Effects of High Acidity on Phase Transitions of Organic Aerosol

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A: Environmental, Combustion, and Atmospheric Chemistry; Aerosol Processes, Geochemistry, and Astrochemistry

Effects of High Acidity on Phase Transitions of Organic Aerosol Delanie J. Losey, Emily-Jean E. Ott, and Miriam Arak Freedman J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b00399 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 26, 2018

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The Journal of Physical Chemistry

Effects of High Acidity on Phase Transitions of Organic Aerosol Delanie J. Losey, Emily-Jean E. Ott, and Miriam Arak Freedman* Department of Chemistry, The Pennsylvania State University, University Park, PA 16802 Revised Manuscript Submitted to the Journal of Physical Chemistry A March 16, 2018 ABSTRACT Aerosol particle morphology influences the effect of particles on climate. Recent studies have documented the high acidity found in many ambient aerosol particles. The effect of this acidity on phase transitions of mixed organic/inorganic aerosol particles has not been addressed. To investigate this effect, six organic compounds and ammonium sulfate are investigated individually with varying amounts of sulfuric acid to determine the role of low pH on the separation, efflorescence, and deliquescence transitions. All phase transitions were affected by the addition of sulfuric acid. This effect was attributed primarily to the change in the identity of the inorganic component as the ammonium:sulfate ratio (ASR) was changed from two to one and a half to one. The separation relative humidity (SRH) decreased with increasing amounts of sulfuric acid for each system studied, with the largest effect seen in compounds that have a lower SRH when mixed with ammonium sulfate. Control studies without an inorganic salt revealed that for some systems, phase separation occurs for mixtures of sulfuric acid and an organic acid.

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Overall, it was found that for aerosol particles at low pH (≤ 0.35) composed of organic acids and inorganic salts, phase separation can be impeded in some cases. Introduction Aerosol particles can affect the environment directly by scattering or absorbing light in the atmosphere and indirectly by serving as cloud condensation nuclei. Because the scattering, absorption, and cloud condensation properties of aerosol particles are greatly affected by the physical state of the particle, it is important to know in which phase the particles are present in the atmosphere. Some aerosol particles exhibit a deliquescence relative humidity (DRH), an 1

efflorescence relative humidity (ERH), a phase separation relative humidity (SRH), and a mixing relative humidity (MRH), which are affected by the composition of the particle. Phase separation can influence the optical properties of the aerosol particle, the heterogeneous chemistry in which the particle participates in the atmosphere, and evaporation and condensation rates.2-11 One factor that could impact the phase transitions of aerosol particles is the aerosol pH, which is influenced by particle composition, size, and location. Studies of fine particles (< 2.5 µm) have determined pH ranges from as low as -0.51 and up to 3.1 depending on the season and location.

12-14

Some events such as haze or biomass burning can cause aerosol pH to be as high as 3.0-4.9.

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These pH measurements have been shown to be very sensitive to particle size as well.

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The

influence of this low pH on phase transitions has not yet been explored. The O:C ratio of the organic component is often used to predict if a system is capable of phase separating, while the relative humidity at which the phase transition occurs depends on the composition including the identity of the inorganic salt and the solution pH.

17-18

The salting-out

ability of different inorganic salts, as predicted by the Hofmeister Series, has been shown to directly correlate to the SRH in aerosol particles. In You et al., the SRH trend was as follows: 18


2

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(NH ) SO ≥ NH HSO ≥ NaCl ≥ NH NO , which is consistent with the Hofmeister series where 4 2

4

4

4

4

3

the salt with the highest salting-out ability has the highest SRH. A previous study from our 19

group explored the effect of high pH (3.65-6.45) on SRH. This study found that when the 17

organic component is deprotonated at high pH, the SRH decreases due to the increased solubility of the organic compound. Ammonium sulfate aerosol is ubiquitous in the continental atmosphere due to its industrial and agricultural emission sources.

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The neutralization state of ammonium sulfate aerosol is

dependent on multiple factors, including temperature, ambient gas concentrations (i.e. NH and 3

SO ), and particle size. Consequently, the neutralization of sulfate in ammonium sulfate particles 2

can vary greatly over a given region. (NH ) H(SO ) (called letovicite if in crystalline form), and 21

4 3

4 2

ammonium bisulfate, NH HSO , have also been shown to be present in high amounts.

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4

4

(NH ) H(SO ) is an intermediate form between ammonium sulfate and ammonium bisulfate due 4 3

4 2

to its intermediate ammonium:sulfate ratio (ASR) of 1.5, verses 2.0 and 1.0 for the other compounds, respectively. (NH ) H(SO ) is commonly studied along with ammonium sulfate and 4 3

4 2

ammonium bisulfate as its efflorescence and deliquescence behavior can influence the behavior of ammonium sulfate and ammonium bisulfate if they are found in the same environment.

2, 27-34

It

has also been well studied as an ice nucleus for heterogeneous ice nucleation and as a component for homogeneous ice nucleation of cirrus clouds.

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While (NH ) H(SO ) and its occurrence in 4 3

4 2

atmospheric aerosol particles has been extensively studied, its role in the phase separation of aerosol particles has not yet been investigated. The pKa of bisulfate is 2.00, and as a result, the 42

low pH of atmospheric aerosol indicates that bisulfate should be the primary ion present. In addition, the volatilization of ammonia and/or organic coatings on inorganic particles have been predicted to prevent full neutralization.

