Responses of Ammonium Sulfate Particles Coated with Glutaric Acid

Here, we measured the hygroscopicity and Raman spectra of solid ammonium sulfate ((NH4)2SO4) particles initially coated with water-soluble glutaric ac...
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Environ. Sci. Technol. 2006, 40, 6983-6989

Responses of Ammonium Sulfate Particles Coated with Glutaric Acid to Cyclic Changes in Relative Humidity: Hygroscopicity and Raman Characterization MAN NIN CHAN, ALEX K. Y. LEE, AND CHAK K. CHAN* Department of Chemical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

Atmospheric particles, which may have an organic coating, exhibit cyclical phase changes of deliquescence and crystallization in response to changes in the ambient relative humidity (RH). Here, we measured the hygroscopicity and Raman spectra of solid ammonium sulfate ((NH4)2SO4) particles initially coated with water-soluble glutaric acid in two consecutive cycles of deliquescence and crystallization utilizing an electrodynamic balance. (NH4)2SO4 particles with glutaric acid coating (49 wt % glutaric acid) had different hygroscopicity and morphology in the two cycles. Once the particles deliquesced, the dissolution of the solid (NH4)2SO4 core and the glutaric acid coating formed mixed (NH4)2SO4-glutaric acid solution droplets, which was confirmed by Raman characterization. Coating studies with either deliquescence or crystallization measurements, or one complete cycle of these two measurements may not fully assess the effects of the organic coatings on aerosol hygroscopicity. We also present an analysis on the kinetic and chemical effects of organic coating on aerosol hygroscopicity. Glutaric acid coating does not impede the evaporation and condensation rates of water molecules compared to the rates of (NH4)2SO4 particles in the two cycles. The coating likely affects the hygroscopicity of aerosol particles through dissolution and its chemical interactions with (NH4)2SO4.

Introduction Atmospheric particles play an important role in radiative forcing and global climate changes (1). It has been reported that atmospheric particles can be coated with organic compounds (2-5). These organic coatings have significant effects on the physics, optics, and chemistry of aerosols (6). They also affect cloud condensation nuclei activity (7, 8) and the hygroscopicity of atmospheric particles (9-12). Organic coatings can hinder the transport rate of water molecules across the aerosol/air interface (kinetic effect) and can also interact with the solutes in the bulk aerosol phase (chemical effect), altering the equilibrium thermodynamics of the particles (6, 13). Organic compounds in atmospheric particles have a wide range of solubility in water (14). Waterinsoluble organic coatings have been found to retard the * Corresponding author phone: (852) 2358-7124; fax: (852) 23580054; e-mail: [email protected]. 10.1021/es060928c CCC: $33.50 Published on Web 10/18/2006

 2006 American Chemical Society

evaporation and condensation rates of water molecules from planar solution surfaces, water droplets, and aerosol particles (15). They may cause kinetic limitations in the hygroscopic measurements of laboratory-generated or atmospheric particles if insufficient time is provided for the particles to attain their equilibrium sizes (16). Organic coatings composed of water-soluble organic compounds may affect both the kinetics of water sorption and the equilibrium thermodynamics of the particles (13). For example, water-soluble organic coatings initially on the surface of preexisting inorganic particles may act as a barrier to impede the transport rate of water molecules. Once the inorganic components deliquesce or the solution droplets grow, the coatings may dissolve. The chemical interactions between dissolved organic compounds and the inorganic solutes in the bulk aerosol phase may affect the water uptake of the mixed particles such as the organic and inorganic species, which do not uptake water independently. This may change the hygroscopicity of the particles compared to those of the initially coated particles. Investigating and modeling the effects of interactions between water-soluble organic compounds and inorganic salts on aerosol hygroscopicity have been the subjects of a number of recent studies (1724). So far, most experimental investigations have been limited to only either measurements of the crystallization or deliquescence route, or one complete cycle of these two measurements. Atmospheric particles may undergo continuous water evaporation and condensation processes or phase changes in response to the changes in ambient relative humidity (RH). The phase of atmospheric particles may change incessantly throughout the particles’ lifetimes. In this paper, we focus on the roles of organic coatings composed of water-soluble organic compounds in the hygroscopicity of inorganic salt particles and choose ammonium sulfate ((NH4)2SO4) - glutaric acid as a model system. We investigate individual levitated single particles by utilizing an electrodynamic balance (EDB) to measure the hygroscopicity of the particles initially comprised of a solid (NH4)2SO4 core and a glutaric acid coating in two consecutive cycles of deliquescence and crystallization. Our aims are to (1) understand the kinetic and chemical effects of glutaric acid coating on aerosol hygroscopicity and (2) examine if there are changes in aerosol hygroscopicity in the two cycles. The change in the state of the particles in the two cycles is characterized utilizing Raman spectroscopy. (NH4)2SO4 is chosen as the hygroscopic core. Glutaric acid is soluble in water (116 g per 100 cm3 H2O) and has been found in atmospheric particles (25). The physical properties of (NH4)2SO4 and glutaric acid are given in Supporting Information (Table S1).

