Water Uptake and Hygroscopic Growth of Organosulfate Aerosol

Article pubs.acs.org/est

Water Uptake and Hygroscopic Growth of Organosulfate Aerosol Armando D. Estillore,† Anusha P. S. Hettiyadura,‡ Zhen Qin,‡ Erin Leckrone,§ Becky Wombacher,§ Tim Humphry,§ Elizabeth A. Stone,*,‡ and Vicki H. Grassian*,†,∥,⊥ †

Department of Chemistry and Biochemistry, ∥Department of Nanoengineering, and ⊥Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093, United States ‡ Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States § Department of Chemistry, Truman State University, Kirksville, Missouri 63501, United States ABSTRACT: Organosulfates (OS) are important components of secondary organic aerosol (SOA) that have been identified in numerous field studies. This class of compounds within SOA can potentially affect aerosol physicochemical properties such as hygroscopicity because of their polar and hydrophilic nature as well as their low volatility. Currently, there is a dearth of information on how aerosol particles that contain OS interact with water vapor in the atmosphere. Herein we report a laboratory investigation on the hygroscopic properties of a structurally diverse set of OS salts at varying relative humidity (RH) using a Hygroscopicity-Tandem Differential Mobility Analyzer (HTDMA). The OS studied include the potassium salts of glycolic acid sulfate, hydroxyacetone sulfate, 4-hydroxy-2,3-epoxybutane sulfate, and 2-butenediol sulfate and the sodium salts of benzyl sulfate, methyl sulfate, ethyl sulfate, and propyl sulfate. In addition, mixtures of OS and sodium chloride were also studied. The results showed gradual deliquescence of these aerosol particles characterized by continuous uptake and evaporation of water in both hydration and dehydration processes for the OS, while the mixture showed prompt deliquescence and effloresce transitions, albeit at a lower relative humidity relative to pure sodium chloride. Hygroscopic growth of these OS at 85% RH were also fit to parameterized functional forms. This new information provided here has important implications about the atmospheric lifetime, light scattering properties, and the role of OS in cloud formation. Moreover, results of these studies can ultimately serve as a basis for the development and evaluation of thermodynamic models for these compounds in order to consider their impact on the atmosphere.

I. INTRODUCTION Secondary organic aerosol (SOA) particles are formed in the atmosphere from the oxidation products of anthropogenic and biogenic volatile organic compounds (BVOC).1 They comprise a significant fraction of atmospheric particulate matter and consist of numerous complex chemical compounds with different physicochemical properties.2 SOA particles can scatter radiation and act as cloud condensation or ice nuclei and therefore influence the Earth’s radiation balance and climate.1−5 The identification, quantification, and exploration of how SOA particles are altered in the atmosphere through physical and chemical processes remain an active area of field studies, laboratory investigations, and atmospheric and chemical modeling. Organosulfates (OS) are important constituents of SOA that contain a sulfate ester functional group. Laboratory studies have shown that organosulfates can be generated via the oxidation of BVOCs such as isoprene, α-pinene, limonene, and β-pinene with OH, NO3, and O3.6−8 They can also be formed through the reactive uptake of volatile aldehydes like glyoxal and pinonaldehyde after exposure to acidic mixtures of (NH4)2SO4 and H2SO4.9,10 Mechanisms for the formation pathways of OS have been compiled elsewhere.1,11,12 These laboratory studies were complemented by several field investigations that © 2016 American Chemical Society

identified and quantified different types of OS to be present in the atmosphere from samples collected from the different parts of the world. Among the types of OS detected are hydroxyl and carboxy OS,6,13 nitrooxy OS,14−17 aromatic OS,18−21 and long chain aliphatic OS.22 The atmospheric abundance of OS is estimated to be in the upper limit of 5− 10% of the organic aerosol mass over the continental US.23 While laboratory and field studies have successfully characterized OS, quantitation of OS is often fraught with challenges due to the lack of standards. The interaction of water vapor with most atmospherically relevant inorganic salts as a function of varying relative humidity (RH) is well established. The results obtained from these studies provide important insights on the mechanisms involved during particle phase transition and cloud-droplet efficiency, as well as its aerosol optical depth.24 However, equivalent studies associated with organosulfates are severely limited, largely because of the nonavailability of OS standards. Recently, Hansen and co-workers measured the water uptake Received: Revised: Accepted: Published: 4259

