Environ. Sci. Technol. 2000, 34, 4313-4319
Deliquescence and Hygroscopic Growth of Mixed Inorganic-Organic Atmospheric Aerosol CELIA N. CRUZ AND SPYROS N. PANDIS* Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213
Deliquescence and hygroscopic growth measurements were performed for four internally mixed aerosol mixtures: NaCl-glutaric acid, NaCl-pinonic acid, (NH4)2SO4-glutaric acid, and (NH4)2SO4-pinonic acid with varying organic mass fractions (0, 0.2, 0.5, 0.8, and 1.0). No effect on the deliquescence relative humidity of the salts was observed for any of the organic mixtures tested. The NaCl-organic mixed aerosols deliquesced at a relative humidity (DRH) 75 ( 1% and the (NH4)2SO4-organic aerosol at 79 ( 1% independent of organic mass fraction. The growth factors at RH ) 85 ( 1%, G(85%), were also measured for the different aerosol mixtures. There was an observed decrease in G(85%) with increasing mass fraction of the organic. Measured G(85%) for the mixtures can be approximated as a first step with the assumption that the species absorb water independently. Overall, the organic portion was observed to enhance the water uptake of the (NH4)2SO4organic aerosol systems by as much as a factor of 2-3 for particles consisting of 80% organic acids. The NaClorganic mixtures presented evidence of positive and negative interaction depending on organic mass fraction, ranging from a 40% decrease to an 20% increase in water uptake as compared to that by the inorganic fraction alone.
1. Introduction The ability of atmospheric aerosol to absorb water and grow with increasing relative humidity influences the light scattering, cloud condensation nuclei properties, lifetime, and chemical reactivity of these particles. Traditionally, the water uptake of atmospheric aerosol has been exclusively associated with their inorganic fraction, e.g. NaCl and (NH4)2SO4. The hygroscopic growth of these salts is well understood (1-4). However, atmospheric aerosol is a complex mixture of inorganic and organic components, where organic species can represent close to 50% of the aerosol mass depending on location (5-8). Knowledge of the water uptake of the organic portion of atmospheric aerosol has been limited. Recently Saxena et al. (9) concluded that the organic fraction in atmospheric aerosol can alter the water uptake by the particles depending on the chemical composition of the aerosol. They estimated that up to 25-40% of the total water content of atmospheric aerosol at a nonurban location was accounted for by the organic fraction. However, similar measurements at an urban location indicated that organic matter resulted in a 25-35% reduction in water absorption by the inorganic atmospheric aerosol. McMurry and Stolzenburg (10) concluded that atmospheric aerosol can be divided * Corresponding author phone (412)268-3531; fax: (412)268-7139; e-mail:
[email protected]. 10.1021/es9907109 CCC: $19.00 Published on Web 09/07/2000
2000 American Chemical Society
into “hygroscopic” and “less hygroscopic” modes according to their change in mass when humidified and that these classifications are dependent on location and chemical composition of the aerosol. Since the aerosol chemical composition in these studies was not established, it was difficult to assess the contribution of specific organic species to water absorption. Recently, a study by Virkkula et al. (11) showed that secondary organic aerosol formed from the oxidation of monoterpenes was slightly hygroscopic, possibly corresponding to the “less hygroscopic” portion observed in field measurements (10, 12-14). A few laboratory studies have investigated the effect of organic coatings on the hygroscopic growth and deliquescence of atmospheric aerosol. Deliquescence, or the phase transition between solid particle and dissolved droplet, occurs at the deliquescence relative humidity (DRH). This process and its dependence on temperature is well understood for inorganic salts (2, 15, 16, 4). The effect of organic coatings on the hygroscopic behavior of NaCl particles was investigated by Andrews and Larson (17) who concluded that a film of organic surfactant lowered the deliquescence relative humidity for the salt particles from 75% to 70-73%, depending on film thickness. The coated particles, however, showed a decrease in the maximum relative mass gain from water adsorption at the lower deliquescence point. This indicated a decrease in the hygroscopic behavior of the inorganic salt particles due to the presence of the surfactant film. Hameri et al. (18) also studied the effect of organic coatings on NaCl deliquescence and found that lauric and octanoic acid, two organics potentially found in the atmosphere, did not hinder the hygroscopic growth nor affect the relative humidity at which the salt particles deliquesced. In an electron microscopy study, Posfai et al. (19) suggested that organics were most likely responsible for water uptake of ambient (NH4)2SO4 at low relative humidities. Xiong et al. (20) reported that film coatings of lauric acid, stearic acid, and oleic acid delayed the hygroscopic growth