Hygroscopicity and Compositional evolution of Atmospheric Aerosols

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Environmental Processes

Hygroscopicity and Compositional evolution of Atmospheric Aerosols Containing Water-Soluble Carboxylic Acid Salts and Ammonium Sulfate: Influence of ammonium depletion Na Wang, Bo Jing, Pan Wang, Zhen Wang, Jiarong Li, Shu-Feng Pang, Yun-Hong Zhang, and Maofa Ge Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b07052 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 3, 2019

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Environmental Science & Technology

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Hygroscopicity and Compositional evolution of Atmospheric

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Aerosols Containing Water-Soluble Carboxylic Acid Salts and

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Ammonium Sulfate: Influence of ammonium depletion

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Na Wang,1 Bo Jing,2 Pan Wang,1 Zhen Wang,2,3 Jiarong Li, 1 Shufeng Pang,*,1 Yunhong Zhang*,1

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and Maofa Ge2,3,4

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1School

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Republic of China

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2State

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Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing

of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, People’s

Key Laboratory for Structural Chemistry of Unstable and Stable Species, CAS Research/Education

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100190, People’s Republic of China

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3University

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4Center

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Academy of Sciences, Xiamen 361021, People’s Republic of China

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* Corresponding author E-mail: [email protected], [email protected]

of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China

for Excellence in Regional Atmospheric Environment, Institute of Urban Environment, Chinese

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1 ACS Paragon Plus Environment


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Abstract

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Water-soluble organic salts are important components of atmospheric aerosols. Despite their

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importance, it is still not clear how water-soluble organic salts influence interactions between

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aerosols and water vapor in the atmosphere. In this study, the hygroscopic behaviors and chemical

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compositions of aerosol particles containing water-soluble organic salt ((CH2)n(COONa)2, n = 0, 1,

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2) and (NH4)2SO4 were measured using in-situ attenuated total reflectance Fourier transform infrared

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spectroscopy (ATR-FTIR). The ammonium depletion due to release of gaseous NH3 was found in

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mixed aerosols composed of (CH2)n(COONa)2 (n = 1, 2) and (NH4)2SO4 upon dehydration. The

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ammonium loss could modify the aerosol composition, resulting in the formation of corresponding

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organic acid and monosodium dicarboxylate in mixed particles with low and high (NH4)2SO4

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content, respectively. Due to the weaker hydrolysis of oxalate anions, the ammonium depletion was

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not observed for the Na2C2O4/(NH4)2SO4 mixtures. The changes in the particle composition led to

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the decreased water uptake upon hydration as compared to that upon dehydration. Our findings

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reveal that interactions between water-soluble organic salts and (NH4)2SO4 in aqueous aerosols may

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affect the repartition of NH3 between the condensed and gas phases, thus modifying composition

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and physicochemical properties of aerosols as well as relevant chemical processes.

32 33


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1. Introduction

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Atmospheric aerosols are generally complex mixtures containing both organic and inorganic

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molecules, which are widely present in the troposphere.1, 2 The chemical compositions of the aerosol

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particles become more complicated during the atmospheric aging process, influencing aerosol’s

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physicochemical properties such as volatility, chemical reactivity, optical properties and

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hygroscopicity as well as cloud condensation nuclei (CCN) activity.

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ammonium and sulfate species, ammonium sulfate (AS) is a typical and significant component of

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tropospheric aerosol. Water-soluble organic acids have been identified as significant components of

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the organic matter in urban and remote atmospheric aerosols.8-10 Water-soluble organic salts could

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be formed through the neutralization of water-soluble organic acids with gas phase NH3 or

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displacement of hydrogen ions by metal ions in the particle phase.11, 12 The field measurements

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observed a significant fraction of water-soluble organic salts transformed from water-soluble

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organic acids in ambient particulates.9, 13 The formation of organic salts has been found to decrease

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the volatility and increase the hygroscopicity of corresponding organic acids.12, 14

2-7

Due to the prevalence of

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The previous studies have fully explored the influence of water-soluble organic acids on the

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hygroscopic behaviors of AS and other typical atmospheric inorganic salts.15-26 The organic acids

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were found to affect phase transitions and water uptake of AS aerosols, depending upon the kinds

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of organic acids.15, 16, 21, 27 In contrast to the organic acids, some organic salts have enhanced water

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uptake, even comparable to that of typical inorganic salts such as NaCl and (NH4)2SO4.28 Despite

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the distinct hygroscopic properties, there is a dearth of information on the role of water-soluble

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organic salts in the overall hygroscopicity of AS. Wu et al. investigated the effects of

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atmospherically relevant water-soluble carboxylic salts on the deliquescence process of AS using a 3 ACS Paragon Plus Environment


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hygroscopicity tandem differential mobility analyzer (H-TDMA).14 They found a clear shift in

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deliquescence relative humidity (DRH) of AS to lower RH for mixtures of AS with organic salts

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and enhanced water uptake of mixtures relative to mixtures with organic acids.14 Schroeder and

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Beyer also observed that the onset DRH of organic salt/AS mixtures was always lower than that of

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pure component, independent of the fraction of organic salts in the mixture.29 However, it is still not

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clear how water-soluble organic salts affect the efflorescence behavior of AS during the dehydration

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process. In fact, efflorescence processes play a critical role in determining hygroscopic behaviors