October 13, 2015 March 9, 2016 March 11, 2016 March 11, 2016 DOI: 10.1021/acs.est.5b05014 Environ. Sci. Technol. 2016, 50, 4259−4268

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Environmental Science & Technology

Table 1. Molecular Structures and Relevant Properties of Organosulfates Considered in the Water Uptake Experimentsa

a

The asterisk denotes the purity of propyl sulfate and benzyl sulfate as determined by 1H NMR.

western US and coastal areas reveal that the highest OS levels are found in areas that have the highest concentration of sulfate and sodium. Moreover, sodium and potassium positively correlate with OS in the entire region that the study was conducted.26 With this in mind, the sodium and potassium salts are considered to be atmospherically relevant, although organosulfates of ammonium salts in the atmosphere, due to the high abundance of ammonia, are also present and of great importance. The synthesis and characterization of potassium 2-butenediol sulfate (C4H7SO5K) and potassium 4-hydroxy-2,3-epoxybutane sulfate (C4H7SO6K) are given below. Aerosol particles were generated from a 0.5 or 1% wt/v aqueous solution using ultrapure water prepared on site (Thermo, Barnsted EasyPureII; > 18.2 MΩ cm resistivity). Mixed NaCl-OS particles are prepared from solution containing 0.5 wt % NaCl and 0.5 wt % organosulfate. The molecular structures and relevant properties of organosulfates considered in this study are listed in Table 1. B. Synthesis and Characterization. Potassium 2Butenediol Sulfate. The synthesis of 2-butenediol sulfate was conducted similarly to previously published syntheses.13 In 25 mL of dry pyridine, cis-2-butenediol (2.6 mL 0.032 mol, 2 equiv) was stirred under nitrogen with a sulfur-trioxide pyridine complex (2.57 g, 0.016 mol, 1 equiv). After 12 h, the reaction mixture was reduced to a pale peach-colored oil using rotary evaporation with the temperature not exceeding 40 °C and then ion exchanged for potassium using 120 mL (>10 equiv) of Dowex 50WX8 cation exchange resin in the potassium form. The resulting clear, colorless aqueous solution was evaporated to a white solid using rotary evaporation, with the temperature not exceeding 40 °C. The solid was stirred vigorously with about 40 mL of methanol, and an insoluble white solid was filtered out that produced an NMR spectrum consistent with disubstituted butenediol sulfate. Diethyl ether (40 mL) and acetone (20 mL) were added to the clear, colorless mother liquor, precipitating a white matted fibrous solid (2.160 g,

properties on sub- and supersaturated conditions of limonenederived OS and their mixtures with ammonium sulfate. They reported that the values of the hygroscopic parameter depend on particle diameter and surface effects.25 Organosulfates are low-volatility, highly amphiphilic SOA compounds. This property of organosulfates may significantly affect the physicochemical properties of aerosols such as hygroscopicity. The dynamic response of the interaction of OS with water vapor is of paramount importance in understanding the thermodynamic and kinetic factors governing the absorption and desorption of water on OS. In this work, the water uptake properties of a structurally diverse set of organosulfates using a hygroscopic tandem differential mobility analyzer (H-TDMA) is measured. We explore the effect of chain length, presence of aromatic rings, degree of unsaturation, and oxygenation on the hygroscopicity of these organosulfate compounds. In addition, internally mixed systems of sodium chloride and organosulfates are also studied.

II. EXPERIMENTAL METHODS A. Chemicals and Reagents. Ammonium sulfate ((NH4)2SO4, >99%) and sodium chloride (NaCl, >99%) were purchased from Fisher Scientific and were used as received. Eight organosulfate standards were used in the water uptake experiments, three of which are commercially available: sodium salts of methyl sulfate (CH3SO4−), ethyl sulfate (C2H5SO4−), and propyl sulfate (C3H7SO4−). Potassium salts of hydroxyacetone sulfate (C3H5SO5−) and glycolic acid sulfate (C2H3SO6−) were synthesized as described in Hettiyadura et al. 13 Sodium salts of benzyl sulfate (C7 H 7SO 4− ) was synthesized similar to the method described in Hettiyadura et al. with the exception of using NaOH in place of KOH.13 Sodium and potassium salts of organosulfates are good standards for these measurements as they are highly stable and can be made or purchased in pure solid form. In addition, a recent field study of measurements of organosulfur over the 4260

