 K behaved somewhat differently from those at 295 K, exhibiting a greater tendency to retain water below the deliquescence point. The cause of this behavior is unclear, but it emphasizes the need to completely dry out particles before measuring the thermodynamic parameters such as the location of the deliquescence point. Measurements were performed with mixtures of MA and GA with NaCl to test the ability of models to predict the hygroscopicity of mixed organic-inorganic aerosol. The agreement between the model calculations and laboratory hygroscopic studies demonstrates that a simple approach, in which the properties of the aqueous mixtures are estimated from those of the pure solutions, gives good results. In general, the predicted efflorescence curves agreed with the laboratory data within the 1 σ error. This work illustrates, in agreement with other studies, that some knowledge of the relative humidity history of aerosols is generally required to accurately model their water content. Inorganic aerosols that experience relative humidity below their efflorescence points (and therefore dry out) are likely to remain solid up to their deliquescence points if relative humidity increases. However, our results for MA at low temperature, and for MA with NaCl, demonstrate that it is also possible for aerosol particles to contain liquid water at all relative humidities. These results are consistent with what was observed experimentally by Choi and Chan,13 and later modeled by Clegg and Seinfeld.9 The overall consistency of the laboratory results with the modeling studies gives confidence that the properties of simple two-component mixed inorganic/organic aerosols can be predicted by thermodynamic models. Acknowledgment. We thank Dr. Christine Braban (Centre for Ecology and Hydrology, Edinburgh, U.K.) for her help in the initial set up of the EDB. Prof. Jonathan Reid (University of Bristol, UK) is thanked for his continual input throughout the project. The EDB is owned by the Laboratory for Global Marine and Atmospheric Chemistry, University of East Anglia, and was set up and run at the Department of Chemistry, University of Cambridge as part of a collaborative project funded by the NERC thematic program APPRAISE. B.J.D.-S. thanks the corporate associates scheme for funding for his summer studentship. Supporting Information Available: A table of the mass growth factors (m/m0) for MA, GA, 1:1 molar mixture of MA-NaCl, 1:2.4 molar mixture of GA-NaCl taken at 295 K is provided. This information is available free of charge via the Internet at http://pubs.acs.org.
Hygroscopic Growth of Single Aerosol Particles References and Notes (1) Forster, P.; Ramaswamy, V.; Artaxo, P.; Bernsten, T.; Betts, R.; Fahey, D. W.; Haywood, J.; Lean, J.; Lowe, D. C.; Myhre, G.; Nganga, J.; Prinn, R.; Raga, G.; Schulz, M.; Van Dorland, R. Changes in atmospheric constituents and in radiative forcing. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the IntergoVernmental Panel on Climate Change; Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K. B., Tignor, M., Miller, H. L., Eds.; Cambridge University Press: Cambridge, U.K. and New York, 2007. (2) Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change; Wiley Interscience: New York, 1998. (3) Pope, C. A., III; Dockery, D. W. J. Air Waste Manage. Assoc. 2006, 56, 709. (4) Kanikidou, M.; Seinfeld, J. H.; Pandis, S. N.; Barnes, I.; Dentener, F. J.; Facchini, M. C.; Van Dingenen, R.; Ervens, B.; Nenes, A.; Nielsen, C. J.; Swietlicki, E.; Putaud, J. P.; Balkanski, Y.; Fuzzi, S.; Horth, H.; Moortgat, G. K.; Winterhalter, T.; Myhre, C. E. L.; Tsigaridis, K.; Vignati, E.; Stephanou, E. G.; Wilson, G. Atmos. Chem. Phys. 2005, 5, 1053. (5) Fuzzi, S.; Decesari, S.; Facchini, M. C.; Matta, E.; Mircea, M.; Tagliavini, E. Geophys. Res. Lett. 2001, 20, 4079. (6) McFiggans, G. B.; Alfarra, M. R.; Allan, J.; Bower, K. N.; Coe, H.; Cubison, M. J.; Topping, D. O.; Williams, P. I.; Desesari, S.; Facchini, C.; Fuzzi, S. Faraday Discuss. 2005, 130, 341.
J. Phys. Chem. A, Vol. 114, No. 16, 2010 5341 (7) Petters, M. D.; Kreidenweis, S. M. Atmos. Chem. Phys. 2007, 7, 1961. (8) Martin, S. T. Chem. ReV. 2000, 100, 3403. (9) Clegg, S. L.; Seinfeld, J. H. J. Phys. Chem. A 2006, 110, 5692. (10) Peng, C.; Chan, M. N.; Chan, C. K. EnViron. Sci. Technol. 2001, 35, 4495. (11) Cruz, C. N.; Pandis, S. N. EnViron. Sci. Technol. 2000, 34, 4313. (12) Choi, M. Y.; Chan, C. K. J. Phys. Chem. A 2002, 106, 4566. (13) Choi, M. Y.; Chan, C. K. EnViron. Sci. Technol. 2002, 36, 2422. (14) Hanford, K. L.; Mitchem, L.; Reid, J. P.; Clegg, S. L.; Topping, D. O.; McFiggans, G. B. J. Phys. Chem. A 2008, 112, 9413. (15) Pope, F. D.; Dennis-Smither, B. J.; Griffiths, P. T.; Clegg, S. L.; Cox, R. A. J. Phys. Chem. A 2010. (16) Davis, E. J.; Buehler, M. F.; Ward, T. L. ReV. Sci. Instrum. 1989, 61, 1281. (17) Clegg, S. L.; Brimblecombe, P.; Wexler, A. S. J. Phys. Chem. A 1998, 102, 2155. (18) Clegg, S. L.; Brimblecombe, P.; Liang, Z.; Chan, C. K. Aerosol Sci. Technol. 1997, 27, 345. (19) Tang, I. N. J. Geophys. Res. 1996, 101, 19245. (20) Clegg, S. L.; Seinfeld, J. H.; Brimblecombe, P. J. Aerosol Sci. 2001, 32, 713. (21) Archer, D. G. J. Phys. Chem. Ref. Data 1992, 21, 793. (22) Tang, I. N. J. Aerosol Sci. 1976, 7, 361. (23) Apelblat, A.; Manzurola, E. J. Chem. Thermodyn. 1990, 22, 289.
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