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and chemical compositions of atmospheric aerosols, as the supersaturated droplets formed upon

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dehydration may favor potential aqueous chemistry between aerosol components.30-33 The early

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studies have confirmed the changes in aerosol composition due to chemical reactivity of inorganic

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salts with organic acids induced during the dehydration process.30,

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efflorescence processes of sea salt aerosols containing sodium salts (NaCl and NaNO3) and water-

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soluble organic acids can lead to the formation of organic salts due to the chloride and nitrate

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depletion driven by the release of gaseous HCl and HNO3, which could significantly affect water

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uptake of aerosols during the deliquescence process.22, 30, 33, 36 In addition, it was found that the

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drying of mixed oxalic acid/AS droplets may facilitate the formation of ammonium hydrogen

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oxalate (NH4HC2O4) and ammonium hydrogen sulfate (NH4HSO4) in aerosols, thus altering the

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aerosol composition and hygroscopic growth.23 Considering the synergistic effect of NH4+

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dissociation and carboxylic anion hydrolysis, potential interactions between water-soluble organic

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salts and AS likely occur in their mixed aerosols. However, the impacts of such interactions on

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particles’ hygroscopicity are not well understood.

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33-36

For instance, the

In this study, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) 4 ACS Paragon Plus Environment

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was used to monitor the hygroscopic behavior and chemical composition of dicarboxylate

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/(NH4)2SO4 mixed aerosols during a dehydration-hydration cycle. We investigated the effect of

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dicarboxylate salts (Na2C2O4, CH2(COONa)2 and (CH2COONa)2) on the hygroscopicity of

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ammonium sulfate aerosols with varying organic-inorganic mixing ratios. The thermodynamic

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method was applied to simulate the influence of NH4+ depletion on the droplet pH and organic acid

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formation.

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2. Experiments

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AS, sodium succinate ((CH2COONa)2), sodium malonate (CH2(COONa)2), sodium oxalate

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(Na2C2O4), monosodium succinate (HOOC(CH2)2COONa), succinic acid ((CH2COOH)2), malonic

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acid (CH2(COOH)2) and oxalic acid (H2C2O4) were purchased from Sinopharm Chemical Reagent

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Co. Ltd. (99.0 % purity) and used without further purification. Monosodium malonate

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(HOOCCH2COONa) was prepared from internally mixed NaOH-malonic acid solutions with the

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molar ratio of 1:1. 0.1 M aqueous solutions of pure components including AS, (CH2COONa)2,

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CH2(COONa)2 and Na2C2O4 were prepared by dissolving their respective solids in ultrapure water

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(18.2 MΩ cm, Barnstead Easy pure II). The binary mixed solutions were prepared following organic-

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inorganic molar ratios of 3:1, 1:1 and 1:3 for each mixed system of (CH2COONa)2-AS,

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CH2(COONa)2-AS and Na2C2O4-AS. The aerosols generated from the solutions in a nebulizer

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flowed into the sample cell and deposited on the ZnSe substrate. The size of aerosol droplets on the

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substrate was less than 10 μm. The RH in the sample cell was adjusted to be higher than 90% to

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ensure the aerosols in an aqueous phase.

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Detailed description of the FTIR-ATR setup has been reported elsewhere.37, 38 Briefly, a Nicolet Magna-IR

model

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FTIR

spectrometer

equipped


with

a

liquid-nitrogen

cooled


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mercury−cadmium−telluride (MCT) detector was used to collect the FTIR absorbance spectra of the

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aerosol particles deposited on a horizontal ATR substrate. A reaction chamber was composed of an

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ATR (Spectra-Tech Inc. USA) accessory with a ZnSe crystal. The RH inside the chamber was

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controlled by a mixing stream of dry and humidified nitrogen gas with a total flow rate of 400

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mL·min-1. The exposure RH and temperature of aerosol particles were monitored by a RH&T meter

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(Centertek Center 310, accuracy of ± 2.5%) near the exit of the chamber. FTIR spectra were gained

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by collecting typical 32 scans between 4000 – 600 cm-1 at a resolution of 4 cm-1. Liquid particles

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were exposed to different RHs in dehydration/hydration experiments with the equilibration time of

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10 min at each RH setting. The condensed-phase water content in the particles was quantified from

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the integrated absorbance of the OH stretching band. The water content was defined as area ratio of

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the integrated peak area (A) between 3660 and 3300 cm-1 at a given RH to that (A0) at the initial

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high RH during the dehydration process. In addition, water-to-solute molar ratios (WSRs) of AS,

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Na2C2O4, CH2(COONa)2 and (CH2COONa)2 particles as a function of RH are calculated according

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to the literature, 39, 40 and the calculative process is supplied in SI. The FTIR-ATR setup was verified

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by measuring the hygroscopicity curve of AS (seen in Fig. S1). The DRH of AS aerosols measured

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in our study was ~ 83% ± 1.5% RH, which agreed with literature values of 80.1 ± 1.5% RH.21, 23, 41

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The ERH of AS aerosols was determined to range from 47.5% to 41.2% RH, close to the ERH value

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of 43.5 ± 2.15% for supermicron AS particles deposited on a substrate.42 The measured ERH values

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in the range of 47.5%-41.2% RH are largely attributed to the inhomogeneous particle sizes.