DOI: 10.1021/acs.est.5b05014 Environ. Sci. Technol. 2016, 50, 4259−4268

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Environmental Science & Technology 0.0105 mol; 67% yield), which was collected by filtration and proved to be the analytically pure potassium salt of the sulfate ester of 2-butenediol. 1H NMR (400 MHz, DMSO-d6): δ 3.98 (t, 2H, 3J HH = 5.5, methylene), δ 4.30 (d, 2H, 3J HH = 6.2, methylene), δ 4.72 (t, 1H, 3J HH = 5.4, alcohol), δ 5.47−5.52 (m, 1H, vinyl), δ 5.55−5.60 (m, 1H, vinyl). 13C NMR (100.6 MHz, DMSO-d6): δ 57.01 (methylene), δ 61.92 (methylene), δ 126.00 (alkene), δ 132.76 (alkene). Tandem mass spectrometric characterization using negative electrospray ionization ((−)ESI) quadrupole time-of-flight mass spectrometer (Q-TOF MS), observed m/z (error (mDa), relative abundance (%), formula): 167.0005 (−0.9, 28.98, C4H7SO5−), 79.9556 (−1.2, 32.48, SO3•−), 80.9633 (−1.3, 8.13, HSO3−), 95.9505 (−1.2, 100, SO4•−), 96.9585 (−1.1, 87.36, HSO4−), 136.9900 (−0.9, 43.15, C3H5SO4−). Elemental analysis (EA) calculated for C4H7SO5K: (%) C, 23.29; H, 3.42; S, 15.55. Found: C, 23.43; H, 3.34; S, 15.50. Potassium 4-Hydroxy-2,3-epoxybutane Sulfate. Epoxidation of 2-butenediol sulfate was carried out using mchloroperoxybenzoic acid (approximately 5 g, 0.021 mol, 2 equiv), dissolved, and stirred in 50 mL of acetonitrile (not dry). To this, powdered potassium 2-butenediol sulfate (2.16 g, 0.0105 mol, 1 equiv) was added at once and stirring was continued for 8 h. At that time, the solution was thick with white precipitate. The solid was isolated from the mother liquor by filtration, then rinsed by stirring with 30 mL of acetonitrile, and filtered again. The solid proved to be the potassium salt of 1,2-epoxybutandiol sulfate (2.100 g; 0.0102 mol, a 97% yield). Analytically pure crystals were obtained by recrystallizing the product from hot ethanol. 1H NMR (400 MHz, DMSO-d6): δ 3.03 (q, 1H, 3J HH = 5.8, methylene), δ 3.14−3.17 (m, 1H, methylene), δ 3.40−3.45 (m, 1H, oxirane), δ 3.54−3.61 (m, 1H, oxirane), δ 3.68 (dd, 1H, 3J1 HH = 12.5, 3J2 HH = 6.9, methylene), δ 3.96 (dd, 1H, 3J1 HH = 12.8, 3J2 HH = 3.9, methylene), δ 4.93 (t, 1H, 3J HH = 5.7, alcohol). 13C NMR (100.6 MHz, DMSO-d6): δ 53.99 (methylene), δ 56.12 (oxirane), δ 59.16 (oxirane), δ 64.27 (methylene). Q-TOF MS (−) ESI observed m/z (error (mDa), relative abundance (%), formula): 182.9957 (−0.6, 10.01, C4H7SO6−), 79.9557 (−1.1, 8.33, SO3•−), 96.9585 (−1.1, 100, HSO4−), 138.9693 (−0.8, 12.70, C2H3SO5−). EA calculated for C4H7SO6K: (%) C, 21.62; H, 3.17; S, 14.32. Found: C, 21.67; H, 3.12; S, 14.31. C. Water Uptake Experiments. Experiments on the water uptake properties of organosulfates were conducted on a MultiAnalysis Aerosol Reactor System (MAARS). Details of this apparatus have been described in detail previously.27,28 Briefly, the instrument is equipped with an aerosol generator, relative humidity (RH) sensors, a reaction or hydration chamber, a differential mobility analyzer (DMA), and a condensation particle counter (CPC) and a cloud condensation nuclei counter (CCNC). MAARS can measure heterogeneous chemistry, phase transitions, cloud condensation nuclei activity, and infrared extinction of aerosol particles. In this study, we focus on the subsaturation (RH < 100%) water uptake of OS aerosol particles. For the water uptake experiments, a stream of polydisperse aerosol particles were generated from an atomizer (TSI Inc., Model 3062), and the particles passed through dual diffusion dryers in series to dry the particle to RH ≤ 5% before sampling and size selection. The dry particles were subsequently charged by a TSI 85Kr charger (TSI Inc., Model 3077) to attain an equilibrium charge distribution. A monodisperse population of dry particle was selected by the first DMA (DMA1, TSI Inc.,

