Removal of Sea Salt Hydrate Water from Seawater-Derived Samples

Article pubs.acs.org/est

Removal of Sea Salt Hydrate Water from Seawater-Derived Samples by Dehydration Amanda A. Frossard and Lynn M. Russell* Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, United States S Supporting Information *

ABSTRACT: Aerosol particles produced from bubble bursting of natural seawater contain both sea salts and organic components. Depending on the temperature, pressure, and speed of drying, the salt components can form hydrates that bind water, slowing evaporation of the water, particularly if large particles or thick layers of salts undergo drying that is nonuniform and incomplete. The water bound in these salt hydrates interferes with measuring organic hydroxyl and amine functional groups by Fourier transform infrared (FTIR) spectroscopy because it absorbs at the same infrared wavelengths. Here, a method for separating the hydrate water in sea salt hydrates using freezing and then heating in warm, dry air (70 °C) is evaluated and compared to other methods, including spectral subtraction. Laboratory-generated sea salt analogs show an efficient removal of 89% of the hydrate water absorption peak height by 24 h of heating at atmospheric pressure. The heating method was also applied to bubbled submicrometer (Sea Sweep), generated bulk (Bubbler), and atomized seawater samples, with efficient removal of 5, 22, and 39 μg of hydrate water from samples of initial masses of 11, 30, 58 μg, respectively. The strong spectral similarity between the difference of the initial and dehydrated spectra and the laboratory-generated sea salt hydrate spectrum provided verification of the removal of hydrate water. In contrast, samples of submicrometer atmospheric particles from marine air masses did not have detectable signatures of sea salt hydrate absorbance, likely because their smaller particles and lower filter loadings provided higher surface area to volume ratios and allowed faster and more complete drying.

■

mean radius of 0.18 μm and a standard deviation of 1.8 μm) at 1% RH had distinct IR spectral absorbance signatures of MgCl2 hydrates, including sharp peaks at about 3390 and 3210 cm−1 and a double peak at about 1650 and 1630 cm−1. The low RH spectra did not change with increases or decreases in temperature, indicating that MgCl2 hydrates have similar absorption spectra or only hexahydrate was formed.7 The aerosol passed through a diffusion drier (with a residence time of 2 min to obtain 1% RH) that likely did not allow sufficient time to reach a stable equilibrium if the hydrate water was prevented from diffusing out. FTIR spectroscopy has been used to identify the composition of ambient submicrometer marine particles collected and dried in thin layers on Teflon filters.8−11 This technique has not been used previously to quantify the organic composition of samples of artificially generated submicrometer and supermicrometer sea spray particles formed from model ocean systems that may be deposited in thicker layers on filters. Because the evaporation of hydrate water at 0% RH can be slowed if diffusion is limited, these larger, thicker samples retain substantially higher quantities of hydrate water as they dry,

INTRODUCTION Seawater and freshly emitted sea spray particles are 97% water, by mass, but they also contain trace amounts of important organic components. A macroscopic sample of seawater dried to 0% RH can leave up to 15% of the mass of dry sea salt as bound water molecules in hydrates,1 depending on the morphology of the salt mixture and the method of drying. The rationale for this study was to develop a method for characterizing the organic components of particles produced by seawater bubble bursting (and by other sources such as dust that retain substantial amounts of water bound as hydrates). FTIR spectroscopy has been shown effective for measuring the organic components of salt and dust-containing refractory particles, but the presence of hydrate water interferes with the measurement of organic hydroxyl groups. Evaporated seawater can include halite (NaCl), kieserite (MgSO 4 ·1H 2 O), carnallite (KMgCl 3 ·6H 2 O), anhydrite (CaSO4), and bischofite (MgCl2·6H2O).2 The most abundant salt formed from standard seawater evaporation is NaCl (∼90%) followed by MgCl2 and MgSO4 (∼3−4% each) and KMgCl3 and CaSO4 (∼2% each).2 Previous studies have shown absorbance in the infrared (IR) region of hydrate bound and liquid water in salt water mixtures using Fourier transform infrared (FTIR) spectroscopy at differing RH values.3−6 Cziczo and Abbatt7 observed that a low flow, directly into the sample chamber, of MgCl2 aerosol (log-normal distribution with a © 2012 American Chemical Society

Received: Revised: Accepted: Published: 13326

August 8, 2012 November 14, 2012 November 26, 2012 November 26, 2012 dx.doi.org/10.1021/es3032083 | Environ. Sci. Technol. 2012, 46, 13326−13333