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3. Results and Discussion

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3.1 Hygroscopic behaviors of individual components

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Fig. 1(b) shows the water uptake of Na2C2O4 particles during a humidity circle. The complete 6 ACS Paragon Plus Environment

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efflorescence transition can be observed at 77.4%-75.6% RH, slight higher than the literature value

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of 75.2-72% RH measured by the electrodynamic balance (EDB).28 Efflorescence is a kinetically

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controlled process. The discrepancy on the ERHs of Na2C2O4 compared to those reported by Peng

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et al. 28 is likely due to the effects of substrate. The substrate supporting droplets may promote the

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heterogeneous nucleation of Na2C2O4 while the levitated droplets in EDB study can avoid induced

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nucleation by the substrate. Song et al.

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efflorescence phase transition of pure AS and its mixture with organic fraction. No deliquescence

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transition occurs upon hydration due to the deliquescence point of Na2C2O4 beyond our RH

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measurement range, consistent with the previous results from the EDB and H-TDMA

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measurements.14, 28 As shown in Fig. 1(a), the infrared spectra of Na2C2O4 particles show the prompt

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peak shift from 1637 cm-1 to 1629 cm-1 and from 1309 cm-1 to 1315 cm-1 with RH decreasing to

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75.6% RH, also indicating the crystallisation of Na2C2O4. As shown in Fig. 1(d), CH2(COONa)2

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particles loses water continuously with decreasing RH. As RH increases, CH2(COONa)2 continues

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to absorb water reversibly without any prompt deliquescence transition. The other literature have

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also reported such reversible and continuous water condensation-evaporation characteristics for

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CH2(COONa)2, which can be explained by the fact that the particles under dry conditions stay in an

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amorphous state rather than in a crystalline state.14, 28 Fig. 1(c) indicates that the liquid water peak

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around 3400 cm-1 is still observed even at the lowest 1.3% RH, suggesting the amorphous state of

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dry CH2(COONa)2 particles.28 As shown in Fig. 1(f), (CH2COONa)2 particles exhibit the

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efflorescence transition at 59.3%-57.7% RH, which deviates from the reported value of 46.7 - 47.9%

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measured by EDB technology.28 The substrate effect likely accounts for this discrepancy, which

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may promote the heterogeneous nucleation of (CH2COONa)2 in this study. The infrared spectra of

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also reported the potential effects of substrate on the



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(CH2COONa)2 particles show that upon dehydration the νs(COO-) band blue-shifts from 1398 cm-1

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to 1438 cm-1 between 59.3% RH and 57.7% RH, along with the narrowing of the band located at

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1552 cm-1, indicating the formation of (CH2COONa)2 crystal.44 During the hydration process, the

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deliquescence transition of (CH2COONa)2 particles is observed to occur at 69.5% RH, close to the

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DRH value between 63.5% and 66 % reported in the literature.28

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3.2 Hygroscopic Behaviors and Chemical Compositional Modification of Mixed Aerosols of

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Dicarboxylate salt ((CH2)n(COONa)2) and (NH4)2SO4

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3.2.1 Na2C2O4/AS Mixed System

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Fig. 2 presents hygroscopic behaviors of mixed Na2C2O4/AS aerosols with molar ratios of 3:1,

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1:1 and 1:3 during a dehydration - hydration process. For the 3:1 Na2C2O4/AS mixed system, the

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water content in aerosols decreases continuously upon dehydration with a distinct efflorescence

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transition between 77% RH and 70% RH, as shown in Fig. 2(b). The spectra of mixed particles in

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Fig. S2(a) also demonstrate that Na2C2O4 efflorescence occurs between 77% RH and 70% RH, as

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indicated by the peak shift from 1309 cm-1 (liquid) to 1317 cm-1 (solid) and 1338 cm-1 (solid) (also

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seen in Fig. 2(a)). The infrared spectra in Fig. S2(a) show the shifts of SO42- band from 979 cm-1 to

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995 cm-1 and 975 cm-1 at 61.4% RH and 55.3% RH, respectively, suggesting the crystallisation of

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Na2SO4 and AS. The formation of Na2SO4 has also been observed in the bulk Na2C2O4/AS

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samples.29 During the hydration process, a slight increase in water content is observed below 79.8%

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RH due to the surface adsorption of water on mixed particles. The mixed particles exhibit obvious

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water uptake above 79.8% RH. Nonetheless, the persistent existence of 1317 cm-1 and 1338 cm-1

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bands in Fig. S2(b) indicates that Na2C2O4 still remains in a crystalline state at high RH, resulting

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in the significantly decreased water uptake as compared to that upon dehydration. 8 ACS Paragon Plus Environment

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As shown in Fig. 2(c), the obvious efflorescence transition is observed between 54.9% RH and

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48.9% RH for 1:1 Na2C2O4/AS mixture. The infrared spectra of mixed particles (seen in Fig. S2(c))

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identify the occurrence of efflorescence of Na2C2O4, Na2SO4 and AS at 76.2% RH, 54.9% RH and

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48.9% RH, respectively, as indicated by corresponding feature shifts. During the hydration process,

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the mixed Na2C2O4/AS particles (1:1) exhibit the deliquescence transition at 80.4% RH and

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dramatically decreased water uptake relative to that upon dehydration. The infrared spectra (seen in

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Fig. S2(d)) suggest the unchanged feature of 1317 cm-1 and 1338 cm-1 for undissolved Na2C2O4 until