Experimental Section This work made use of the single particle levitation method in an EDB to measure the phase transitions and hygroscopic growth of (NH4)2SO4 particles coated with glutaric acid in RH cycling. The size of the levitated particles was estimated to be about 20 microns in diameter with a microscope. The EDB is suitable for investigating aerosol hygroscopicity in consecutive deliquescence and crystallization cycles because a single particle levitated in the EDB can be exposed to cycles of increasing and decreasing RH. The experimental procedures for coating, hygroscopic measurements, and Raman spectroscopy measurements are given below. Organic Coating. Initially, a single (NH4)2SO4 solution droplet was levitated at the center of the EDB by adjusting VOL. 40, NO. 22, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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the electric fields and was then equilibrated at about 5%RH to obtain a solid (NH4)2SO4 particle. Glutaric acid vapor, produced by heating solid particles at a constant temperature (70-110 °C), was introduced into the EDB through the bottom opening of the EDB. The vapor condensed onto the surface of the solid (NH4)2SO4 particle. We note that there was light scattering from the laser light path when the vapor was introduced in the EDB. It is possible that much smaller glutaric acid particles were formed and attached to the solid (NH4)2SO4 particle forming a coating. The mass of the glutaric acid coating was controlled by monitoring the coating time and was determined by measuring the particle mass change before and after coating. The mass fraction (or weight percentage) of the glutaric acid coating was defined as the ratio of the mass of the condensed glutaric acid to the total mass of the freshly coated particle. Hygroscopic Measurement. After coating, hygroscopic measurements were conducted utilizing the scanning EDB (SEDB) method developed by Liang and Chan (26). It involves continuous in situ measurements of the particle mass as a function of well-characterized increasing (or decreasing) RH as a result of a single step change in the feed RH to the EDB. The SEDB method allows the particle mass measurement to be performed over a range of RH within an hour and minimizes the evaporative loss of semi volatile organic solutes (27, 28). The evaporative loss of glutaric acid from dry particles and solution droplets was minimized. The evaporative loss was insignificant since the variation in the particle mass before and after the experiment was found to be within 3%. In the first cycle of deliquescence and crystallization measurements, the freshly coated particle was first exposed to increasing RH until it deliquesced, followed by decreasing RH to initiate crystallization. In the deliquescence measurements, the particle was first equilibrated at a low RH1 (RH1 ) 20%). The RH of the stream of the feed to the EDB was then changed from RH1 to RH2 (RH2 ) 90%) in a single step, and the mass of the particle was simultaneously measured as a function of time. The same procedure was used in the crystallization measurements, in which case the particle initially equilibrated at a high RH (RH1 ) 80% RH) and RH2 was set to be 20% RH. After completing the first cycle, the deliquescence and crystallization measurements were repeated on the same particle in a second cycle. The RH as a function of time inside the EDB in the deliquescence and crystallization measurements had been previously calibrated utilizing calcium chloride (CaCl2) particles. The mass of the particle as a function of RH can be obtained from a RH-time calibration curve. For comparison with the literature data reported in the form of the diameter growth factor, Gf, our relative mass measurements are converted to Gf as a function of RH in the two cycles. Details of the Gf calculation are discussed in the Supporting Information (S3). The RH of the feed stream to the EDB was varied by adjusting the flow ratio of the dry gas stream and the water saturated gas stream. The RH was determined by measuring the dew point with a dew point hygrometer (EdgeTech Dew Prime I) and the temperature with a digital thermocouple thermometer. The uncertainty in the determination of RH is estimated to be (1%. The experiments were made at temperatures between 21 and 24 °C. Kinetic Effect. The principle of the SEDB method is that a quasi-equilibrium between a levitated particle and its surroundings can be achieved given that the RH change inside the EDB is the rate-limiting step (26). The balancing voltage of the particles was continuously monitored in situ and recorded as a function of time. If the quasi-equilibrium assumption is valid, the fractional change in the balancing voltage, δ ) (Vi - V(t))/(Vi - Vf), as a function of time for different types of particles have been shown to follow the same trend in previous SEDB studies (26, 27), when the 6984