of ultrafine H2SO4 aerosol during the first few seconds of growth. These apparently contradicting results point to different overall effects depending on the organic species and the particle morphology and chemistry. Therefore, more information is necessary to better understand the water uptake properties of typical organics in the atmosphere. In the present study, we attempt to answer two main questions: (1) what is the effect of typical atmospheric organic species on the deliquescence of inorganic aerosol and (2) what is the effect of the organic on the water uptake of the inorganic components. Another goal of this study is to provide experimental data for inorganic-organic mixed aerosol systems, which would assist in the development and evaluation of the corresponding theoretical tools. These questions have been previously addressed for internal aerosol mixtures of two inorganic salts, e.g. NaCl-(NH4)2SO4 and NaCl-Na2SO4. The results show that the mixed inorganic aerosol deliquesces at a RH which is lower than the deliquescence relative humidity (DRH) of either of the individual salts. The hygroscopic growth proceeds in two stages: first a gradual dissolution of the solid core and then complete dissolution and growth of the homogeneous solution droplet (1, 15, 16, 4). However, due to lack of thermodynamic data for the inorganic-organic mixtures of interest and due to the complexity of the mixtures, there is no theoretical framework for predicting such behavior for atmospheric aerosol. Early theoretical attempts have focused on the reproduction of water activity coefficients in simple mixtures (21). VOL. 34, NO. 20, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1: Summary of Physical Propertiesa chemical compound
mol wt, density, solubility, g per g/mol g/cm3 100 cm3 H2O
NaCl 58.14 2.165 (NH4)2SO4 132.14 1.769 glutaric acid 132.11 1.424 pinonic acid 146.14 0.786d
26.0 75.4 116b 0.6d
vapor pressure at 25 °C, Pa
5.55 × 10-4 c 10 s. Similar observations for TDMA experiments were reported by Ristovski et al. (35). The good agreement with previous work also indicates that the RH to which the particles were exposed to was consistent throughout the system and that deliquescence and not efflorescence was being measured. Therefore, pockets of high relative humidity, which would have resulted in Dp/Dp0 values above the deliquescence curve, were unlikely throughout the flow system. Deliquescence curves for the internally mixed aerosol were measured for NaCl-glutaric acid, NaCl-pinonic acid, (NH4)2SO4-glutaric acid, and (NH4)2SO4-pinonic acid mixtures. The organic mass fraction in each inorganic-organic aerosol mixture was also varied; values of 0.2, 0.5, and 0.8 organic mass fraction were sampled. Figure 5 shows sample deliquescence curves for two NaCl-glutaric acid mixtures. Two observations can be drawn from these experiments: (1) glutaric acid does not significantly affect the DRH of NaCl (within ( 1%), regardless of organic mass fraction and (2) the total water uptake at a given RH decreases with increasing organic mass fraction. The same behavior was observed for the NaCl-pinonic acid mixtures (not shown): no observable effect on DRH and decreased water absorption with increasing organic mass fraction. Figure 6 shows sample deliquescence curves for (NH4)2SO4-organic mixtures of glutaric acid and pinonic acid. As in the NaCl experiments, no effect on 4316
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FIGURE 6. Sample deliquescence curves measured for (NH4)2SO4glutaric acid and (NH4)2SO4-pinonic acid mixtures. Initial diameter is 100 nm. the DRH of (NH4)2SO4, i.e., particles deliquesced at RH ) 79 ( 1.0%, regardless of the type or mass fraction of organic acid used. Also, a decrease in particle growth due to water absorption is observed with the presence of the organics. Further analysis of the organic effect on total water content will be discussed in the following sections. Given that the gradual growth of the organics occurred at a higher RH than the pure salts, the observed deliquescence behavior for the mixed organic-inorganic particles (no effect on salt deliquescence) could also be explained by a lowering of the salt DRH by less than 1% (experimental error). This is based on deliquescence measurements of multicomponent salts where the DRH of the mixture is observed to be lower than that of any of its constituents (1, 15, 16). The same results viewed from the point of view of the organic component suggest that mixing of the inorganic salt lowers the DRH of the organic particle, something consistent with the predictions of Wexler and Seinfeld (15). In other words, the data best supports the conclusion that the DRH of these mixed particles is practically equal to the DRH of the inorganic component and lower than the DRH of the organic component. Effect of Organics on the Hygroscopic Growth of Inorganics. The total hygroscopic growth of a particle at a given RH is usually expressed by the growth factor G(RH), defined as the ratio of the wet particle diameter at a given RH over the dry particle diameter (RH < 10%).