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high RH, which should be responsible for the decreased water uptake of mixed particles upon

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hydration. The mixed 1:3 Na2C2O4/AS particles have similar hygroscopic curve with 1:1 mixed

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particles in the whole RH range. The infrared spectra (seen in Fig. S2(e)) indicate the crystallisation

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of Na2C2O4 at ~82% RH, followed by crystallisation of Na2SO4 and AS at ~ 51% RH and ~ 45%

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RH, respectively. During the hydration process, the 1:3 mixed particles absorb considerable water

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above 81% RH. The infrared spectra (seen in Fig. S2(f)) upon hydration show the solid feature of

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Na2C2O4 and disappearance of 995 cm-1 peak for crystalline Na2SO4 at high RH, suggesting the

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dissolution of Na2SO4 in the deliquescent AS phase.

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It can be concluded that the Na2C2O4 content has negligible effects on the deliquescence

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transition of AS while it could enhance the efflorescence point of AS in the mixed particles with

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dominated Na2C2O4 fractions. The initial ERH of AS in the mixed Na2C2O4/AS particles increases

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from 44.9%, 48.9% to 55.3% RH with increased Na2C2O4 fraction. For mixed particles with a

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relatively large fraction of Na2C2O4, the efflorescent Na2C2O4 likely provides enough heterogeneous

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nuclei to facilitate the crystallization of AS. The previous study observe that the addition of H2C2O4

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into AS aerosols has similar effects on phase transitions of AS.23 Also, the mixed H2C2O4/AS 9 ACS Paragon Plus Environment


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particles have less water uptake upon hydration than that upon dehydration due to the presence of

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undissolved H2C2O4 that has a high deliquescence point.23

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3.2.2 CH2(COONa)2/AS Mixed System

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Fig. 3 illustrates hygroscopic behaviors of mixed CH2(COONa)2/AS aerosols with molar ratios

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of 3:1, 1:1 and 1:3 during a dehydration - hydration process. The mixed CH2(COONa)2/AS (3:1)

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particles have gradual water uptake during the humidity circle. In addition, less water content at high

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RH can be observed upon hydration as compared to that upon dehydration. As shown in Fig. S3 (a),

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a new peak located at 1710 cm-1 due to the stretching mode of COOH, along with peak shift from

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1570 cm-1 to 1596 cm-1, corresponds to the features of monosodium malonate (HOOCCH2COONa,

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seen in Fig. 3(a)), suggesting the formation of monosodium malonate.45 The occurrence of

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absorption of 995 cm-1 at 66. 7% RH indicates the initial crystallization of Na2SO4. In addition, the

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absence of NH4+ feature between 2800 cm-1 – 3400 cm-1, along with the appearance of the peak at

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995 cm-1 for solid Na2SO4, suggests that the reactivity of CH2(COONa)2 with AS likely leads to the

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complete NH4+ depletion through the release of gaseous NH3 upon dehydration. The Raman spectra

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in Fig. S4 also show the Na2SO4 absorption and absence of (NH4)2SO4 features including both

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features of NH4+ centered at 3200 cm-1 and SO42- at 975 cm-1 in 3:1 CH2(COONa)2/AS aerosols

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under dry conditions. Thus, the compositions of initial mixed CH2(COONa)2/AS (3:1) particles are

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transformed into HOOCCH2COONa, Na2SO4 and residual CH2(COONa)2. During the hydration

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process, Na2SO4 remains in a crystalline state even at high RH, as indicated by the persistence of

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995 cm-1 peak (seen in Fig. S3(b)). The previous study found that the hygroscopicity of monosodium

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malonate was lower than that of disodium malonate.46 As a result, the undissolved Na2SO4 and

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transformation of disodium malonate into monosodium malonate should be responsible for the 10 ACS Paragon Plus Environment

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decreased water uptake relative to that upon dehydration.

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As shown in Fig. 3(c), the 1:1 mixed CH2(COONa)2/AS particles undergo an efflorescence

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transition between 68.8% RH and 64.7% RH. The spectra in Fig. S3(c) show the occurrence of 995

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cm-1 peak at 68.8% RH, corresponding to the initial efflorescence phase transition of Na2SO4. In

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addition, new peaks at 1726 cm-1 and 916 cm-1 assigned to COOH, combined with the absence of

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1596 cm-1 due to HOOCCH2COONa, indicate the formation of CH2(COOH)2 rather than

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HOOCCH2COONa. The Raman spectra in Fig. S4 show the presence of residual AS in 1:1 mixed

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CH2(COONa)2/AS particles. The νs(-COO-) band at 1353 cm-1 for CH2(COONa)2 suggests the

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presence of residual CH2(COONa)2, seen in Fig. 3(a). Thus, the particle composition is modified by

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the partial reaction between CH2(COONa)2 and AS upon dehydration, resulting in the formation of

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CH2(COOH)2 and Na2SO4. During the hydration process, water content in particles increases more

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quickly above 69.2% RH due to the deliquescence of AS and partial dissolution of Na2SO4. As can

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be seen in Fig. S3(d)), a new band of 979 cm-1 observed above 69.2% RH and the decrease in

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intensity of 995 cm-1 peak indicate the formation of aqueous SO42- and incomplete dissolution of

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Na2SO4, which likely contributes to the obvious decrease in water content as compared to that upon

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dehydration.