Environmental Science & Technology

Article

Table 1. Conditions for Particle Generation and Collection mass (μg m−3) aerosol source

particle size

samples

drying during collection

generated

submicrometer

6

Sea Sweep RH controlled

generated

sub- and supermicrometer

12

Bubbler none

generated

sub- and supermicrometer

6

atomized seawater inline drier

ambient

submicrometer

6

ambient aerosol RH controlled

a,b

hydrate watera,b

Nab

OC/Nab

3.50 5.00 4.62 3.96 2.16 2.50 2.74

2.44 2.20 3.10 2.87 1.53 2.31 2.62

4.59 NA 5.74 4.07 2.33 5.29 5.49

0.37 NA 0.42 0.42 0.39 0.29 0.26

16.6 14.0 10.3 9.8 16.6 13.0 13.9 10.4 25.4 36.4 17.9 16.0 15.5

47.7 11.3 24.3 26.7 40.5 33.9 23.8 16.3 164.0 173.2 31.1 16.7 10.4

53.2 30.8 37.9 NA 24.5 NA 28.6 NA NA 170.7 NA NA 26.6

0.15 0.24 0.16 NA 0.30 NA 0.26 NA NA 0.07 NA NA 0.34

487.5 285.2 362.9 535.4 494.6 847.7 399.1

995.9 952.0 1231.8 766.7 970.3 1388.8 665.6

NA NA NA NA NA NA NA

NA NA NA NA NA NA NA

1.15 2.87 0.75 1.45 0.78 0.73 0.31

NA NA NA NA NA NA NA

0.07 0.05 0.03 0.04 0.12 0.16 0.04

8.35 30.9 9.9 20.0 3.6 1.9 3.3

OM

OM and hydrate water were determined using the dehydration procedures outlined in the paper. bThe values in the first row of each section are the averages for each particle generation type.

a

components, generated submicrometer marine aerosol, generated bulk marine aerosol, and atomized seawater. IR spectroscopy was used here to assess the extent to which hydrate water mass was removed by each process. Spectral subtraction was also investigated to determine if it is a sufficient technique to isolate the hydrate water absorbance from the organic absorbance.

which are more difficult to remove and may be part of salt hydrates that have formed into gels.12 Multiple techniques for extracting hydrate water from seawater-derived sea salt hydrates, while retaining the organic components, have been proposed including the following: (i) freezing which causes the hydrate water to break apart from the salt hydrate to form solid water;13 (ii) heating which causes the hydrate water to evaporate directly14 but may alter the chemical composition of reactive or volatile organic components; (iii) vacuum drying after freezing which reduces the pressure then allows the frozen water to sublime without directly heating the sample;15 and (iv) desiccating which pushes the equilibrium of the hydrate water and RH of the air toward water evaporation.16 Here we evaluate several approaches for removing hydrate water from seawater-derived samples while retaining intact the organic components for chemical characterization. The amount of hydrate water removed by each technique was measured for four types of seawater derived samples: standards of seawater

■

METHODS Particle Generation and Collection. The five types of seawater-related samples studied here are standards of seawater components, generated submicrometer marine aerosol, generated bulk marine aerosol, atomized seawater, and ambient marine aerosol particles (Table 1). For all of the following sampling conditions, particles were collected by drawing air through 37-mm Teflon filters. Samples collected in the field were frozen and transported to San Diego for analysis. Prior to analysis, each sample was equilibrated in a temperature- (20

13327

dx.doi.org/10.1021/es3032083 | Environ. Sci. Technol. 2012, 46, 13326−13333

Environmental Science & Technology

Article

Figure 1. FTIR absorption spectra of standards, Sea Sweep, Bubbler, and atomized seawater samples. (a) Spectra of NaCl, MgCl2, and sea salt (black), spectra of the respective salts added to glucose (gray), and the initial spectra of glucose (purple). These spectra were normalized by the maximum absorption of the glucose and salt mixtures. (b) Initial spectra for each sampling condition. (c) Difference spectra of the initial and dehydrated spectra. (d) Final spectra of the mass remaining in the dehydrated samples. The initial, difference, and dehydrated spectra were all normalized by the maximum absorption of the initial spectrum of each sample to show the relative differences in absorption. Gaps in the spectra represent artifacts from the dehydrator at 2917 ± 12 and 2850 ± 6 cm−1 (dehydrator and difference). Vertical lines indicate sea salt hydrate signature peaks at 3380, 3235, and 1640 cm−1 (solid) and a signature local minimum at 3260 cm−1 (dashed). In the Glucose + MgCl2 plot, the vertical lines represent the MgCl2 hydrate signature peaks at 3340, 3235, and 1640 cm−1 (solid).