G(RH) )
Dp(RH) Dp(RH < 10%)
(1)
The final diameter for the internally mixed inorganic-organic particles at RH ) 85.0 ( 1.0% was compared to the initial particle diameter in order to calculate G(85%). In these experiments, RH was maintained at a single value, and different initial diameters of the distribution were sampled. The initial diameters sampled for the different inorganicorganic mixtures were 50, 80, 100, and 120 nm. The results for NaCl and (NH4)2SO4 are shown in Tables 2 and 3, respectively. The error reported with each value of G(85%) indicates the uncertainty of the measurements at each diameter sampled. This uncertainty represents three sources of error: the DMA sizing error, the fluctuations in RH, and the resolution of the SMPS system, which becomes lower with increasing diameter. The G(85%) values in Tables 2 and 3 show no dependence on initial diameter at any composition, since the water needed to dissolve the solute per mass basis is the same at a given RH. Also, the lack of dependence on diameter is secondary evidence that the
[
composition
Dp0 (nm) G(85.0 ( 1.0%)
100% NaCl
50 80 100 120 80% NaCl 50 20% glutaric acid 80 100 120 50% NaCl 50 50% glutaric acid 80 100 120 20% NaCl 50 80% glutaric acid 80 100 120 80% NaCl 50 20% pinonic acid 80 100 120 50% NaCl 50 50% pinonic acid 80 100 120 20% NaCl 50 80% pinonic acid 80 100 120 a
2.09 ( 0.11 2.12 ( 0.08 2.13 ( 0.10 2.09 ( 0.08 1.65 ( 0.06 1.72 ( 0.06 1.75 ( 0.04 1.65 ( 0.06 1.54 ( 0.05 1.54 ( 0.05 1.64 ( 0.10 1.43 ( 0.05 1.29 ( 0.05 1.29 ( 0.05 1.31 ( 0.05 1.29 ( 0.08 1.78 ( 0.06 1.84 ( 0.06 1.84 ( 0.06 1.71 ( 0.06 1.65 ( 0.07 1.72 ( 0.05 1.71 ( 0.05 1.72 ( 0.05 1.43 ( 0.05 1.43 ( 0.05 1.43 ( 0.05 1.43 ( 0.05
Mwater/ Msolutea
ξw
1 3.94 ( 0.2 1 1 1 0.5 ( 0.2 1.87 ( 0.2 0.6 ( 0.2 0.7 ( 0.2 0.5 ( 0.2 0.6 ( 0.2 1.57 ( 0.2 0.6 ( 0.2 0.8 ( 0.3 0.5 ( 0.2 0.7 ( 0.2 0.79 ( 0.2 0.7 ( 0.2 0.8 ( 0.2 0.7 ( 0.3 0.7 ( 0.2 2.75 ( 0.2 0.8 ( 0.2 0.8 ( 0.2 0.6 ( 0.2 0.8 ( 0.3 2.99 ( 0.2 1.0 ( 0.2 1.0 ( 0.2 1.0 ( 0.2 1.2 ( 0.2 1.76 ( 0.2 1.2 ( 0.2 1.2 ( 0.2 1.2 ( 0.2
Values calculated using average G(85.0 ( 1.0%).
TABLE 3. Growth Factor for (NH4)2SO4-Organic Mixtures at RH ) 85% composition 100% (NH4)2SO4
Dp0 (nm) G(85.0 ( 1.0%)
100 50 80% (NH4)2SO4 80 20% glutaric acid 100 120 50% (NH4)2SO4 50 50% glutaric acid 80 100 120 20% (NH4)2SO4 50 80% glutaric acid 80 100 120 80% (NH4)2SO4 50 20% pinonic acid 80 100 120 50% (NH4)2SO4 50 50% pinonic acid 80 100 120 20% (NH4)2SO4 50 80% pinonic acid 80 100 120 a
1.49 ( 0.07 1.33 ( 0.03 1.33 ( 0.03 1.38 ( 0.03 1.38 ( 0.03 1.38 ( 0.03 1.33 ( 0.03 1.37 ( 0.03 1.33 ( 0.03 1.24 ( 0.06 1.20 ( 0.04 1.17 ( 0.07 1.19 ( 0.06 1.33 ( 0.03 1.38 ( 0.03 1.41 ( 0.04 1.40 ( 0.03 1.38 ( 0.04 1.33 ( 0.04 1.41 ( 0.04 1.43 ( 0.03 1.33 ( 0.03 1.33 ( 0.03 1.37 ( 0.03 1.37 ( 0.03
Mwater/ Msolutea
ξw
1.26 ( 0.2 1 0.8 ( 0.2 0.82 ( 0.2 0.8 ( 0.2 0.9 ( 0.2 0.9 ( 0.2 1.4 ( 0.3 0.93 ( 0.2 1.2 ( 0.3 1.4 ( 0.3 1.2 ( 0.3 2.0 ( 0.5 0.47 ( 0.2 1.6 ( 0.4 1.3 ( 0.7 1.5 ( 0.6 0.8 ( 0.2 1.04 ( 0.2 0.9 ( 0.2 1.0 ( 0.2 1.0 ( 0.2 1.4 ( 0.3 1.36 ( 0.2 1.2 ( 0.3 1.6 ( 0.3 1.7 ( 0.3 2.9 ( 0.4 1.38 ( 0.2 2.9 ( 0.4 3.4 ( 0.4 3.4 ( 0.4
Values calculated using average G(85.0 ( 1.0%).