Model 3080) with mobility diameters of ca. 100 nm. After size selection, the aerosol particles were sent to a hydration chamber where the particles were exposed and equilibrated to a defined RH. The aerosol particles grew (or shrank) in this chamber before they entered the second DMA (DMA2). The RH was adjusted by varying the ratio of wet and dry air controlled by a commercial dry air generator (Parker Balston, model 75−62). Multiple RH sensors are used to monitor the RH to within ±1% throughout the hydration chamber. After exiting the reaction chamber, the aerosol particles exposed to specific RH conditions were directed to a scanning mobility particle sizer (SMPS) consisting of DMA2 (TSI, Inc., model 3080) and a condensation particle counter (CPC, TSI 3776) for the measurement of the humidified size distribution. The water vapor was supplied through a Nafion system to the DMA2 sheath air, in which the RH is controlled to nearly equal to that of the RH of the hydration chamber. The RH in DMA2 is monitored at the DMA inlets of sample and sheath air and at the outlet of the sheath air to confirm and ensure the uniformity of the RH in the DMA2. The RH is changed stepwise, and the size distribution is measured when the RH stabilizes (5−20 min). For all measurements, the sheath and sample flow rates of the two DMAs were 3.0 and 0.3 liters per minute, respectively. The H-TDMA measurements were carried out in two modes: hydration and dehydration. For the hydration process, the dry particles exiting from DMA1 were exposed to a monotonically increasing RH until the resulting change in size was measured by the SMPS system. For the dehydration process the particles were first deliquesced at RH ≤ 85% in the hydration chamber, and the RH in DMA2 was monotonically lowered enabling the efflorescence relative humidity (ERH) to be measured.

III. RESULTS AND DISCUSSION A. Interaction of Water Vapor with an Inorganic Sulfate: Ammonium Sulfate. The hygroscopic properties of ammonium sulfate have been characterized and well studied using different techniques.29−34 Figure 1 shows the hygroscopic growth of ammonium sulfate as a function of RH as the test molecule in validating the MAARS system. The hygroscopic growth factor (GF) measures the amount of water taken up by the aerosol particles as a ratio of the diameter at ambient RH

Figure 1. Hydration and dehydration curves for D0 = 100 nm aerosol particles composed of (NH4)2SO4 particles as a function of RH. 4261


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Figure 2. Hydration and dehydration curves for D0 = 100 nm aerosol particles composed of (A) potassium glycolic sulfate and (B) potassium hydroxyacetone sulfate particles as a function of RH.

Figure 3. (A) Hydration curves for D0 = 100 nm aerosol particles composed of several different organosulfates including sodium methyl sulfate, sodium ethyl sulfate, potassium 2-butenediol sulfate, and potassium 4-hydroxy-2,3-epoxybutane sulfate when exposed to increasing relative humidity. (B) Fitting the hydration data of several organosulfate salts when exposed to increasing relative humidity. The continuous lines in the plot are the model fit from eq 4, and the dotted lines are the model fit from eq 5 using the parameters provided in Table 2. See text for additional details.

relative to the original dry particle and is calculated by the equation GF(RH) =

GF(RH) ≡

Dp(RH) Do

(1)

Do

⎛ ⎞1/3 ρ =⎜C s ⎟ ⎜ sol ρ ⎟ ⎝ 100 sol ⎠

(3)

At RH < 10% the ammonium sulfate particle exists as a dry crystal in its stable state. Upon hydration (increasing RH), a prompt deliquescence transition from the crystalline to aqueous phase was observed at the deliquescence relative humidity (DRH) of 79.9 ± 0.10%. On further increase of humidity, the particles become a fully liquid droplet resulting in further increase in size. Upon dehydration (decreasing RH), water partially evaporates from the particles and the particles remain aqueous, exhibiting hysteresis, until they undergo efflorescence transition where the particles crystallize at the efflorescence relative humidity (ERH) of 36.7 ± 1.8%. The hysteresis during dehydration is due to the persistence of metastable supersaturated solution droplets relative to the crystalline ammonium sulfate.24,36 When RH was further reduced beyond the efflorescence point, the particles exist as dry crystals in its stable state. The deliquescence and efflorescence values determined in this study agree well with the reported values for ammonium sulfate in the literature.29,37

where Dp(RH) is a particle diameter at a particular RH value, and D0 is the diameter of the dry particle. The resulting GF is then plotted as a function of RH. The growth of aqueous solution droplets in humid air is described by the Köhler theory29,35 which gives the relation between the droplet diameter and the equilibrium RH ⎛ 4M σ ⎞ %RH w sol ⎟ = a w exp⎜⎜ ρ 100 RT D⎟ ⎝ w p⎠

