The Effects of Organic Species on the Hygroscopic Behaviors of

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Environ. Sci. Technol. 2002, 36, 2422-2428

The Effects of Organic Species on the Hygroscopic Behaviors of Inorganic Aerosols MAN YEE CHOI AND CHAK K. CHAN* Department of Chemical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

Water-soluble organic compounds have recently received much attention because of their ability to absorb water and alter the hygroscopic properties of inorganic aerosols. The effects of glycerol, succinic acid, malonic acid, citric acid, and glutaric acid on the water cycles (water activities during particle evaporation and growth), crystallization relative humidities (CRH), and deliquescence relative humidities (DRH) of sodium chloride (NaCl) and ammonium sulfate (AS) were studied using an electrodynamic balance (EDB). The growth factors of these inorganic and organic mixtures were lower than those of the pure inorganic species. The presence of all these organics in the mixed particle reduce the water absorption of NaCl but enhance that of AS relative to that of the pure inorganic salts. Glycerol and succinic acid did not affect the deliquescence properties of NaCl and AS, although succinic acid increased the CRH of NaCl and AS. Malonic acid and citric acid, behaving as nondeliquescent species in single particle studies, caused NaCl and AS particles to absorb a significant amount of water before deliquescence. Glutaric acid caused NaCl and AS to deliquesce gradually, spanning a wide range of relative humidity. The ZSR model was found to be useful in predicting the water activity of the mixtures and the growth ratios. However, the detailed crystallization and deliquescence behaviors of the organic/inorganic mixtures cannot be easily predicted from the hygroscopic properties of the individual components.

Introduction Atmospheric aerosols have a direct impact on the earth’s radiation balance (1), air pollution (2), fog formation and cloud physics (3), and visibility degradation (4-6) as well as human health (7). Of central importance to these effects is the hygroscopic nature of the atmospheric aerosols as they undergo size changes in moist air. To accurately assess the role that aerosols play in these phenomena, it is imperative to determine the physical and chemical properties of atmospheric aerosols at various humidities. Atmospheric aerosols often consist of mixtures of organic and inorganic substances. Organic matter contributes to 2060% of fine particles, depending on location (8-11). They have varying degrees of polarity with a significant fraction that is water-soluble (12). Saxena et al. (13) have pointed out the importance of atmospheric water-soluble organic com* Corresponding author phone: (852)2358-7124; fax: (852)23580054; e-mail: [email protected]. 2422

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pounds (WSOC) in the hygroscopicity of atmospheric aerosols. Recently, some WSOC have been postulated to have roles similar to that of inorganics in visibility degradation and the formation of cloud condensation nuclei (14-16). The organic fraction can have a profound effect on the light scattering, hygroscopicity, and phase transition properties of multicomponent atmospheric aerosols (17). Although there have been recent measurements of the hygroscopic properties of organic salts and carboxylic acids of atmospheric relevance (18, 19), thermodynamic data on WSOC are scarce. Only a few organic species have recently been incorporated into aerosol thermodynamic models (20). Studies of the effect of organics on the hygroscopicity of inorganic aerosols are, in general, limited and somewhat contradicting in the literature. Some researchers have suggested that organics, which most of them are sparingly soluble compounds or surfactants, have a negative effect on the growth factor or evaporation rate of inorganics (e.g., refs 17, 21-24). However, some researchers have found that organics can both positively and negatively affect the hygroscopicity of inorganic aerosols. Saxena et al. (13) found that organic aerosols appeared to diminish the water uptake of inorganic aerosols at urban locations and enhance inorganic aerosol water absorption at nonurban locations. Cruz and Pandis (25) found that glutaric acid and pinonic acid, in general, enhance water sorption of ammonium sulfate (AS) but reduce that of NaCl on a volume basis relative to that of the pure inorganic salts. Dick et al. (26) found that organic-associated water content is less than that of sulfate compounds at high relative humidity (RH) but comparable or greater at low RH. The effects of organics on inorganic aerosols appear to be species specific. While the positive effect on growth is likely due to water absorption of the organics (and/or the chemical effect with the inorganics, i.e., the chemical interactions between the inorganic and organic species), the negative effect can be a mass transfer effect or a chemical effect. The distinction between a mass transfer effect and a chemical effect is an important one. Mass transfer effects may not necessarily affect the water absorption of ambient particles in the atmosphere because of their much longer residence time compared to that used in laboratory studies, such as a few seconds in TDMA equipments. In contrast, the chemical effects are expected to affect the hygroscopic properties of atmospheric particles. Peng et al. (19) have recently found that glutaric acid took much longer time to deliquesce than other organic acids of similar size using an electrodynamic balance. They further confirmed that their measured growth factors for glutaric acid are larger than the literature data based on TDMA (25). The role of organics on the crystallization RH (CRH) and the deliquescence RH (DRH) of inorganic aerosols is not well understood (17, 25, 27). Here the CRH means the RH for which the particle crystallizes from a supersaturated solution, so that the CRH value is much lower than the DRH value in this case. This study investigated the effect of selected organics on the hygroscopic properties (water activity during evaporation and growth, CRH and DRH, and the growth ratio) of NaCl and AS using an electrodynamic balance. NaCl and AS were chosen because of their abundance in coarse mode and fine mode aerosols in the atmosphere. The organics studied were glycerol, succinic acid, malonic acid, citric acid, and glutaric acid. The low molecular weight dicarboxylic acids (malonic acid, succinic acid, and glutaric acid) are most commonly found in field measurements (28-31). Citric acid and glycerol were also chosen because they are expected to exist in the atmosphere (32). The water sorption character10.1021/es0113293 CCC: $22.00