reanalyzed before heating to 30, 40, 50, and 70 °C for 20 min and reanalyzing after each 20 min heating period. The filters were frozen for an additional 5.4 months and then placed in a desiccator containing silica gel for 4.0 days, rescanned, and returned to the desiccator box for 7.0 more days. After scanning, the filters were placed in a dehydrator (Nesco/ American Harvest FD-80) at 70 °C in the cleanroom and rescanned after 1.8 and 14.9 total days. 3. Bubbler Sea Spray Particles (Submicrometer and Supermicrometer). The Bubbler consisted of a large glass cylinder filled with seawater, and bubbles were produced by flowing zero air through two sets of ceramic frits (fine and coarse) or by dropping jets of seawater on to the model seawater surface.21 The continuously regenerated seawater surface contained ambient seawater constantly pumped by the R/V Atlantis. The bubbles burst at the seawater surface, which lofted the resulting aerosol into a continuous air stream. Submicrometer and supermicrometer particles were collected on filters without drying. After the initial FTIR scan, the filters were stored frozen for 13.8 months and then rescanned. They were then placed in a desiccator for and rescanned after 2.0, 6.0, and 13.0 total days. Then, the filters were placed in a dehydrator at 70 °C in the cleanroom and rescanned after 2.7, 6.7, 22.4, and 28.0 total days. 4. Atomized Seawater. Ambient seawater was collected during the CalNex 2010 cruise and frozen. The seawater was thawed and atomized to produce a broad mode of submicrometer particles, dried by passing through a drier containing drierite (average residence times of 2−7 s), and deposited on Teflon filters. After the initial FTIR scan, the filters were frozen for 15.9 months and then rescanned. They were then placed in a desiccator for 15 days and rescanned. After, the filters were placed in a dehydrator at 70 °C in the cleanroom and rescanned after 1.8, 5.7, 14.4, 27.4, and 42.7 total days.

°C) and humidity-controlled ( 0⎟⎟⎟ ⎝ SSS ⎠⎠ ⎝

(1)

where Sdehy1 is the final spectra with hydrate water removed, S0 is the initial spectrum, Sss is the sea salt hydrate spectrum, and x1 and x2 are the wavenumber bounds of the region of interest (described in the SI). This method was applied to the Sea Sweep, Bubbler, and atomized seawater samples, and the resulting dehydrated spectra are shown in Figure S2b. The scaling of the sea salt spectrum was used to calculate the hydrate water mass removed (SI). With this method, an average of 6.2, 18.3, and 21.7 μg of hydrate water were removed for the Sea Sweep, Bubbler, and atomized seawater samples, respectively (Table 2). Equation 1 gives an upper bound of the hydrate water in the sample by assuming that all of the absorption at 3700−3100 cm−1 that fits the hydrate water spectral shape could be due to hydrate water. Because primary marine aerosol particles have been found to contain significant fractions of hydroxyl functional groups,10 this assumption provides an upper bound on, and likely an overestimate of, the hydrate water mass. The spectra resulting from eq 1 compare well in peak height and shape with the dehydrated spectra from freezing and heating for the Sea Sweep and frit configuration Bubbler samples (Figure S2b; Figure 1d), although there is less remaining absorbance area, as is consistent with eq 1 as an upper bound. The spectra resulting from eq 1 for the atomized seawater and jet configuration Bubbler samples do not compare as well with the spectra from the dehydration because the peak at 3500 cm−1 was removed by dehydration but not by eq 1. This spectral subtraction does not remove the peak at 3500 cm−1 because that peak is associated with trapped liquid water rather than hydrate water. 13331

dx.doi.org/10.1021/es3032083 | Environ. Sci. Technol. 2012, 46, 13326−13333

Environmental Science & Technology

Article

The second type of estimate used eqs 2, 3, and 4 to calculate the dehydrated spectra for each sample by subtracting a scaled sea salt hydrate spectrum based on the XRF or IC-measured elemental masses. In eq 2, the measured mass of Na (MNa, μg) for each sample was used to calculate the dehydrated spectra: ⎛ rMNaFhwAbs ⎞ Sdehy2 = S0 − ⎜SSS × ⎟ FNam w PSS ⎠ ⎝