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As seen in Fig. 3(d), the 1:3 mixed particles exhibit similar hygroscopic behaviors with that of

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1:1 mixed particles. The efflorescence transition is observed between 51.9% RH and 45% RH. All

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the bands for CH2(COONa)2 (seen in Figs. 3(a) and S3(e)) disappear in the 1:3 mixed particle spectra,

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suggesting the complete CH2(COONa)2 loss in the reaction between CH2(COONa)2 and AS with a

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mixing ratio of 1:3. The new peak at 1726 cm-1 combined with the absence of 1596 cm-1 due to

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HOOCCH2COONa, indicates the formation of CH2(COOH)2. Braban et al. has reported that the 11 ACS Paragon Plus Environment


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absorption features of CH2(COOH)2, excepting the peak at 1726 cm-1, are not discernable in its

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mixture with AS when the mass fraction of CH2(COOH)2 is less than 0.75.47 Thus, the CH2(COOH)2

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mode at 916 cm-1 is not observed due to the minor mass fractions of CH2(COOH)2 (~ 0.20) in the

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mixed particles. The composition of CH2(COONa)2/AS (1:3) aerosols is changed into a mixture

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consisting of AS, Na2SO4 and CH2(COOH)2 during the dehydration process with about 33.3% NH4+

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depletion compared to initial amount of AS. The CH2(COONa)2/AS (1:3) aerosols undergo the

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deliquescence transition at ~ 71.2% RH, lower than that of AS. As RH increases to 84%, the shift of

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ν1(SO42-) band for Na2SO4 from 995 cm-1 to 979 cm-1 illustrates the deliquescence of Na2SO4, seen

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in Fig. S3(f). The malonic acid has been found to markedly lower or even obscure the deliquescence

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transition of AS.15, 47 Similar to malonic acid, CH2(COONa)2 could also induce water uptake and

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deliquescence transition of AS at lower RH. However, it is clear that the interaction between

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CH2(COONa)2 and AS could modify aerosol compositions, forming Na2SO4, HOOCCH2COONa or

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CH2(COOH)2 dependent upon the AS content. The formation of HOOCCH2COONa was also found

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in the reaction of CH2(COOH)2 with NaCl in mixed particles upon dehydration.46

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3.2.3 (CH2COONa)2/AS Mixed System

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Fig. 4 shows hygroscopic behaviors of mixed (CH2COONa)2/AS aerosols with molar ratios of

248

3:1, 1:1 and 1:3 during a dehydration - hydration process. For the 3:1 mixed (CH2COONa)2/AS

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particles, the efflorescence transition is observed at ~ 58 % RH. As shown in Fig. S5(a), a new band

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at 1712 cm-1 attributed to asymmetric stretching mode of -COOH group, along with the peak shift

251

from 1552 cm-1 to 1575 cm-1, is coincident with the HOOC(CH2)2COONa features (seen in Fig.

252

4(a)), implying the formation of HOOC(CH2)2COONa. The shift from 1097 cm-1 to 1134 cm-1 and

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occurrence of the 995 cm-1 band at 55.1% RH suggest the completely crystallisation of Na2SO4. The 12 ACS Paragon Plus Environment

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absence of the broad NH4+ stretching between 3400 and 2800 cm-1 under dry conditions indicates

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the NH4+ depletion in the particle phase, seen in Fig. 4(a). During the hydration process, the 3:1

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mixed particles exhibit continuous water uptake from low RH. The spectra of mixed particles upon

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hydration (seen in Fig. S5(b)) show the deliquescence of Na2SO4 at 76.9% RH, as indicated by the

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shift of 995 cm-1 to 979 cm-1. The decreased water uptake at high RH relative to that upon

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dehydration can be attributed to the changes in the particle composition. The HOOC(CH2)2COONa

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component has gradual hygroscopic growth (seen in Fig. S6), which could promote the water uptake

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of mixed particles at low RH compared to the other components such as (CH2COONa)2 and Na2SO4.

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Also, the HOOC(CH2)2COONa component has less water uptake than (CH2COONa)2 at high RH

263

(seen in Fig. S6), thus the partial conversion of (CH2COONa)2 into HOOC(CH2)2COONa may lower

264

the water uptake of mixed particles at high RH relative to that upon dehydration.

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As shown in Fig. 4(c), the mixed 1:1 (CH2COONa)2/AS particles undergo the efflorescence

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transition at ~ 65% RH. The spectrum of 1:1 (CH2COONa)2/AS particles in Fig. 4(a) shows two

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bands at 1726 cm-1 and 1695 cm-1 due to the vibration mode of -COOH group, along with the band

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at 1201 cm-1, suggesting the formation of (CH2COOH)2 upon dehydration.48. The shift from 1097

269

cm-1 to 1134 cm-1 and a new peak at 995 cm-1 confirm the formation of Na2SO4 (seen in Fig. S5(c)).