 2002 American Chemical Society Published on Web 04/30/2002

istics of the organic acids and glycerol were taken from Peng et al. (19) and Choi and Chan (33). The measured water activities of the mixtures and the calculated growth factors were compared with the predictions of the Zdanovskii-StokesRobinson (ZSR) model (34, 35), which is one of the most widely used empirical mixing rules for the water activity of atmospheric inorganic aerosols.

Methods In this study, we have employed a technique that enables the measurement of a set of water activity in an hour (33, 36). The technique has been found to be useful to measure hygroscopic properties of semivolatile species of vapor pressure less than 1E-4 mmHg at ambient temperature (33). Compared to the conventional application of the EDB that the RH is changed in discrete steps, the technique involves the scanning of RH. Hence, we call the technique scanning EDB (SEDB). The SEDB measurements employed a step increase of 20-90% to study deliquescence and hygroscopic growth and a step decrease of 80-20% to study evaporation. All measurements were made at 20-23 °C. In each individual study, the temperature varied less than 0.2 °C. Deviations of water activity and DRH within such small temperature changes are negligible. The error in the determination of RH depends on the flow rate of the feed stream, and it is estimated to be (0.86% at RH ) 40-80%. The overall experimental error in this work was within (0.01 in mass fraction of the solute (mfs ) mass of solute on a dry basis/mass of solution) for droplets, although it can be as large as (0.03 in mfs for partially crystallized or solid particles. In the SEDB method, the densities of the salts are required to calculate the mfs. Expressions for the densities of NaCl and AS, including supersaturated solutions at 25 °C, were taken from the work of Tang and Munkelwitz (37) and Tang et al. (38). The densities of the organics were calculated from the mass fraction of organics in the droplet and the pure solid density provided by the manufacturer. The densities of the mixtures were then estimated from the simple volume additivity rule (39), which has been found to be adequate for most aqueous multicomponent solutions. Bulk water activity measurements were made to provide the reference state concentrations for the calculation of mfs in the EDB measurements. The water activities of bulk solutions were measured using an AquaLab water activity meter (Model 3TE, Decagon devices, U.S.A.), and the procedures used in measuring the aw of the bulk solutions were described in detail by Peng and Chan (18). All bulk measurements were made at 22 °C.