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particles are in a droplet state. Vi is the initial balancing voltage at the beginning of the experiment, V(t) is the balancing voltage at time t, and Vf is the final balancing voltage at the end of the experiment (tf ) 60 min). In that case, the differences between the δ values of the tested particles (e.g., (NH4)2SO4) and that of the respective calibration curves utilizing CaCl2 particles, ∆ ) [(Vi - V(t))/(Vi - Vf)]exp - [(Vi - V(t))/(Vi - Vf)]cal, are close to zero. Within the experimental time scale, the hygroscopic growth (or mass change) for different compounds in response to the RH change have been considered in the (Vi - Vf) term. Hence, the hygroscopicity of the droplets will not affect the value of ∆ and the trend of the ∆ will follow the calibration curve. The uncertainty in the calculations of the ∆ values was estimated to be (0.08. If the ∆ values significantly deviate from zero, quasi-equilibrium assumption is no longer valid and a new rate-limiting step exists. Negative ∆ values suggest the evaporation or condensation rates of the water molecules from the particles are slower compared to those of CaCl2 particles and (NH4)2SO4 particles in the deliquescence and crystallization measurements. The δ and ∆ values of (NH4)2SO4 particles initially coated with glutaric acid were plotted as a function of time in the two deliquescence and crystallization cycles. Raman Spectroscopy. The Raman spectra of (NH4)2SO4 particles coated with glutaric acid in two cycles of deliquescence and crystallization were recorded utilizing another EDB system coupled with a Raman spectroscopy system. Raman spectroscopy has been found to be a useful tool for characterizing the physical and chemical properties of levitated particles (29, 30). Briefly, a Raman spectroscopy system consisting of a 5W argon ion laser (Coherent I90-5) and a 0.5 m monochromator (Acton SpectraPro 500) attached to a CCD (Andor Technology DV420-OE) was integrated with another EDB system. The 514.5 nm line of an argon ion laser was used as the source of excitation. A pair of lenses was used to focus the 90° scattering of the particles in the EDB onto the slit of a monochromator, which was attached to the CCD detector and was used for spectroscopic measurements. A 514.5 nm Raman notch filter was placed between the two lenses to remove any strong Rayleigh scattering. A 300 g/mm grating of the monochromator was selected. The integration time for each spectrum was 30 s (10 frames, each with an accumulation time of 3 s). The resolution of the obtained spectra was about 6 cm-1. The Raman spectra of the particles in the two cycles were recorded after equilibration for about an hour at a given RH. Only one measurement at the solid and one at the liquid state were measured in each cycle to minimize the evaporative loss of glutaric acid due to laser heating. Nevertheless, the spectra of these “final states” are sufficient to show the structural differences of the particles due to the cyclical changes of RH. The Raman spectra of (NH4)2SO4 particles and mixed (NH4)2SO4-glutaric acid particles, generated from a premixed solution of known (NH4)2SO4 and glutaric acid concentration, were also recorded for comparison.