270

Meanwhile, the characteristic peaks of AS (ν3 and ν1 stretching modes of the SO42- at 1089 cm-1 and

271

975 cm-1 and the broad band of NH4+ stretching between 3400 and 2800 cm-1) and (CH2COONa)2

272

(-COO- absorption at 1552 cm-1) almost disappear, seen in Figs. 4(a) and S5(c). These results

273

indicate that the original components in the mixed particles are fully transformed into Na2SO4 and

274

(CH2COOH)2 upon dehydration, thus resulting in the NH4+ loss and NH3 release from the particle

275

phase. During the hydration process, the deliquescence transition is observed at ~ 85% RH, 13 ACS Paragon Plus Environment


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consistent with the DRH of Na2SO4.49 Also, a considerable decrease in water content of 1:1 mixed

277

particles can be observed upon hydration in comparison to that upon dehydration. It can be explained

278

by the fact that (CH2COOH)2 remains in a solid state even at 90% RH, as indicated by persistence

279

of solid feature at 1726 cm-1 and 1695 cm-1 in Fig. S5(d). In fact, succinic acid has a deliquescence

280

point higher than 99% RH.50, 51 As shown in Fig. 4(d), the mixed (CH2COONa)2/AS (1:3) particles

281

are observed to effloresce at ~ 54% RH. The spectra indicate the formation of (CH2COOH)2 (1726

282

cm-1 and 1695 cm-1) and Na2SO4 (995 cm-1), as well as residual AS (1089 cm-1 and 975 cm-1), seen

283

in Fig. S5(e) and Fig. 4(a). Then, the deliquescence transition occurs at 80.5% RH, close to the DRH

284

of pure AS. Again, the undissolved (CH2COOH)2 component may reduce the hygroscopic growth

285

of mixed particles upon hydration. The previous studies also found that (CH2COOH)2 could still

286

remain as solid in the deliquescent aerosols, resulting in the decreased water uptake.22, 25, 52

287

The integrated area of bands located at 1454 cm-1 for NH4+ ions, 1552 cm-1 for -COO- and 1726

288

cm-1 for -COOH group within aqueous aerosols above 65% RH, are used to evaluate the changes in

289

component contents of AS, (CH2COONa)2 and (CH2COOH)2 in the 1:1 mixed aerosols, respectively.

290

Because of the complexity of these overlapping IR absorption bands, differential spectra obtained

291

by the every spectrum at various RHs subtracting that at the initial high RH, are utilized to acquire

292

the changes in peak areas of NH4+ ions, -COO- and -COOH group (gray shadows in Fig. S7(a)). Fig.

293

S7(b) shows that the peak areas of NH4+ ions and -COO- group decrease with decreasing RH while

294

-COOH group has the inverse tendency, indicating the enhanced NH4+ loss or NH3 release and the

295

increased conversion of (CH2COONa)2 to (CH2COOH)2 in the aqueous aerosols upon dehydration.

296

3.3 The mechanism for ammonium depletion of mixed dicarboxylate/(NH4)2SO4 particles

297

The infrared analysis suggests the formation of monosodium dicarboxylate or dicarboxylic acid 14 ACS Paragon Plus Environment

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in the mixed (CH2)n(COONa)2 (n = 1, 2)/AS particles upon dehydration, accompanied by the NH3

299

release. The aqueous chemistry between disodium dicarboxylate (Na2A) and (NH4)2SO4 in solutions

300

can be expressed as follows:

301 302 303 304 305

A2 - /HA - (aq) + NH4 + (aq)

K

NH3(aq, g) + HA - /H2A(aq)

NH4 + (aq) ↔ NH3(aq) + H + (aq) A2 - (aq) + H2O ↔ HA - (aq) + OH - (aq)

(1)

(1a) (1b)

HA - (aq) + H2O ↔ H2A(aq) + OH - (aq) (1c) NH3(aq) ↔ NH3(g)

(1d)

306

Although the reaction (1) can remain in a thermodynamical equilibrium in bulk solutions, it would

307

be gradually shifted to the right with the continuous degassing of NH3 upon drying of the aqueous

308

aerosols. As shown by the reactions of (1a) – (1c), the hydrolysis of dicarboxylic anions could

309

enhance the NH4+ dissociation, thus promoting the formation of NH3 (aq). The evaporation of NH3

310

will drive the equilibrium of reaction (1) towards formation of HA- and H2A. In our study, the

311

NH4+/NH3 depletion is not observed for Na2C2O4/(NH4)2SO4 mixtures. This can be explained by the

312

equilibrium constant (K) of reaction (1). Assuming the formation of HA- in reaction (1), the

313

calculated K value is 8.91×10-6 for the Na2C2O4/(NH4)2SO4 system, considerably lower than that of

314

2.82×10-4 and 1.74×10-4 for CH2(COONa)2/(NH4)2SO4 system and (CH2COONa)2/(NH4)2SO4

315

system, respectively. As the K values determined by the hydrolysis constants of carboxylic anions

316

(seen in Supporting Information (SI)), the hydrolysis ability of organic salts likely plays a role in the

317

ammonium depletion. In addition, our results indicate the less formation of dicarboxylic acid and

318

lower NH4+/NH3 depletion in the 1:1 CH2(COONa)2/(NH4)2SO4 system relative to the 1:1

319

(CH2COONa)2/(NH4)2SO4 system. Assuming the formation of H2A in reaction (1), the calculated K 15 ACS Paragon Plus Environment


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value is 1.09×10-10 for the CH2(COONa)2/(NH4)2SO4 system, lower than that of 1.54×10-9 for

321

(CH2COONa)2/(NH4)2SO4 system. The lower K value would limit the yield of H2A. For the organic

322

salt/AS mixtures with varying mixing ratios, the relative content of AS in the mixture could affect

323

the formation of NaHA or H2A due to the availability of H+ supplied by NH4+. Thus, the formation

324

of NaHA can be expected in CH2(COONa)2/AS and (CH2COONa)2/AS mixed particles with an

325

organic-inorganic ratio of 3:1.