Results and Discussion 1. The Growth Factor and the Change in the Water Absorption of the Inorganics Due to the Presence of Organics. Figures 1-10 show the mfs-aw data for the mixtures of NaCl or AS and organics. The raw data can be obtained from http://ihome.ust.hk/∼keckchan. Using these data, the growth factor (Gf) from RH1 ()10%) to RH2 ()85%) was calculated

Gf(RH2) ≡

( )

Rp,2 mfs1F1 ) Rp,1 mfs2F2

1/3

(2)

where Rp is the particle radius and F is the particle density at the corresponding RH. Rp was in the range of 5-10 µm in this study. Cruz and Pandis (25) quantified the effect of the organic fraction on the hygroscopicity of NaCl and AS using ξw, the change in the water uptake of NaCl or AS because of the

TABLE 1. Growth Factors for the WSOC and Their Mixture with NaCl/AS Calculated from Measurements and ZSR Predictions and ξw for the NaCl/AS and WSOC Mixtures Studied NaCl + organicb chemical

Gfa

no organic glycerol succinic acid malonic acid citric acid glutaric acid

N/Ad 1.36f 1.00 1.40 1.28 1.09

Gf(85%) Gf,ZSRc 1.93e 1.61 1.57 1.59 1.42 1.49

N/A 1.61 1.57 1.63 1.51 1.49

ξw N/A 0.75 0.61 0.75 0.33 0.51

AS + organicb Gf(85%) Gf,ZSRc 1.51e 1.45 1.43 1.45 1.41 1.38

N/A 1.45 1.43 1.45 1.41 1.38

ξw N/A 2.07 1.66 1.99 1.01 1.36

a The growth ratio was taken from Peng et al. (19) and was calculated from RH ) 10-85% unless specified otherwise. b All mixtures are in 1:1 mole ratio except that NaCl + glutaric acid is 1:1 by mass. c The growth ratio was calculated from RH ) 10-85% by assuming the mfs at RH ) 10% is 1 and the mfs at RH ) 85% was determined by the ZSR equation. d Not applicable. e The growth ratio was calculated from RH ) 1085%. f The growth ratio was taken from ref 33.

presence of the organic species on volume basis

ξw )

Gf3 - 1

(3)

(1 - )(GINORG3 - 1)

where Gf is the measured growth factor for the mixture,  is the volume fraction of the organic, and GINORG is the growth factor for the pure inorganic component at the same RH. The parameter  is calculated from the mass and density of each component in the mixture. Table 1 lists the calculated Gf and ξw of all mixtures studied. Cruz and Pandis (25) found that Gf evaluated from RH < 10% to RH ) 85% for NaClglutaric acid (50:50 by mass) and AS-glutaric acid (50:50 by mass, which is equivalent-to-mole ratio of 1:1) are 1.43-1.64 and 1.33-1.38, respectively, for particles with diameters ranging from 50 to 120 nm. They calculated ξw of the above systems to be 0.5-0.8 and 1.2-1.4. In the calculation of ξw, it is assumed that only the inorganic fraction of the particle interacts with water, while the organic fraction remains inert. Values of ξw > 1.00 and 94%) (19) and acts almost inert during deliquescence of NaCl and AS. Lightstone et al. (17) also reported a very small decrease in the DRH of ammonium nitrate, from 61.5% to about 60%, when adding as much as 50% by mass of succinic acid. Although glycerol and succinic acid have different hygroscopic properties and solubility in water, they have very similar effects in affecting the DRH of NaCl and AS. Overall, because of its low solubility and high DRH, succinic acid behaves as an “inert” species. It is most surprising to find that glycerol, a species that absorbs water even at RH much lower than the DRH of NaCl (or AS), does not affect the deliquescence properties of the inorganics. Ge et al. (45) examined the crystallization of droplets of a two-electrolyte system and pointed out that the solid is surrounded by an aqueous coating of the eutectic composition of the salt mixture. Upon further evaporation of water, the dried particle contains a pure salt core surrounded by a solid coating with the eutectic composition. When RH increases to the mutual DRH of the two-salt system, which is lower than the DRH of each of the individual salts, the coating begins to dissolve with absorption of water. However, this is not applicable to the current system. Glycerol does not form a mixed salt with NaCl (or AS). Furthermore, if glycerol in the dried particle is exposed to water vapor, it would absorb water and therefore significantly reduce the DRH of the mixed particle. On the other hand, if crystallization of NaCl takes place at the droplet surface, engulfing a solution of NaCl, glycerol, and water, the dried particle would be a solid NaCl crust trapping glycerol inside. Leong (46) studied the morphology of solid NaCl particles after crystallization and suggested evaporation occurred at the surface and resulted in an increase of solute concentration near the surface, which ultimately led to the formation of a shell of NaCl on the surface. In the glycerolNaCl system, glycerol is about 200 times more viscous than water and may have caused mass transfer effects in the diffusion of ions from the surface to the center of the particle. This could result in a particle with a core of glycerol enclosed by a crust of NaCl. These can explain why the particle does not absorb water until the DRH of pure NaCl is reached. b. Malonic Acid and Citric Acid. Figures 5 and 6 show the water cycles of mixtures of malonic acid and NaCl and AS, respectively. Malonic acid shows no crystallization or deliquescence in single particle studies, and it has a mfs ) 1 at RH < 10% (19). It absorbs water reversibly without hysteresis. Peng et al. (19) measured the water cycles of malonic acid particles using the stepwise EDB method and reported that the solute evaporates at about 3% per hour at room temperature and pressure for particles 10-15 µm in diameter, averaged over the course of the water cycle measurement. VOL. 36, NO. 11, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. Evaporation and growth profile of an AS and malonic acid (mole ratio ) 1:1) mixture.