Results and Discussions Prior to the discussion of the results, it is useful to note the hygroscopicity of (NH4)2SO4 particles. Two or three (NH4)2SO4 particles for each mass fraction of glutaric acid were investigated in the EDB and Raman experiments. Representative data are shown and discussed here. The data (mass fraction of solute as a function of RH) are available on the webpage: http://ihome.ust.hk/∼keckchan/ hygroscopic.html. Hygroscopicity of (NH4)2SO4 Particles. We measured the hygroscopicity of (NH4)2SO4 particles in two consecutive deliquescence and crystallization cycles utilizing the SEDB

TABLE 1. Crystallization RH (CRH), Deliquescence RH (DRH), and Hygroscopic Growth of (NH4)2SO4 Particles Coated with Different Mass of Glutaric Acid in Two Deliquescence and Crystallization Cycles. cycle 1 organic mass (wt %)

system single component particles (NH4)2SO4 glutaric acida (NH4)2SO4 particles coated with glutaric acid

13 49 a

Data obtained from Peng et al. (34).

b

cycle 2

CRH (%)

DRH (%)

Gf (80%)

CRH (%)

DRH (%)

Gf (80%)

45.6 29-33

80.1 83.5-85

1.45 1.16b

45.4

80.5

1.46

50.3 40.9

80.7 80.9

1.42 1.36

51.0 40.6

81.1 79.0

1.43 1.34

Gf is derived from the crystallization data of glutaric acid particles from Peng et al. (34).

method. Very similar hygroscopic properties of (NH4)2SO4 particles were observed in the two cycles (Table 1). Deliquescence data and hygroscopic growth were consistent with the literature data (deliquescence RH (DRH) ) 79-81% RH) (31-33). The crystallization RH (CRH) value lies within the upper range of CRH (CRH ) 33-48% RH) reported for (NH4)2SO4 particles (33). These results demonstrate that the SEDB method is appropriate for measuring aerosol hygroscopicity in cycles of deliquescence and crystallization. The hygroscopic data of (NH4)2SO4 particles in the two cycles are given in the Supporting Information (Figure S1). In the following, we assess the effects of the glutaric acid coating on the hygroscopicity of (NH4)2SO4 particles. The hygroscopicity of (NH4)2SO4 and glutaric acid particles obtained from previous EDB measurements (31, 34) are incorporated in the figures. Hygroscopicity of (NH4)2SO4 Particles Coated with Glutaric Acid. Figure 1a illustrates the hygroscopicity of (NH4)2SO4 particles coated with 13 wt % glutaric acid in two deliquescence and crystallization cycles. In the first cycle, the freshly coated particle (comprised of a solid (NH4)2SO4 core and a coating of glutaric acid) did not uptake any water at low RH and exhibited an abrupt increase in particle size at 80.7% RH, close to the measured DRH of (NH4)2SO4 particles (DRH ) 80.1-80.5% RH) but lower than that of glutaric acid particles (DRH ) 83.5-85% RH). The particle continued to grow with increasing RH above the deliquescence point. Upon decreasing RH, the particle formed after deliquescence of the freshly coated particle crystallized at 50.3% RH, which was slightly higher than the CRH of (NH4)2SO4 particles in our study (45.4-45.6% RH). In the second cycle, the hygroscopicity of the particle (CRH ) 51.0% RH and DRH ) 81.1% RH) was very similar to that observed in its first cycle. Generally, (NH4)2SO4 particles with a thin coating (13 wt % glutaric acid) had very similar hygroscopicities in the two cycles. The hygroscopic growth of the particles was comparable with that of (NH4)2SO4 particles. Figure 2a illustrates the hygroscopicity of (NH4)2SO4 particles coated with 49 wt % glutaric acid in two deliquescence and crystallization cycles. In the first cycle, the freshly coated particle did not show any change in particle size with increasing RH until 80.9%RH (close to the DRH of (NH4)2SO4 particles but lower than the DRH of glutaric acid particles) at which point it absorbed a significant amount of water spontaneously. After deliquescence, the particle further grew as the RH increased. Once the freshly coated particle deliquesced, upon decreasing RH, the particle did not crystallize until the RH reached 40.9% RH. The CRH values fell in between the CRH of (NH4)2SO4 in our study (45.445.6% RH) and glutaric acid particles (29-33% RH). In the second cycle, the particle formed after one cycle of deliquescence and crystallization exhibited different deliquescence characteristic compared to that of freshly coated particle in the first cycle. It absorbed a small amount of water at about 50% RH before deliquescence. It started to deliquesce at about 74% RH, earlier than that of the freshly coated particle