326

The interactions between dicarboxylic acids and NaCl (and nitrate) within aerosols upon

327

dehydration could result in chloride (and nitrate) depletion. In our study, ammonium depletion

328

results from interactions between dicarboxylic acid salts and AS. The previous studies revealed that

329

chloride (and nitrate) depletion is dependent upon the volatility, acidity and fraction of organic acids

330

in aerosols.30, 33, 36 Generally, the higher acidity and larger content of organic acids in the organic

331

acid/ NaCl (and nitrate) particles could contribute to the greater chloride (and nitrate) depletion.33,

332

36

333

conjugate acid with lower acidity may favor the ammonium depletion. For instance, for the

334

CH2(COONa)2/AS and (CH2COONa)2/AS mixtures with a molar ratio of 1:1, the greater ammonium

335

depletion is observed for the latter relative to the former. Also, the larger organic salt content

336

corresponds to the higher ammonium depletion.

However, for the dicarboxylic acid salts/AS mixed system, the organic salt corresponding to the

337

The effects of NH4+ depletion on pH and the formation of organic acid are shown in Fig. 5. The

338

simulation process has been supplied in SI. Fig. 5 illustrates the dependence of solution pH and the

339

conversion fraction of A2- to H2A on the NH4+ depletion for the CH2(COONa)2/AS and

340

(CH2COONa)2/AS mixed solutions with the molar ratio of 1:1. The solution acidity and conversion

341

fraction of A2- to H2A are significantly increased with NH4+ depletion. As the NH4+ depletion 16 ACS Paragon Plus Environment

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approaches 100%, the formation fraction of CH2(COOH)2 and CH2(COOH)2 is 97.5% and 88.3%,

343

respectively, which indicates the incomplete reaction between CH2(COONa)2 and AS. Previous

344

studies reveal that the reactivity of water-soluble organic acids with sea salts can lead to the chloride

345

and nitrate depletion in aerosols upon dehydration due to the high volatility of HCl and HNO3. 22, 30,

346

33, 36

347

aerosols, affecting the repartition of NH3 between particle phase and gas phase as well as the aerosol

348

pH. Field measurements observed a low ammonium–sulfate ratio (< 2) for the sulfate aerosol in the

349

eastern United States despite excess ammonia in the gas phase, which is at odds with thermodynamic

350

models.53 In addition, it was found that despite the high NH3 concentrations, the fine particulates

351

were highly acidic in the southeastern United States.54 Guo et al. found that the low ammonium–

352

sulfate ratio (< 2) is strongly correlated with the concentration of sodium (Na+).55 Our findings reveal

353

that the water-soluble organic salts such as sodium salts likely contribute to the low ammonium–

354

sulfate ratio of sulfate aerosol in the atmosphere. Our results highlight a potential mechanism of NH3

355

recycling, which results in the modification of condensed phase species and thus further affects the

356

physicochemical properties and chemical processes of aerosols.

357

Supporting Information

358

This section includes: infrared spectra of AS aerosol particles during the dehydration process and

359

Variations of WSRs in AS aerosol particles on dehydration and hydration process (Fig. S1); and the

360

FTIR spectra of mixed Na2C2O4/AS particles (Fig. S2) and CH2(COONa)2/AS particles (Fig. S3)

361

with molar ratios of 3:1, 1:1 and 1:3 during dehydration - hydration processes, respectively; the

362

Raman spectra of mixed CH2(COONa)2/AS particles with molar ratios of 3:1, 1:1 and 1:3, as well

363

as that of CH2(COOH)2, NaOOCCH2COOH, CH2(COONa)2 and AS under dry conditions (Fig. S4);

Similarly, the water-soluble organic salts may induce the ammonium depletion of (NH4)2SO4



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and the FTIR spectra of mixed (CH2COONa)2/AS particles with molar ratios of 3:1, 1:1 and 1:3

365

during dehydration - hydration processes (Fig. S5); and the water uptake by HOOC(CH2)2COONa

366

aerosol particles as a function of RH upon hydration and the water content of HOOC(CH2)2COONa

367

and (CH2COONa)2 at 80% RH (Fig. S6); and the differential spectra of (CH2COONa)2/AS (1:1)

368

mixed aerosols upon dehydration and the integrated areas of differential spectra for COOH, -COO-

369

and NH4+ as a function of RH upon dehydration (Fig. S7); Calculation method for water-to-solute

370

ratios (WSRs) of AS, Na2C2O2, CH2(COONa)2 and (CH2COONa)2 aerosol particles; ATR-FTIR

371

spectra of AS solutions with various WSR and a plot on the basis of the Beer-Lambert law used for

372

calculation of the integrated cross section ratio of σ (solute) / σ (OH) (Fig. S8); Simulation method

373

for pH and composition estimation dependent upon NH4+ depletion in sodium dicarboxylate

374

/ammonium sulfate mixtures.

375

Acknowledgment

376

We gratefully appreciate financial support from the National Natural Science Foundation of China

377

(Nos. 91544223, 91644101 and 41875144) and the Ministry of Science and Technology of China

378

(No.2016YFC0203000).