Dp(RH)

(2)

where aw is the water activity, Mw is the molar mass of water (kg/mol), ρw is the density of water (kg/m3), R is the ideal gas constant (J/K·mol), T is the temperature (K), and σsol is the surface tension of the solution (N/m). Theoretical growth factors can be calculated from the solution concentration Csol (mass percent), the solution density ρsol (kg/m3), and the salt density ρs (kg/m3) by 4262


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Table 2. Summary of the Fitting Parameters Used in Eqs 4 and 5 and the Growth Factors at 85% Relative Humidity of the Organosulfates That Show Gradual Deliquescence fitting parameters from eq 4

fitting parameters from eq 5

organosulfate

A

B

C

a

b

c

growth factor at 85% RH

sodium methyl sulfate sodium ethyl sulfate potassium 2-butenediol sulfate potassium 4-hydroxy-2,3-epoxybutane sulfate potassium glycolic acid sulfate potassium hydroxyacetone sulfate

0.1435 0.1835 0.2755 0.3084 0.3084 0.3504

0.5702 0.4818 0.3572 0.2202 0.2202 0.2602

1.7025 2.1225 2.1592 1.8225 1.9825 3.0825

0.6025 0.2025 0.1025 0.0825 0.0825 0.0725

0.7522 0.7022 0.7222 0.6022 0.1622 0.6022

−1.0095 −0.5495 −0.5095 −0.5295 −0.0205 −0.5385

1.50 1.45 1.40 1.30 1.29 1.30

magnesium salt forms.41 Together, these results suggest that methyl sulfate will continuously uptake water regardless of their counterion. The hygroscopic growth of OS aerosol particles can be fitted to functional forms. Figure 3B shows the results of the fitting of the experimental hygroscopic growth of OS using the threeparameter functional form used by Varutbangkul and coworkers44

B. Interaction of Water Vapor with Single Component Organosulfates. Figures 2A and 2B show the measured hygroscopic growth curves of potassium glycolic acid sulfate (GAS, C2H3SO6K) and potassium hydroxyacetone sulfate (HAS, C3H5SO5K), respectively. Olson et al. reported GAS as the most abundant OS detected and quantified in the ambient particulate matter (PM2.5) samples collected from Mexico City, Pakistan, and the United States.38 GAS was also detected over the continental US during the Deep Convective Clouds and Chemistry Experiments (DC3) and the Studies of Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC4RS) field campaigns.39 HAS is the third most abundant organosulfate detected and quantified using an authentic standard in the range of 2.7−5.8 ng/m3 from the samples collected in Centreville, AL during the Southeast Oxidant and Atmosphere Study (SOAS) field campaign in summer 2013.13 Unlike ammonium sulfate, potassium salts of GAS and HAS aerosol particles do not show distinct deliquescence and efflorescence transitions. Instead, the particles show gradual deliquescence associated with continuous growth in size upon hydration. This suggests that both potassium salts of GAS and HAS aerosol particles readily absorb water even at low RH that leads to an increase in aerosol volume as evidenced by the increasing growth factor. With decreasing RH, the aerosol particles slowly and steadily lose water content. During the reverse process, the growth factor is a little higher than the values obtained during the hydration process. This can be due to the kinetic limitations of evaporation and efflorescence. The aqueous droplets persist during the reduction of RH and that the liquid water content of the aqueous droplets did not immediately evaporate upon drying in DMA2.40 Figure 3A displays the hygroscopic growth of sodium methyl sulfate (CH3SO4Na), sodium ethyl sulfate (C2H5SO4Na), potassium 2-butenediol sulfate (C4H7SO5K), and potassium 4-hydroxy-2,3-epoxybutane sulfate (C4H7SO6K) with increasing RH. Methyl sulfate was detected in the ambient PM2.5 samples collected from Centreville, AL during the SOAS field campaign in July 10−11, 2013. Ethyl sulfate was below detection limit using mass spectrometry (MS) detection.13 Recent high-resolution mass spectrometric analyses of these samples suggest the presence of 2-butenediol sulfate and 4hydroxy-2,3-epoxybutane sulfate and their positional or functional group isomers. As can be seen from Figure 3B, the particles do not undergo distinct phase transitions but exhibit a continuous, reversible uptake and evaporation of water under the influence of changing RH. In contrast to a prior study of sodium methanesulfonate,41−43 sodium methyl sulfate shows continuous water uptake instead of discrete deliquescence and efflorescence. This finding is similar to the continuous water uptake of methanesulfonate in the ammonium, calcium, and