(2)

where r is the ratio of μmol OH per μmol of water (2 OH per H2O), Fhw is the observed mass fraction of hydrate water in dried seawater (0.15),1 Abs is the sea salt hydrate absorptivity (SI), FNa is the mass fraction of Na in sea salt (0.31), mw is the molecular weight of water (18 μg μmol−1), and PSS is the total peak area of the standard sea salt hydrate spectra. In eq 3, the ratio of Mg to Na (rNa, 0.088) in seawater and MNa were used to calculate the dehydrated spectra for each sample: ⎛ rMNarNaAbs ⎞ ⎟ Sdehy3 = S0 − ⎜⎜SSS × rMgmMg PSS ⎟⎠ ⎝

Figure 3. Normalized, FTIR spectra from atmospheric particles collected during CalNex in clean marine air masses (gray) and marine derived particle spectra from Scripps Pier 2008 (gold). Vertical lines indicate hydrate signatures that are present in spectra in Figure 1a but absent here: peaks (solid) and local minimum (dashed). Spectral noise for these spectra was smoothed using Savitzky−Golay with 31 points and second order.

(3)

atomized seawater, and reference samples, rapid collection of a larger mass of salt may have inhibited drying by crystallization of salt layers on top of layers of undried water pockets.7 A third possibility, if the particles were more aged, is that as ambient marine aerosol ages and reacts with sulfuric and nitric acids in the atmosphere, it becomes depleted in chlorine and enriched in sulfur.26 The average ratio of Na to Cl in the CalNex ambient marine samples is 2.01, which is greater than the ratio in seawater (0.56) showing Cl depletion. Since Cl is needed to form MgCl2 and KMgCl3, less water is bound in Cl-containing hydrates. In summary, low heating at atmospheric pressure enabled dehydration of metastable hydrate water from sea salts. Initial FTIR spectra of Sea Sweep, Bubbler, and atomized seawater samples showed absorbance signatures characteristic of sea salt hydrates, which were removed by dehydration that included freezing and heating the samples. The dehydrated spectra resemble atmospheric particles from clean marine regions and have spectral shapes consistent with previously identified marine organic components. High cosine similarity values between the difference of the spectra before and after dehydration with the spectra of sea salt hydrates confirmed that the mass removed during dehydration was hydrate water and not volatile organic species. While not recommended for quantifying hydrate water or for recovering the nonwater organic hydroxyl spectrum, spectral subtraction of scaled reference spectra of sea salt hydrate provides a useful indicator of the presence and potential contribution of hydrate water. Since minor artifacts from contamination were caused by the dehydrator, we recommend that future seawater-derived aerosol sampling include drying particles by heating and desiccation before collection on filters to reduce the need for dehydration after sampling. Another prudent sampling technique is limiting the mass of salt-containing particles that are deposited on the filters so that the thickness of the salt layer does not slow mass transfer by trapping bound water in hydrate pockets.

where rMg is the molar ratio of Mg to hydrate water in sea salt hydrates (1:6) and mMg is the molecular weight of Mg (24.3 μg μmol−1). In eq 4 the mass of Mg (MMg, μg) for each sample was used to calculate the dehydrated spectra: ⎛ rMMgAbs ⎞ ⎟ Sdehy4 = S0 − ⎜⎜SSS × rMgmMg PSS ⎟⎠ ⎝

(4)

The resulting averages for the hydrate water removed with eqs 2, 3, and 4 are 5.7, 3.8, and 6.8 μg from the Sea Sweep samples and 12.3, 10.0, and 23.4 μg of hydrate water from the Bubbler samples (Table 2 and Figure 2). The dehydrated spectra from the different spectral subtraction equations compare well to each other in shape and magnitude (Figure S2). The atomized seawater samples were not analyzed by XRF or IC, so hydrate water was not calculated with eqs 2, 3, or 4. The quantification of the organic components remaining is discussed in SI. Comparison to Atmospheric Particles from SeawaterDerived Spray. The submicrometer atmospheric aerosol particles collected during the CalNex cruise, in airmasses of marine origin, had FTIR spectra that lacked the signatures of sea salt hydrate absorption (Figure 3), even without the dehydration procedures described here. The characteristic double peak at 3380 and 3235 cm−1 and the local minimum at 3260 cm−1 are absent. Using eq 2 to calculate an upper bound on the amount of hydrate water present, hydrate water could contribute at most less than 5% of the organic mass. There are two likely explanations for the absence of hydrate water in the submicrometer atmospheric aerosol particles collected. First, they were sampled through an RH-controlled (heated) inlet that dried particles to approximately 60% RH before collection on the filter, which is in contrast to the reference, Bubbler, and atomized seawater samples that were not heated. This meant that there was less liquid water present when the salts dried. Second, the lower atmospheric concentrations of salt particles relative to the generated samples meant the mass of salt collected on filters was lower (ambient Na: 0.07 μg m−3; Sea Sweep Na: 4.59 μg m−3), even though sampling times were longer. Hence the collected samples were thinner and collected more slowly, providing for uniform and complete drying of the sample. For the Sea Sweep, Bubbler,