FIGURE 8. Evaporation and growth profile of an AS and citric acid (mole ratio ) 1:1) mixture.

FIGURE 7. Evaporation and growth profile of an NaCl and citric acid (mole ratio ) 1:1) mixture.

FIGURE 9. Evaporation and growth profile of an NaCl and glutaric acid (mole ratio ) mass ratio ) 1:1) mixture.

The evaporation problem was negligible in the SEDB measurements (33). Evaporation was also not a problem in the mixture study here. In NaCl-malonic acid mixtures, the presence of malonic acid slightly increases the CRH of the mixture to about 45%. The mixture loses water continuously after crystallization and still contains about 7% water by mass at RH < 9%. However, it is possible that the particle can eventually become anhydrous at sufficiently low RH. Malonic acid does not have a significant effect on the CRH of AS. The mixture still loses water as RH decreases and becomes anhydrous (mfs ) 1) at RH < 20%. Both the NaCl-malonic acid and the AS-malonic acid mixtures absorb a significant amount of water before their respective DRH, because malonic acid absorbs water reversibly without crystallization. Here the DRH of the mixtures is defined as the point where the particles are completely deliquesced, i.e., the point where the measured data follow the evaporation data (DRHcomplete in Table 2). It is clear that the presence of malonic acid decreases the DRH of NaCl and AS. This follows the theoretical prediction by Wexler and Seinfeld (47) that the DRH of one electrolyte is lowered by the addition of a second electrolyte, even though the second electrolyte is an organic acid in this case. Figures 7 and 8 show the water cycles of NaCl-citric acid and AS-citric acid mixtures. Citric acid, similar to malonic acid, shows neither crystallization, deliquescence nor hysteresis, and it still contains 5% of water by mass at RH ) 5% in single particle experiments (19). According to Figure 9, the mixture of NaCl and citric acid crystallizes at about RH

) 40% (evaporation 1), but sometimes it persists as a supersaturated solution even at RH ) 28% (evaporation 2). Stepwise EDB data (not shown) show that the mixture contains about 8% water by mass even at RH ) 10%. The DRH of this mixture is about 60%, which is much lower than that of pure NaCl (70-75%). The addition of citric acid changes the hygroscopicity of AS completely, as shown in Figure 8. The mixture loses water continuously and finally reaches mfs ) 1 at RH ) 10%. The mixture may be in a liquid state, as no significant jump in mfs, i.e., no efflorescence, was found. Afterward, the mixture absorbs water reversibly with a slightly different trend as RH increases. The experiments were repeated several times, and a slight difference between the evaporation and condensation data was consistently observed. Generally, the presence of malonic acid and citric acid reduce the DRH and increased the CRH of NaCl and AS. In these mixtures, the particles still contain a lot of water and have only a slight jump in mfs after crystallization (of presumably a fraction of NaCl). This makes the hysteresis very small compared to that of NaCl or AS. Furthermore, because of the large amount of water present at low RH, these mixtures could easily be observed to be nondeliquescent in hygroscopic growth measurements at a limited number of RH settings, e.g. stepwise EDB or TDMA measurements. c. Glutaric Acid. Figures 9 and 10 show the water cycles of mixtures of NaCl-glutaric acid and AS-glutaric acid. Glutaric acid is very soluble in water and it crystallizes at RH ) 29-33% (19). Surprisingly, glutaric acid has a stronger