−A ⎡⎛ ⎛ RH ⎞C ⎤ RH ⎟⎞ ⎟ ⎥ × B⎜ GF = 1 + ⎢⎜1 − ⎝ 100 ⎠ ⎦ 100 ⎠ ⎣⎝

(4)

where RH is the relative humidity, and A, B, and C are positive empirical parameters. The first part in the product term was used to represent the hygroscopicity of water-soluble organic compounds45 and was also used on a field campaign measuring the hygroscopic properties of aerosol particles in the northeastern Atlantic.46 The second part is a general power law used to model the hygroscopic growth of dicarboxylic acids.47 The parameters used in the fitting and the growth factor at 85% are summarized in Table 2. In addition, the following expression by Kreindenweis et al.35 was used to model the hygroscopic growth factor of OS 1/3 ⎡ aw ⎤ GF = ⎢1 + (a + b·a w + c·a w2 ) ⎥ 1 − aw ⎦ ⎣

(5)

where a, b, and c are adjustable parameters, and aw is the water activity which is equal to RH(%)/100. This is the same functional used by Dick and co-workers48 and Brooks et al.49 to describe the growth of particles that undergo gradual deliquescence and efflorescence following interaction with water vapor. These two functionals are used to extrapolate the growth factor at RH = 85%. The fitting parameters are compiled in Table 2. The results from the H-TDMA measurements show that upon hydration, the OS absorbs water even at low RH and retains water over the full RH range considered in this study. Likewise, during drying, water is steadily desorbed from the aerosol particles. Figure 3B clearly demonstrates that alkyl OS (methyl and ethyl) have the greatest growth factors, while the oxygenated compounds had consistently lower growth factors. This is directly supported by the comparison of the 2butenediol and 4-hydroxy-2,3-epoxybutane sulfate comparison, in which the alkene has a higher growth factor than the epoxide. In addition, comparing hydroxyacetone sulfate and glycolic acid sulfate suggests that there is little difference in hygroscopic growth when switching between oxygenated functional groups, specifically the methyl ketone to a carboxyl group. The observed gradual water uptake and loss behavior, and the absence of prompt phase transition, has been observed in the 4263


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Figure 4. Hydration and dehydration curves for D0 = 100 nm aerosol particles composed of (A) sodium propyl sulfate and (B) sodium benzyl sulfate hygroscopicity as a function of increasing relative humidity. Note the different scale in the y-axis. Most likely, these samples were contaminated by chloride. See text for additional details. (C) Hydration and dehydration curves for D0 = 100 nm aerosol particles composed of NaCl particles as a function of RH and (D) hydration and dehydration curves for D0 = 100 nm aerosol particles composed of mixed sodium chloride and sodium methyl sulfate as a function of RH.

case of alkylaminium sulfates 50 as well as sulfonates: CH3SO3NH4, (CH3SO3)2Mg, and (CH3SO3)2Ca with the exception of CH3SO3Na, which exhibits distinct deliquescence and efflorescence transitions.41,42 Secondary organic aerosols formed by oxidation of cycloalkenes, monoterpenes, and sesquiterpenes also show smooth hygroscopic growth when exposed to different RH.44 One possible explanation for the continuous water accommodation of OS is the presence of physical defects of OS particles such as surface cracks, pores, and grain boundaries that could facilitate water adsorption and absorption even at low RH. The smooth water uptake curves of OS without deliquescence and efflorescence transitions is characteristic of that of amorphous aerosol particles.40,51 There is evidence that points to formation of amorphous phases in several inorganic aerosol components (e.g., NaNO3 and Ca(NO3)2) that do not exhibit discrete deliquescence and efflorescence transitions.52,53 Furthermore, many organic aerosol components have showed continuous uptake and loss of water corresponding to changes in RH without prompt phase transitions.40,54 Virtanen and co-workers55 reported that biogenic SOA particles produced in plant chamber experiments can adopt an amorphous solid state form. Amorphous aerosol particles interact with water vapor and undergo gradual deliquescence and hygroscopic growth at lower relative