■

ASSOCIATED CONTENT

S Supporting Information *

Description of the procedures used to quantify the masses of organic functional groups for the samples; description of the calculation of the absorptivity for hydrate water and the 13332

dx.doi.org/10.1021/es3032083 | Environ. Sci. Technol. 2012, 46, 13326−13333

Environmental Science & Technology

Article

(12) Chan, C. K.; Ha, Z. Y.; Choi, M. Y. Study of water activities of aerosols of mixtures of sodium and magnesium salts. Atmos. Environ. 2000, 34 (28), 4795−4803. (13) Wang, A.; Freeman, J. J.; Jolliff, B. L. Phase transition pathways of the hydrates of magnesium sulfate in the temperature range 50 degrees C to 5 degrees C: Implication for sulfates on Mars. J. Geophys. Res.-Planets 2009, 114. (14) Ni, Q.-Y.; Wu, Y.-L.; Yang, M.-D.; Hu, H.-S.; He, Y.-F.; Zhang, L.-J.; Dang, J. TG-FTIR Analysis of Thermal Degradation of Bischofite’s Crystals with Aniline Hydrochloride. Spectrosc. Spectral Anal. 2011, 31 (7), 1747−1751. (15) Popp, M.; Lied, W.; Meyer, A. J.; Richter, A.; Schiller, P.; Schwitte, H. Sample preservation for determination of organic compounds: Microwave versus freeze-drying. J. Exp. Bot. 1996, 47 (303), 1469−1473. (16) Weis, D. D.; Ewing, G. E. Infrared spectroscopic signatures of (NH4)(2)SO4 aerosols. J. Geophys. Res.-Atmos. 1996, 101 (D13), 18709−18720. (17) Maria, S. F.; Russell, L. M.; Turpin, B. J.; Porcja, R. J.; Campos, T. L.; Weber, R. J.; Huebert, B. J. Source signatures of carbon monoxide and organic functional groups in Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) submicron aerosol types. J. Geophys. Res.-Atmos. 2003, 108, D23. (18) Berner, A.; Lurzer, C.; Pohl, F.; Preining, O.; Wagner, P. Size distribution of the urban aerosol in Vienna. Sci. Total Environ. 1979, 13 (3), 245−261. (19) Quinn, P. K.; Coffman, D. J.; Kapustin, V. N.; Bates, T. S.; Covert, D. S. Aerosol optical properties in the marine boundary layer during the First Aerosol Characterization Experiment (ACE 1) and the underlying chemical and physical aerosol properties. J. Geophys. Res.Atmos. 1998, 103 (D13), 16547−16563. (20) Bates, T. S.; Quinn, P. K.; Frossard, A. A.; Russell, L. M.; Hakala, J.; Petäjä, T.; Kulmala, M.; Covert, D. S.; Cappa, C. D.; Li, S.-M.; Hayden, K.; Nuaaman, I.; McLaren, R.; Massoli, P.; Canagaratna, M. R.; Onasch, T. B.; Sueper, D.; Worsnop, D. R.; Keene, W. C. Measurements of Ocean Derived Aerosol off the Coast of California. J. Geophys. Res.-Atmos. 2012, 117, D00V15. (21) Keene, W. C.; Maring, H.; Maben, J. R.; Kieber, D. J.; Pszenny, A. A. P.; Dahl, E. E.; Izaguirre, M. A.; Davis, A. J.; Long, M. S.; Zhou, X. L.; Smoydzin, L.; Sander, R. Chemical and physical characteristics of nascent aerosols produced by bursting bubbles at a model air-sea interface. J. Geophys. Res.-Atmos. 2007, 112, D21. (22) Draxler, R. R.; Rolph, G. D. HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory); Air Resources Laboratory, NOAA: Silver Spring, MD, 2003. (Model access via NOAA ARL READY Website http://www.arl.noaa.gov/readyhysplit4.html). (23) Pavia, D. L.; Lapman, G. M.; Kriz, G. S. Introduction to Spectroscopy, 3rd ed.; Brooks/Cole, 2001. (24) Woods, E.; Chung, D.; Lanney, H. M.; Ashwell, B. A. Surface Morphology and Phase Transitions in Mixed NaCl/MgSO(4) Aerosol Particles. J. Phys. Chem. A 2010, 114 (8), 2837−2844. (25) Foster, M.; Ewing, G. E. An infrared spectroscopic study of water thin films on NaCl (100). Surf. Sci. 1999, 427−428. (26) McInnes, L. M.; aaCovert, D. S.; Quinn, P. K.; Germani, M. S. Measurements of chloride depletion and sulfur enrichment in individual sea-salt particles collected from the remote marine boundary-layer. J. Geophys. Res.-Atmos. 1994, 99 (D4), 8257−8268.