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FIGURE 10. Evaporation and growth profile of an AS and glutaric acid (mass ratio ) 1:1) mixture. effect than succinic acid, a relatively insoluble organic acid, in increasing the CRH of the mixtures to about 60%. It is interesting to note that the AS + glutaric acid mixture had about 20% water after crystallization but the NaCl + glutaric acid mixture contained very little water. Peng et al. (19) made stepwise measurements and found that glutaric acid has a DRH of 83%, but it takes over 10 h to completely deliquesce. A strong mass transfer effect in the deliquescence growth of sodium pyruvate has also been reported (18). The mixtures exhibited a gradual deliquescence ranging from RH ) 58-71% and 69-77% for NaCl and AS mixtures, respectively, as shown in Figures 9 and 10. The observed trend of gradual deliquescence was confirmed to be at equilibrium by additional stepwise EDB measurements (also shown in Figure 10). Cruz and Pandis (25) measured the deliquescence growth of a 1:1 (by mass, equivalent to equimolar) AS-glutaric acid mixture and a 1:1 (by mass) NaCl-glutaric acid mixture. They found that the presence of glutaric acid does not affect the DRH of NaCl or AS. However, according to Figures 9 and 10, the mixtures start to absorb water and completely deliquesce at RH lower than the respective DRH of NaCl or AS. Peng et al. (19) have found that the TDMA growth data of glutaric acid reported by Cruz and Pandis (25) may not be equilibrium measurements. It is possible that while the residence time in the humidifier in Cruz and Pandis (25) was sufficient for NaCl and AS, it was not sufficient for the mixtures with glutaric acid to deliquesce, so no growth was detected between RH ) 70 and 75% and RH ) 75 and 80% for the NaCl-glutaric acid mixture and AS-glutaric acid mixture, respectively. Generally speaking, mixtures of NaCl and AS with glycerol and malonic acid have the largest ξw, while mixtures of NaCl and AS with citric acid have the smallest ξw, even though citric acid has a higher growth ratio (i.e. more hygroscopic) than succinic acid (a sparingly soluble compound) and glutaric acid in single particle studies. Glycerol is nondeliquescent and is expected to have effects on NaCl or AS like those of malonic acid and citric acid but in fact it behaved like succinic acid. The hygroscopic properties of the organic alone might not be used to predict its effects on inorganics. The chemical interactions between the organic and the inorganic and the morphology of the particles formed are important factors of the growth of inorganic-organic mixed aerosols. In general, the addition of organic decreases the Gf of the mixture when compare with the Gf of the pure inorganic. Except for citric acid and succinic acid, the more hygroscopic the organic (i.e. with a Gf closer to that of the inorganics), the less reduction of the Gf of the inorganicorganic mixture is, as expected.

While the effect of organics on the hygroscopicity of inorganic aerosols is complex and is species specific, it can be generalized according to RH. (1) At low RH (80%), particles becomes completely deliquesced. The ZSR equation gives good predictions for the water content of the mixture. Most laboratory and field measurements have reported the growth ratio from RH ) 10% to a high RH such as 85%. Hence, for such a ratio alone, the ZSR is adequate. Unfortunately, a big range of ambient RH falls into the moderate RH range between the CRH and the DRH of NaCl and AS, i.e., around RH ) 45-80%, where the effects of organics on deliquescence and crystallization are very complex. It should be noted that a major portion of atmospheric WSOC has not been identified. In the literature, including this paper, much of the focus on the influence of WSOC on inorganics has been on carboxylic acids, which is only a small fraction of the WSOC found in the atmosphere. More measurements of the hygroscopicity of other organics such as humic like substances, which can account for up to 50% of the total WSOC (48-49) and inorganic mixed aerosols, are essential in order to understand completely the roles of organics in the atmosphere. Efforts in detailed thermodynamic modeling of inorganic and organic mixed systems are needed to understand the deliquescence properties of these mixtures (20, 50).

Acknowledgments This work was supported by the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. HKUST6121/97P and HKUST6039/00P).

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Received for review October 2, 2001. Revised manuscript received March 5, 2002. Accepted March 5, 2002. ES0113293