(in the first cycle) and the (NH4)2SO4 particles observed. Deliquescence was completed at 79.0% RH. Upon decreasing RH, the particle lost water and crystallized at 40.6% RH. Very similar crystallization characteristics were observed in the two cycles. The Gf of the particles fell in between those of (NH4)2SO4 particles and glutaric acid particles. There are observations that worth further discussion. One is that upon increasing RH, the freshly coated particles (13 wt % and 49 wt % glutaric acid) exhibited an abrupt increase in particle size at 80.7 and 80.9% RH, which was close to the DRH of (NH4)2SO4 particles in the first deliquescence cycle (Figures 1a and 2a). The abrupt increase in particle size is likely due to the deliquescence of the solid (NH4)2SO4 core, followed by the dissolution of the glutaric acid coating into aqueous solution droplet. As the glutaric acid vapor condensed on the surface of solid (NH4)2SO4 particles, it would be expected that (NH4)2SO4 particles with a glutaric acid coating would exhibit deliquescence similar to that of glutaric acid particles. In this case, the freshly coated particles would deliquesce at a RH comparable with the DRH of glutaric acid particles (DRH ) 83-85% RH) (34). One possible explanation for the observation of early deliquescence is that if the glutaric acid coating completely covered the surface of the solid (NH4)2SO4 core, there were pores/gaps in the coating, making it permeable to water molecules as suggested by Cruz and Pandis (7) in their cloud condensation nuclei measurements of solid (NH4)2SO4 particles coated with glutaric acid. Another explanation is that even at high organic mass loading, the glutaric acid coating may not completely cover the surface of solid (NH4)2SO4 core. A part of the solid (NH4)2SO4 core may be exposed to the air and the water molecules reach the solid (NH4)2SO4 core directly. Another observation is that (NH4)2SO4 particles with a thick coating (49 wt % glutaric acid) exhibited different deliquescence characteristics in the two cycles (Figure 2a). In the first cycle, the freshly coated particle did not uptake any water until it deliquesced at 80.9%RH. In the second cycle, the particle started absorbing water at about 50%RH before deliquescence. It started to deliquesce at a lower RH at 74% and it completely deliquesced at 79.0%RH. The differences in the two cycles can be clearly seen in Figure 2b, where the δ value of the freshly coated particle (in the first cycle) followed the trend of (NH4)2SO4 particles. However, in the second cycle, the δ value started increasing at an earlier time (about 13 min) than in the freshly coated particle and in (NH4)2SO4 particles. The particles completely deliquesced after 29.6 min and after 21.5 min in the first and second cycles, respectively. The above observation may be explained by the suggestion that when the freshly coated particle deliquesced in the first cycle, the solid (NH4)2SO4 core and glutaric acid coating dissolved to form a mixed (NH4)2SO4-glutaric acid solution droplet. Mixing of (NH4)2SO4 and glutaric acid lowers the DRH value in the second cycle, which is consistent with previous studies that used mixed (NH4)2SO4-glutaric acid VOL. 40, NO. 22, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. (a) Hygroscopicity of (NH4)2SO4 particles coated with 13 wt % glutaric acid in two deliquescence and crystallization cycles. (b) The δ and ∆ values of (NH4)2SO4 particles coated with 13 wt % glutaric acid as a function of time in two deliquescence cycles. (c) The δ and ∆ values of (NH4)2SO4 particles coated with 13 wt % glutaric acid as a function of time in two crystallization cycles. particles generated from the premixed solution of known (NH4)2SO4 and glutaric acid concentrations (28, 35, 36). Choi and Chan (28) used an SEDB method to find that mixed (NH4)2SO4-glutaric acid particles (50 wt % glutaric acid) absorbed small amounts of water before deliquescence and completely deliquesced at 76.6% RH. Prenni et al. (35) reported that mixed (NH4)2SO4-glutaric acid particles deliquesced at 79.3% RH with a (NH4)2SO4 to glutaric acid molar ratio of 10:1 (∼9 wt % glutaric acid) and deliquesced at 77.3% RH with a molar ratio of 1:1 (∼50 wt % glutaric acid) at 30 °C utilizing a hygroscopic tandem differential mobility analyzer (HTDMA). Pant et al. (36) measured the DRH and CRH of mixed (NH4)2SO4-glutaric acid particles by depositing 6986