379

References

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(49) Tang, I. N.; Fung, K. H.; Imre, D. G.; Munkelwitz, H. R. Phase-transformation and metastability

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of hygroscopic microparticles. Aerosol Sci. Technol. 1995, 23, (3), 443-453.

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(50) Peng, C.; Chan, M. N.; Chan, C. K. The hygroscopic properties of dicarboxylic and

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multifunctional acids: Measurements and unifac predictions. Environ. Sci. Technol. 2001, 35, (22),

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4495-4501.

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(51) Parsons, M. T.; Mak, J.; Lipetz, S. R.; Bertram, A. K. Deliquescence of malonic, succinic,

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glutaric, and adipic acid particles. J. Geophys. Res.-Atmos. 2004, 109, (D6), D06212.

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(52) Ling, T. Y.; Chan, C. K. Partial crystallization and deliquescence of particles containing

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ammonium sulfate and dicarboxylic acids. J. Geophys. Res.-Atmos. 2008, 113, (D14), D14205.

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(53) Silvern, R. F.; Jacob, D. J.; Kim, P. S.; Marais, E. A.; Turner, J. R.; Campuzano-Jost, P.;

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Jimenez, J. L. Inconsistency of ammonium-sulfate aerosol ratios with thermodynamic models in the

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eastern us: A possible role of organic aerosol. Atmos. Chem. Phys. 2017, 17, (8), 5107-5118.

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(54) Nah, T.; Guo, H.; Sullivan, A. P.; Chen, Y.; Tanner, D. J.; Nenes, A.; Russell, A.; Ng, N. L.;

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Huey, L. G.; Weber, R. J. Characterization of aerosol composition, aerosol acidity, and organic acid

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partitioning at an agriculturally intensive rural southeastern US site. Atmos. Chem. Phys. 2018, 18, 23 ACS Paragon Plus Environment


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(15), 11471-11491.

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(55) Guo, H.; Nenes, A.; Weber, R. J. The underappreciated role of nonvolatile cations in aerosol

540

ammonium-sulfate molar ratios. Atmos. Chem. Phys. 2018, 18, (23), 17307-17323.

541 542


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543

544

545 546

Fig. 1 Infrared spectra of Na2C2O4 (a), CH2(COONa)2 (c) and (CH2COONa)2 (e) particles upon

547

dehydration. Water-to-solute molar ratio (WSR) of Na2C2O4 (b), CH2(COONa)2 (d) and



Page 26 of 31

548

(CH2COONa)2 (f) particles as a function of RH during a dehydration/hydration process. Data from

549

Peng et al. (ref 28) (orange and blue cross) are also included for comparison.


Page 27 of 31


550 551

Fig. 2 (a) The spectra collected at RH less than 3% for mixed Na2C2O4/AS aerosols with molar

552

mixing ratios of 3:1, 1:1 and 1:3, along with the spectra of pure H2C2O4, Na2SO4, Na2C2O4 and AS.

553

Water content of mixed Na2C2O4/AS aerosols with molar ratios of 3:1 (b), 1:1 (c) and 1:3 (d) as a

554

function of RH during a dehydration/hydration process. The ν1(SO42-) band shifts corresponding to

555

the RHs of efflorescence and deliquescence transition are also included.



Page 28 of 31

556 557

Fig. 3 (a) The spectra collected at RH less than 3% for mixed CH2(COONa)2/AS aerosols with molar

558

mixing ratios of 3:1, 1:1 and 1:3, along with the spectra of pure NaOOCCH2COOH, CH2(COOH)2,

559

Na2SO4, CH2(COONa)2 and AS. Water content of mixed CH2(COONa)2/AS aerosols with molar

560

ratios of 3:1 (b), 1:1 (c) and 1:3 (d) as a function of RH during a dehydration/hydration process. The

561

ν1(SO42-) band shifts corresponding to the RHs of efflorescence and deliquescence transition are also

562

included.

563


Page 29 of 31


564 565

Fig. 4 (a) The spectra collected at RH less than 3% for mixed (CH2COONa)2/AS aerosols with molar

566

mixing ratios of 3:1, 1:1 and 1:3, along with the spectra of pure NaOOC(CH2)2COOH, (CH2COOH)2,

567

Na2SO4, (CH2COONa)2 and AS. Water content of mixed (CH2COONa)2/AS aerosols with molar

568

ratios of 3:1 (b), 1:1 (c)and 1:3 (d) as a function of RH during a dehydration/hydration process. The

569

ν1(SO42-) band shifts corresponding to the RHs of efflorescence and deliquescence transition are also

570

included.

571 572 573



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574 575

Fig. 5 The dependence of pH and organic acid fraction on the NH4+ depletion for the three mixed

576

solutions. Here the NH4+ depletion is denoted by the ratio of depleted [NH4+] (CNH4+ – [NH4+]) to

577

the initial concentration of NH4+ ions (CNH4+ = 0.2 mol/L). The organic acid fraction refers to the

578

fraction of conversion from -OOC(CH)nCOO- (organic salt, n = 0, 1, 2) to HOOC(CH)nCOOH

579

(organic acid, n = 0, 1, 2), which is indicated by the ratio of the equilibrium concentration [H2A] to

580

the initial concentration of organic anions.


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Table of contents

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Hygroscopicity and Compositional evolution of Atmospheric Aerosols

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