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Weber et al. suggested that aerosol pH is insensitive to


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decreases in sulfate emissions due to the partitioning of ammonia between the gas and particle phase which effectively buffers the pH between 0 and 2. Likewise, Silvern et al. investigated 43

what prevents the uptake of ammonia onto aerosol particles. By implementing a kinetic barrier 21

to ammonia uptake into particles, the discrepancy between the particle-phase ammonium:sulfate ratio and observed gaseous ammonia concentrations could be minimized. The kinetic barrier 21

was thought to be due to an organic coating present on the particles, a consequence of liquidliquid phase separation.

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The pH of individual particles has been investigated previously using Raman spectroscopy and fluorescent dyes.

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Raman spectroscopy has been used to determine pH of simplified aerosol

particles by tracking acid-base equilibria of different systems such as sulfate/bisulfate, acetic acid/acetate, and nitric acid/nitrate combined with Debye-Hückel activity calculations, but these tools are still being developed and are currently only accurate for well-controlled systems.

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A

fluorescent pH dye has also been employed to track pH of the organic phase during a humidity cycle in an organic/inorganic system that undergoes phase separation. The experiments that 45

have tracked pH during humidity cycles have not observed more than a one pH unit difference during the cycle, and as a result, the pH of the initial system is most important when determining the effects of pH on phase separation.

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Many studies have explored the role of sulfuric acid in the heterogeneous chemistry and phase state of aerosol particles.

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Particle acidity in general has been well documented to affect

reactive uptake of gas species on particles and solar absorption for species such as nitrophenols.

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Sulfuric acid-containing aerosol particles have been studied to understand their effects on the

viscosity of the particles. These studies have found that sulfuric acid-containing aerosol particles become more viscous with decreasing temperatures and/or increased weight percent.

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This


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increase in viscosity with added sulfuric acid could inhibit crystallization or crystal growth and significantly delay water uptake of these particles.

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High molecular weight organic components

(up to 1153 Da) at low temperatures have been used to explore the effect of viscosity on phase separation by You et al., and it was found that viscosity could lower the SRH for certain systems, but there was no strong dependence.

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In this study, systems containing an organic component, ammonium sulfate, and sulfuric acid are investigated to determine the effect of sulfuric acid on phase transitions. Through the addition of sulfuric acid, the stoichiometric ratio between ammonium and sulfate can be manipulated to achieve ASR = 1.5 and ASR = 1. The effect of having excess sulfate is also investigated for systems with an atmospherically relevant low pH of 0.35. The organic compounds investigated were chosen to achieve a wide range of O:C ratios and a range of SRHs. They are 2-methylglutaric acid (O:C = 0.67), 3-methylglutaric acid (O:C = 0.67), 2,2Bis(hydroxymethyl)butyric acid (O:C = 0.67), 3,3-dimethylglutaric acid (O:C = 0.57), diethylmalonic acid (O:C = 0.57), and 1,2,6-hexanetriol (O:C = 0.50). Most of the organic compounds investigated in this study are carboxylic acids. Carboxylic acids are estimated to make up about 25% of non-methane hydrocarbons in the atmosphere and are common products of olefin oxidation or can be emitted directly during combustion.

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Alcohols can be directly

released from biogenic sources and have been identified as products of certain combustion processes.

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The relative humidities at which the phase transitions, ERH, DRH, SRH, and MRH,

occur were observed with optical microscopy.


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Experimental Methods Aerosol particles containing a variety of organic components and ammonium sulfate (EMD, GR ACS) were generated in the laboratory from an aqueous solution. The organic components investigated were 2-methylglutaric acid, 3-methylglutaric acid, 2,2-bis(hydroxymethyl)butyric acid, 3,3-dimethylglutaric acid, diethylmalonic acid, and 1,2,6-hexanetriol (Table 1). A 5.0 wt.% solution of an organic component and ammonium sulfate in a 1:1 ratio by mass was made in ultrapure water (HPLC grade, Fisher), except for 2,2-bis(hydroxymethyl)butyric acid which was in a 0.6:0.4 ratio of inorganic:organic to guarantee phase separation. This ratio was chosen because Bertram and coworkers found that an organic:sulfate mass ratio of less than 1 was required for phase separation on some compounds with O:C near 0.7. The identity of the 66

inorganic component was changed to ASR = 1 or ASR = 1.5 through addition of concentrated sulfuric acid to achieve an ammonium:sulfate stoichiometric ratio of 1:1 and 2:3, respectively. To mimic the low pH in aerosol particles that is expected from recent studies,

13-14, 43

additional

concentrated sulfuric acid was added to reach a pH of approximately 0.35 (ASR 4

H PO > HCl ≈ CH COOH, which mirrors what is expected based on the Hofmeister series. The 3

4

3

ERH of each system varies greatly with pH and is usually controlled by the crystallization of the organic or inorganic component. Several systems do not effloresce with high sulfuric acid content. This result may be due to the high viscosity expected of these particles, which can prevent crystallization. The DRH of each of the systems tends to decrease with increasing sulfuric acid, except for 3,3-dimethylglutaric acid and diethylmalonic acid which never exhibit complete deliquescence. Combined with the results from the high pH studies done previously, these results underline the importance of pH in studies of aerosol phase transitions. The pH and inorganic salts investigated here suggest that phase separation may not occur for certain systems, especially at low pH. In particular, in the ASR

Effects of High Acidity on Phase Transitions of Organic Aerosol

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