calculation of the mass of hydrate water; discussions of primary marine organics and cosine similarity calculations. This information is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was funded by NSF grants ATM-0744636, OCE0948252, and OCE-1129580. The authors gratefully acknowledge Timothy Bates and Patricia Quinn for organizing the CalNex 2010 cruise and for the creation and deployment of the Sea Sweep, and William Keene and John Maben for the design and deployment of the Bubbler. We acknowledge Shang Liu, Anita Johnson, and Robin Modini for assisting in the preparation of laboratory standards. We thank Derek Coffman and Drew Hamilton for their assistance in sample collection and the scientists, captain, and crew of the R/V Atlantis for their support in the field.

■

REFERENCES

(1) Lewis, E. R.; Schwartz, S. E. Sea Salt Aerosol Production: Mechanisms, Methods, Measurements, and Models - A Critical Review; American Geophysical Union: Washington, DC, 2004; Vol. 152. (2) Harvie, C. E.; Weare, J. H.; Hardie, L. A.; Eugster, H. P. Evaporation of SeaWater - Calculated Mineral Sequences. Science 1980, 208 (4443), 498−500. (3) Weis, D. D.; Ewing, G. E. Water content and morphology of sodium chloride aerosol particles. J. Geophys. Res.-Atmos. 1999, 104, D17. (4) Cziczo, D. J.; Nowak, J. B.; Hu, J. H.; Abbatt, J. P. D. Infrared spectroscopy of model tropospheric aerosols as a function of relative humidity: Observation of deliquescence and crystallization. J. Geophys. Res.-Atmos. 1997, 102 (D15), 18843−18850. (5) Hoffman, R. C.; Laskin, A.; Finlayson-Pitts, B. J. Sodium nitrate particles: physical and chemical properties during hydration and dehydration, and implications for aged sea salt aerosols. J. Aerosol Sci. 2004, 35 (7), 869−887. (6) Zhao, L. J.; Zhang, Y. H.; Wei, Z. F.; Cheng, H.; Li, X. H. Magnesium sulfate aerosols studied by FTIR spectroscopy: Hygroscopic properties, supersaturated structures, and implications for seawater aerosols. J. Phys. Chem. A 2006, 110, 3. (7) Cziczo, D. J.; Abbatt, J. P. D. Infrared observations of the response of NaCl, MgCl2, NH4HSO4, and NH4NO3 aerosols to changes in relative humidity from 298 to 238 K. J. Phys. Chem. A 2000, 104 (10), 2038−2047. (8) Russell, L. M. Aerosol organic-mass-to-organic-carbon ratio measurements. Environ. Sci. Technol. 2003, 37 (13), 2982−2987. (9) Russell, L. M.; Bahadur, R.; Ziemann, P. J. Identifying organic aerosol sources by comparing functional group composition in chamber and atmospheric particles. Proc. Natl. Acad. Sci., U. S. A. 2011, 108 (9), 3516−3521. (10) Russell, L. M.; Hawkins, L. N.; Frossard, A. A.; Quinn, P. K.; Bates, T. S. Carbohydrate-like composition of submicron atmospheric particles and their production from ocean bubble bursting. Proc. Natl. Acad. Sci., U. S. A. 2010, 107 (15), 6652−6657. (11) Hawkins, L. N.; Russell, L. M.; Covert, D. S.; Quinn, P. K.; Bates, T. S. Carboxylic acids, sulfates, and organosulfates in processed continental organic aerosol over the southeast Pacific Ocean during VOCALS-REx 2008. J. Geophys. Res.-Atmos. 2010, 115, D13201. 13333

dx.doi.org/10.1021/es3032083 | Environ. Sci. Technol. 2012, 46, 13326−13333