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FIGURE 2. (a) Hygroscopicity of (NH4)2SO4 particles coated with 49 wt % of glutaric acid in two deliquescence and crystallization cycles. (b) The δ and ∆ values of (NH4)2SO4 particles coated with 49 wt % of glutaric acid as a function of time in two deliquescence cycles. (c) The δ and ∆ values of (NH4)2SO4 particles coated with 49 wt % of glutaric acid as a function of time in two crystallization cycles. the particles on a surface of a hydrophobic polytetrafuroroethylene film in a RH controlled cell. They observed that the DRH of mixed (NH4)2SO4-glutaric acid particles slightly decreased to about 77-78% when the mole fraction of glutaric acid increased to 0.5 (∼50 wt % glutaric acid). These studies have shown that the DRH of mixed (NH4)2SO4-glutaric acid particles slightly decreased with an increasing mass fraction of glutaric acid. This is consistent with the observation that (NH4)2SO4 particles with a thin coating (13 wt % glutaric acid) deliquesced at RH close to the DRH of (NH4)2SO4 in the second cycle.

FIGURE 4. Raman spectra of mixed (NH4)2SO4-glutaric acid particles (50 wt % glutaric acid): (a) solid particles; (b) solution droplets; c: The Raman spectra of solid glutaric acid particles.

FIGURE 3. Raman spectra of (NH4)2SO4 particles coated with 47 wt % of glutaric acid in two deliquescence and crystallization cycles: (a) (NH4)2SO4 particles; (b) solid (NH4)2SO4 particles coated with glutaric acid (center); (c) solid (NH4)2SO4 particles coated with glutaric acid (branch/coating); (d) solution droplets in the first deliquescence cycle; (e) solid particles in the first crystallization cycle; f) solution droplets in the second deliquescence cycle; (g) solid particles in the second crystallization cycle. Recently, Clegg and Seinfeld (37) have developed an electrolytic-organic model to predict the hygroscopicity of mixed (NH4)2SO4-glutaric acid particles. From their Figure 27, the DRH value for the 13 wt % glutaric acid does not differ significantly from that of (NH4)2SO4 particles. For the 49 wt % glutaric acid, a lower DRH value was predicted. The model predictions are in good agreement with our observations for the thin and thick glutaric acid coatings. They predicted that mixed (NH4)2SO4-glutaric acid particles (50 wt % glutaric acid) took up a small amount of water before complete deliquescence, assuming that the particles consist of aqueous glutaric acid and solid (NH4)2SO4. We have also compared the hygroscopic growth of the particles with the other experimental data and Zdanovskii-Stokes-Robinson (ZSR) predictions. The discussion is given in the Supporting Information (S3). The ZSR predictions can generally capture the hygroscopic growth of mixed (NH4)2SO4-glutaric acid solution droplets. We recorded the Raman spectra of (NH4)2SO4 particles with a thick glutaric coating in two deliquescence and crystallization cycles in another EDB experiment. A similar amount of glutaric acid (47 wt % glutaric acid) was coated on the surface of the solid (NH4)2SO4 particle. The Raman spectra of solid (NH4)2SO4 particle (Figure 3a) and of the core region of the freshly coated particle (Figure 3b) show that the core region of the freshly coated particle mainly consists of relatively large amounts of solid (NH4)2SO4 compared to glutaric acid, as expected. We also measured the Raman spectrum of the coating by adjusting the position