humidity. This is not surprising given the presence of oligomeric compounds and other organic compounds with high molecular weight and low volatility in the atmosphere.15,56,57 C. Interaction of Water Vapor with Sodium Propyl Sulfate and Sodium Benzyl Sulfate. Figures 4A and 4B show the complete hydration and dehydration cycle of sodium propyl sulfate (C3H7SO4Na) and sodium benzyl sulfate (C7H7SO4Na), respectively. Relative to the hydration and dehydration curves seen in other single component OS, the humidograms for sodium propyl sulfate are surprising and intriguing. It showed discontinuity in its growth factor at a welldefined DRH of 71.0 ± 0.6%, corresponding to a phase transition from a solid to a dissolved state. Upon dehumidification, the dissolved particles slowly shed their water content before it crystallizes at 39.0 ± 0.8% RH. This is in sharp contrast to the smooth and continuous growth curves we have seen for the other single component OS studied. The same behavior is true for benzyl sulfate, albeit at a much lower GF of ∼1.15 at 85% RH compared to the GF of propyl sulfate of ∼1.78 at the same RH. Benzyl sulfate has been detected in the ambient PM2.5 samples collected from Lahore, Pakistan, Godavari, Nepal, and Pasadena, CA.18,19 It is the first confirmed atmospheric OS with an aromatic backbone. This aromatic OS 4264


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mixture that was significantly lower than the DRH of NaCl, and the mobility growth factors for all compositions of SDS/NaCl were also retarded. Molecular dynamics simulations of the interaction of water vapor with SDS/NaCl slabs revealed that the SDS hinders the initial water uptake. In the case of the interaction of sodium methyl sulfate/NaCl with water, ab initio calculations at the MP2/cc-pVDZ level of theory demonstrate that the oxygen from H2O interacts with the sodium cation in the sodium methyl sulfate.64 Additionally, two hydrogen bonds are formed from the hydrogen atoms of H2O to one of the S− O−C oxygen from the methyl sulfate and the other to a chloride ion in NaCl. The overall binding energy is calculated to be −61.5 kcal/mol which exceeds the sum of the binding energies of water and sodium methyl sulfate, suggesting synergy of their interactions.64 Whether the same chemical mechanism is similar to the NaCl/OS mixture certainly warrants further study combining theory, computations, and experiments. Furthermore, it is important to look at the nature of the equilibrium particle morphology of the NaCl-OS mixture using ambient single particle techniques.65 The behavior and properties of atmospheric aerosols are dependent on their interaction with humidity. Water uptake can modify particle size thereby affecting its ability to scatter light and could potentially lead to activation of particles to form cloud droplets. Presented here are the water uptake properties of several organosulfates and a mixture of inorganic salts and organosulfates measured using H-TDMA. Whereas inorganic ammonium sulfate and sodium chloride particles clearly show distinct deliquescence and efflorescence points upon hydration and dehydration, the growth curves of the organosulfates are characterized by a continuous increase of Dp/Do as a function of relative humidity demonstrating the hygroscopic nature of organosulfates even at low RH. In addition, mobility growth factors of the organosulfates and OS-inorganic mixtures are suppressed relative to pure inorganic salts. The DRH and ERH values of the OS-salt mixture are shifted to lower values of RH compared to the inorganic components alone. In a 1:1 by mass inorganic-organosulfate mixture, the hygroscopic growth of the mixture reveals that OS have a substantial influence on the growth factor and deliquescence of inorganic salts such as NaCl. However, such large relative concentrations of organosulfates seem unlikely given that the upper limit of OS contributions to organic aerosol mass is 5−10%.23 The absence of clear transition points of single component OS and their continuous uptake of water even at low RH suggest that this class of compounds plays an important role in extending the range of environmental conditions at which particle-bound water influences optical and physicochemical properties of aerosol particles. In addition, the presence of OS in inorganic salts modifies the hygroscopic properties of inorganic salts thereby modulating the role of these particles in the environment as it interacts with solar radiation. As atmospheric measurement techniques continue to advance, new information on the chemical constitution of particulate matter is discovered which underscores the need to consider their impacts to atmospheric chemistry and physics.5 The detection of organosulfates in the atmosphere, however in its infancy, presents a rich ground for research and merits further studies that include, but not limited to, identifying its sources, deducing formation mechanisms, measuring reactivity, and measuring cloud condensation and ice nucleating activity. Moreover, it is also important to measure the thermodynamic