particles had enough time to reach equilibrium even for the mixed particles, given that larger particles did not experience less growth. The mass of water absorbed by the mixed particles at RH ) 85% per total mass of solute was calculated from the measured growth factors by
]
Ff Mwater ) G(85%)3 * -1 Msolute F0
TABLE 2. Growth Factor for NaCl-Organic Mixtures at RH ) 85%
(2)
The results are also included in Tables 2 and 3. In eq 2 Ff (final wet particle density) and F0 (initial dry particle density) are estimated based on the mass fraction of each component in the mixed particle (38). The density estimates for the organic solution and the mass averaging of component densities add a 5-10% uncertainty to the water absorption calculation. However, such an exercise is important to relate the measured quantities using particle diameters to thermodynamic parameters for atmospheric modeling (39). Using the values for Mwater/Msolute, the final wet mass of a mixed organic-inorganic particle can be estimated and compared to actual atmospheric measurements. To quantify the effect of the organic fraction on the hygroscopic growth of the inorganic fraction, the hygroscopic growth due to the inorganic portion was compared to that of the total growth by the mixed particle. The change in water uptake because of the organic, ξw, was calculated as
ξw )
water volume uptake by mixed particle ) water volume uptake by inorganic fraction 3 G -1 3
(1 - 0)(GINORG - 1)
(3)
where G is the measured growth factor for the mixed particle, 0 is the volume fraction for the organic, and GINORG is the growth factor for the inorganic component at the relative humidity of interest. The change in water uptake, ξw, corresponds to the error incurred by assuming that the inorganic fraction of the particle only interacts with water, and the organic behaves like an inert material. In eq 3, the water volume uptake change can be calculated directly from the growth measurements of the pure and mixed particles. The results for the experiments are also shown in Tables 2 and 3. The discrepancy between the measured growth of the pure inorganic components and the values in the literature (4) can be used together with the standard deviation of the measurements for the calculation of the uncertainty in ξw. The ξw values can be used to determine whether the organic has a positive or negative effect on total water absorption. According to (3), a value of ξw > 1.0 indicates that the organic fraction enhances the water absorption by the particle, by either increasing the hygroscopic growth of the inorganic fraction or absorbing water itself. On the other hand, if ξw is less than 1.0, the organic portion decreases the hygroscopic growth by the inorganic portion and thus decreases total water uptake by the particle. A value of ξw ∼ 1.0 indicates a neutral effect of the organic portion on the water absorption by the inorganic. The results for the NaCl-organic experiments (Table 2) indicate the possibility of positive or negative organicinorganic interactions during hygroscopic growth, depending on the amount of organic present in the particle. For example, the 80% NaCl-20% glutaric acid aerosols increased their diameter by 1.65-1.75 at 85% RH. If the NaCl absorbed the expected amount of water (growth factor of 2.1) and the organic acted as inert, a change in size of approximately 2 would have been observed. The corresponding average ξw value for this experiment, 0.6 ( 0.2, suggests a decrease in the water uptake by the NaCl fraction of nearly 40%, due to the presence of the glutaric acid. A similar negative interaction was observed in the other NaCl-glutaric acid experiments. For the NaCl-pinonic acid experiments, as the mass fraction of the organic increases, the diameter change after humidification decreases by up to 30%, as indicated by the G(85%) measurements (Table 2). Also, at low organic mass fraction (