of the laser beam. In contrast, many strong Raman hydrocarbon signatures (∼2900-3100 cm-1) were obtained in the coating region (Figure 3c), suggesting that the major component of the coating was glutaric acid. It is interesting to note that the Raman spectrum of the glutaric acid coating (Figure 3c), especially between the region of 2900 and 3100 cm-1, differs from that of a solid glutaric acid particle (Figure 4c). The molecular structure of the coating formed from condensational/coagulational growth of glutaric acid might be different from that of solid glutaric acid particle formed from crystallization of glutaric acid solution droplets. Figure 3 depicts the evolution of the Raman spectra of (NH4)2SO4 particles coated with 47 wt % glutaric acid in the two cycles. The Raman spectra of mixed (NH4)2SO4-glutaric acid particles, generated by premixed solutions of a similar amount of glutaric acid (50 wt % glutaric acid), in solid and droplet states are shown in Figure 4a and b. The Raman spectra and features of the solution droplets formed after deliquescence of solid (NH4)2SO4 particles coated with 47 wt % glutaric acid in the two cycles are similar (Figure 3d and f) to those of mixed (NH4)2SO4-glutaric acid solution droplets (50 wt % glutaric acid) (Figure 4b). The Raman spectra of the solid particles formed after cycles of deliquescence and crystallization (Figure 3e and g) are comparable with those of solid mixed (NH4)2SO4-glutaric acid particles (Figure 4a). These results support the observation that mixed (NH4)2SO4-glutaric acid particles are formed after dissolution of the solid (NH4)2SO4 core and the glutaric acid coating during deliquescence. It is noted that the Raman spectra of freshly coated particles (Figure 3b and c) are different from those of the solid particles formed after cycles of deliquescence and crystallization (Figure 3e and g) and from solid mixed (NH4)2SO4-glutaric acid particles (Figure 4a). Upon deliquescence of the freshly coated particles, the formation of a mixed (NH4)2SO4-glutaric acid solution droplets, as confirmed by Raman characterization, may also explain the observation that (NH4)2SO4 particles with a thick coating (49 wt % glutaric acid) crystallized at a RH lower than the CRH of (NH4)2SO4 particles we measured in two crystallization cycles. Glutaric acid has a CRH of 29-33% RH, which is lower than the CRH of (NH4)2SO4 particles (34) and likely suppresses the crystallization of (NH4)2SO4 in mixed (NH4)2SO4-glutaric acid particles. Choi and Chan (28) observed a step decrease in the mass of mixed (NH4)2SO4glutaric acid particles (∼50 wt % glutaric acid) at 55.7-59.2% RH and the particles became completely dry at ∼38-40% RH. A single decreasing step was observed for (NH4)2SO4 particles with a thick coating (49 wt % glutaric acid) at 40.6% RH and 40.9% RH in two crystallization cycles. The difference VOL. 40, NO. 22, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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might be attributed to the presence of a minute amount of impurity in Choi and Chan’s study. We acknowledge that Pant et al. (36) reported glutaric acid suppressed the crystallization of (NH4)2SO4 and the CRH of mixed (NH4)2SO4-glutaric acid particles (∼50 wt % glutaric acid) was about 30% RH, which was lower than those observed in our study. Kinetic Effect. Organic coatings can inhibit the condensation and evaporation rates of water molecules and may shift the deliquescence point of an inorganic core to a higher RH or may shift the crystallization point to a lower RH compared with its equilibrium thermodynamic values. Here, we investigate if the glutaric acid coating had a kinetic effect on the hygroscopic measurements. Figure 1b and c depict the δ and ∆ values of (NH4)2SO4 particles coated with 13 wt % glutaric acid as a function of time in the first and second deliquescence and crystallization cycles, respectively. As shown in Figure 1b, the δ values of the particle followed the trend of (NH4)2SO4 particles in the two deliquescence cycles. Upon decreasing RH, the δ values of the particle also followed the same trend as (NH4)2SO4 particles prior to crystallization and crystallized at an earlier time than that of (NH4)2SO4 particles in two cycles (Figure 1c). Overall, the ∆ values of the particles were close to the zero line within acceptable uncertainty in the two deliquescence and crystallization cycles (the upper panel of Figure 1b and c). This gives us confidence that the assumption of a quasi-equilibrium between the particle and its surroundings is valid. The glutaric acid coating had no significant retardation effect on the evaporation and condensation rates of water molecules from the particles in the two cycles compared to the same rates of (NH4)2SO4 particles. Figure 2b and c depict the δ and ∆ values of (NH4)2SO4 particle coated with 49 wt % glutaric acid. In the deliquescence cycles (Figure 2b), the δ values of the freshly coated particle (red circle, the first cycle) followed the trend of (NH4)2SO4 particles (green inverted triangle) while those of the particle formed after the first deliquescence and crystallization cycle (blue triangle, in the second cycle) increased at a time earlier than that of (NH4)2SO4 particles. These results indicate that the particle absorbed water prior to deliquescence in the second deliquescence cycle, which is similar to the deliquescent behavior of internally mixed (NH4)2SO4-glutaric acid particles (28). Above the deliquescence point, the ∆ values were close to zero, suggesting that the hygroscopic measurements reached equilibrium (upper panel of Figure 2b). In the crystallization cycles (Figure 2c), the δ values of the particle had the same trend as that of (NH4)2SO4 particles before the crystallization point in the two cycles. The time at which crystallization occurred was later than that of (NH4)2SO4 particles, indicating a lower CRH. In the upper panel of Figure 2c, it can be seen that the ∆ values were close to zero down to the crystallization point. Hence, even a thick coating of glutaric acid did not significantly impede the evaporation and condensation rates of water molecules from the particles in the two deliquescence and crystallization cycles compared to the rates of (NH4)2SO4 particles. From the above analysis, the shifts of CRH and DRH observed in (NH4)2SO4 particles with thick coatings (49 wt % glutaric acid) in the two cycles are attributed to chemical effects but not to kinetic effects. We have demonstrated that coated heterogeneous (solid inorganic core with a water soluble organic coating) particles can form internal mixtures after deliquescence and the hygroscopicity and morphology of the particles before and after the deliquescence step can be different. To the best of our knowledge this is the first time in the literature to link the hygroscopicity of coated particles to that of internal mixtures. We observe that (NH4)2SO4 particles with a glutaric acid coating exhibited different hygroscopicity in the two deliquescence and crystallization cycles. The results imply that 6988