has defined transition points when exposed to varying RH at 70% and 40% for its deliquescence and efflorescence points, respectively. The humidograms of both propyl sulfate and benzyl sulfate are very similar to crystalline aerosol particles like the inorganic salts. The reason for this anomalous behavior of these two organosulfates relative to the other organosulfates and the similarity with sodium chloride may result from sample impurities. Indeed, ion chromatography confirms the presence of chloride in the samples. Ion chromatography analyses for chloride in propyl sulfate and benzyl sulfate samples prepared from their sodium salts show that these contain 41.78% and 11.21% of Cl− by mass, respectively. The presence of this high level of chloride seen in these samples (which can be considered an impurity) could potentially give rise to the prompt deliquescence and efflorescence that are otherwise not seen in the other OS studied and that should not be taken as representative of the organosulfate. To confirm this, we performed water uptake measurements of an intentional mixture of NaCl with OS in what follows. D. Interaction of Water with OS-Inorganic Salt Mixtures. To determine the effect of organosulfates on the water uptake of crystalline aerosol particles, we look at the hygroscopicity of a 1:1 wt % mixture of NaCl and sodium methyl sulfate salt. Figure 4C shows the hydration and dehydration curves for pure NaCl with D0 = 100 nm. Like ammonium sulfate, NaCl is known to exhibit prompt deliquescence and efflorescence transition points as experimentally measured in our setup with values at DRH = 75.0 ± 0.50% and ERH = 44.0 ± 1.0%, respectively. These values agree well with what is reported in the literature.29,37 Figure 4D shows the growth factor of NaCl/methyl sulfate sodium as a function of relative humidity. With increasing RH at 5−69% RH, the growth curve of the mixture exhibits a “methyl sulfatelike” character, i.e. the growth factor grows smoothly with increasing RH and then undergoes a sharp deliquescence transition at DRH of 69.6 ± 1.0%. This phase transition is notably lower than the DRH of pure NaCl (see Figure 4C) at 75.0 ± 0.50% RH. During the reverse process, the humidified particles crystallize at 36 ± 0.5% RH, lower than the ERH of pure NaCl 44 ± 1.0% RH shown in Figure 4C. In addition, there is reduction of the growth factor at 85% RH of the mixture relative to the pure NaCl salt. Comparison of the hydration and dehydration curves of sodium propyl sulfate (Figure 4A) and sodium benzyl sulfate (Figure 4B) with that of the NaCl-OS mixture (Figure 4D) confirms that the presence of the defined transition points of the two single component OS salt samples is due to chloride impurity. Laboratory studies on the water uptake properties of mixed inorganic−organic compounds show that the presence of organic compounds can reduce the DRH at which the mixture undergoes phase transitions compared with particles containing only inorganic components.58−62 The results of this study show that the presence of OS affects the deliquescence of NaCl of the same particle size. Previous studies on the mixture of NaCl with long chain organosulfates (surfactants) showed the same effect. Woods et al.63 found that the deliquescence of 100 nm NaCl particles mixed with one coverage layer of sodium dodecyl sulfate (SDS, CH3(CH2)11SO4Na) was shifted to lower DRH by ∼1% RH. Furthermore, Harmon and co-workers64 have shown the effect of sodium dodecyl sulfate (SDS) on the hygroscopic growth and deliquescence of NaCl nanoparticles with a mobility diameter of 14.0 ± 0.2 nm. Gradual deliquescence was observed for the SDS/NaCl nanoparticle 4265


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and kinetic properties of these compounds to constrain their possible role in gas- to particle-phase transport.

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AUTHOR INFORMATION

Corresponding Authors

*Phone: 319-384-1863. Fax: 319-335-1270. E-mail: [email protected]. Corresponding author address: Chemistry Building, Iowa City, IA 52242 (E.A.S.). *Phone: 858-534-2499. Fax: 858-534-6255. E-mail: [email protected]. Corresponding author address: 3030 Urey Hall Addition, 9500 Gilman Dr, Mail Code: 0314, La Jolla, CA 92093 (V.H.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the US EPA Science to Achieve Results (STAR) program grant number 83540101 (E.A.S.). The contents in this study do not necessarily reflect the official views of the US EPA. The US EPA does not endorse the purchase of the commercial products used in this report.

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REFERENCES

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