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studies of only a single deliquescence or crystallization step, or even both may not completely represent the hygroscopicity of the coated particles, especially those coated with watersoluble species in the atmosphere. This can have significant implications in future investigations on the effect of organic coatings on aerosol hygroscopicity, including chamber experiments in which secondary organic aerosols are subjected to an increase in RH, without cyclical changes in RH. From our Raman characterization, we observed that there is also a change in particle morphology in the repeated increasing and decreasing RH conditioning. The change in morphology may affect the physical and chemical properties of aerosol particles. Organic coatings can have kinetic (providing a mass transfer barrier) and chemical effects on aerosol hygroscopicity. The former provides a mass transfer barrier while the latter involves the chemical interactions with other components in the aerosol particles so that different components do not uptake water independently. It is important to know in what ways organic coatings alter aerosol hygroscopicity. Studies have shown that the aerosol particles of organic coatings may require a longer equilibrium time than in their uncoated pure form (15). It is important to confirm that measurements are made without any kinetic limitations (16). Further experiments on particles morphology and on the effects of coatings of organic compounds of different properties (e.g., water solubility, surface activity) onto preexisting solid particles or aqueous droplets in cycles of deliquescence and crystallization are useful.

Acknowledgments This work was funded by earmarked grants (600303 and 610805) from the Research Grants Council of the Hong Kong Special Administrative Region, China

Supporting Information Available Physical properties of (NH4)2SO4 and glutaric acid are summarized in Table S1. Hygroscopicity data of (NH4)2SO4 particles are given in Figure S1. Details of the Gf calculations and the comparison of the hygroscopic growth data and the ZSR estimations and literature data are also given. This material is available free of charge via the Internet at http:// pubs.acs.org.

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Received for review April 18, 2006. Revised manuscript received August 21, 2006. Accepted September 11, 2006. ES060928C

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