The Role of pH in Aerosol Processes and Measurement Challenges

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Feature Article

The Role of pH in Aerosol Processes and Measurement Challenges Miriam Arak Freedman, Emily-Jean E. Ott, and Katherine E. Marak J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b10676 • Publication Date (Web): 26 Dec 2018 Downloaded from http://pubs.acs.org on December 27, 2018

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The Journal of Physical Chemistry

The Role of pH in Aerosol Processes and Measurement Challenges Miriam Arak Freedman*, Emily-Jean E. Ott, Katherine E. Marak Department of Chemistry, The Pennsylvania State University, University Park, PA 16802

Revised Submitted to the Journal of Physical Chemistry A Manuscript Type: Feature Article December 21, 2018

* To whom all correspondence should be addressed: [email protected], 814-867-4267

Abstract pH is one of the most basic chemical properties of aqueous solution, but its measurement in nanoscale aerosol particles presents many challenges. pH of aerosol particles is of growing interest in the atmospheric chemistry community because of its demonstrated effects on heterogeneous chemistry and human health, as well as potential effects on climate. The authors have shown that phase transitions of aerosol particles are sensitive to pH, focusing on systems that undergo liquidliquid phase separation. Currently, aerosol pH is calculated indirectly from knowledge of species present in the gas and aerosol phases through the use of thermodynamic models. From these models, ambient aerosol is expected to be highly acidic (pH ~ 0 – 3). Direct measurements have focused on model systems due to the difficulty of this measurement. This area is one in which physical chemists should be encouraged to contribute because of the potential consequences for aerosol processes in the environment.

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A. Aerosol Composition and Acidity Atmospheric aerosol consists of solid or liquid particles suspended in air. These particles are ubiquitous and have important consequences for human health and climate. In particular, particles < 2.5 µm in diameter can be inhaled into the alveolar region of the lungs, and the concentration of these particulates is directly correlated with mortality rates.1,2 Particles < 100 nm in diameter can cross from the lungs into the blood stream, where they can potentially affect other organs at the cellular level.3 In addition to their health effects, atmospheric particles affect climate through their interactions with light, through heterogeneous chemistry, and through the nucleation of cloud droplets and ice particles.4 Aerosol-radiation and aerosol-cloud-radiation interactions are the least understood aspects of our climate system, in part due to the limited representation of aerosol particles in climate models.5 Atmospheric particles have diverse composition. The two largest emissions by mass are sea spray and mineral dust aerosol, but aerosol particles composed of organic compounds, sulfates, nitrates, soot, and biomass burning aerosol are also common.4 Organic compounds compose 2090 wt. % of particulate matter < 2.5 µm in diameter (PM2.5) in the troposphere.6 Other common species in organic aerosol include ammonium, nitrate, and sulfate.7,8 The organic compounds that are present depend on the location of the aerosol, and are often functionalized due to atmospheric oxidation processes to include alcohol, acid, ketone, and aldehyde groups.9 In urban environments, oxidized organic compounds may originate from compounds like toluene and xylene; and in rural environments, isoprene, mono- and sesquiterpenes.4 The acidity of aerosol particles composed of organic compounds and salts is an open question of great current interest. pH is defined as 𝑝𝐻 = − log 𝑎) *

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where a is the activity. One clue to the acidity is the state of sulfate in the aerosol particle, if sulfate is present. In general, concentrations of sulfate are declining due to regulation of emissions. As a consequence, the total ammonia/ammonium (present in gas and particulate phases) to sulfate ratio is increasing. If sulfate is fully neutralized to ammonium sulfate, then the pH will be higher than if sulfate is only partially neutralized, for example to an ammonium to sulfate ratio of 1.5 or 1. The degree of neutralization can be assessed using thermodynamic models and by determining the partitioning of semivolatile ammonia. A recent study that uses the ISORROPIA-II (meaning “equilibrium” in Greek) model suggests that until sulfate levels reach preindustrial concentrations, organic aerosol is expected to remain highly acidic (summertime pH = 0 - 2).10,11,12 In contrast, in a different thermodynamic model, the Extended Aerosol Inorganic Model (E-AIM), increasing the ammonia to sulfate ratio to > 2 results in a likely pH of 1 - 4.13–15 This example illustrates the discrepancy between different thermodynamic models, and one of the reasons why direct measurements are needed to refine these models. Below, we give a brief overview of the impact of aerosol pH on human health, heterogeneous chemistry, and aerosol effects on climate. Then, we review our work on the impact of pH on aerosol phase transitions. A short overview of models and their results is given. We describe the efforts that have been made to measure aerosol pH. Finally, we discuss open questions and challenges in this area.

B. Effect of pH on Health The acidity of aerosol can have important consequences for health, aerosol reactivity, and climate. Generally, effects on human health have been attributed to PM2.5 without further attribution to particle composition, but some studies have looked in more detail into composition and acidity. Kim et al. notes that various epidemiology studies show that not all particles are

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equally toxic, and the toxicity of PM2.5 may be due to a general increase in acidity with smaller size as well as an increased ability to penetrate the body.16 A modeling study by Lelieveld et al. shows that globally, carbonaceous aerosol is the most toxic aerosol, especially in regions of Asia with indoor cook stove use. Mineral dust aerosol is also important especially in Africa.17 Agriculture is a large contribution to aerosol globally, and power generation is important in some locations and is a large contributor in the USA. These two sources result in the production of sulfate, nitrate, ammonium sulfate, and ammonium nitrate aerosol.17 The mechanism of toxicity of aerosol particles is oxidative stress in lung epithelial cells caused by certain organic components of aerosol particles as well as metals.18 Acidity has been hypothesized to play a role in part due to acidity-driven metal dissolution.19 Additionally, secondary organic aerosol (SOA) particles are prevalent in the atmosphere. Atmospheric processing of SOA leads to higher concentrations of organic acids which have been demonstrated to have more adverse effects on lung epithelial cells than fresh SOA. 20 Several studies have related acidity with particle toxicity (Fig. 1).

For example,

summertime respiratory hospital emissions are correlated with acidity.21 Aerosol acidity was also correlated to respiratory hospital emissions and respiratory mortality over a 2.5 year period in Buffalo, New York.22 A study performed with rats showed significantly more damage to rat lung parenchyma when exercising animals were exposed to acidic aerosol (sulfuric or nitric acid) and ozone than would be caused by ozone alone.23 In addition, decreased pulmonary function by way of reduced forced vital capacity of the lungs (FVC) in children is associated with higher acidity particles.24 Urban regions have been shown to have systematically more acidic aerosols than rural areas, which could exasperate negative health effects of aerosols in cities.25 A study performed on residents of the greater Boston area found participants with diabetes were vulnerable to particles from traffic and coal-burning demonstrated by their susceptibility to decreased flow-mediated and

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nitroglycerine-mediated vascular activity with increases in sulfate concentration.26 It has also been shown that during haze events in Seoul, South Korea aerosol composition is more acidic than clear days, and in Beijing and Gucheng, China there is evidence of heterogeneous reactions forming SO42– and NO3– during winter haze events which may play a role in the negative health effects seen during these periods.27,28

Figure 1. Acidic aerosol can cause damage to the lungs through dissolution of metals, oxidative stress, and inflammation.

C. Effect of pH on Aerosol Reactivity In terms of reactivity, the mass yield of products of reactions of organic compounds increases on acidic seeds due to the formation of high molecular weight species and oligomers.29– 33

This increased mass yield may also result in an altered size distribution of ambient particles

because more organic matter partitions to these acidic particles. Some experiments focus on the products formed when volatile organic compounds such as aldehydes are exposed to acidic seeds (for example, sulfuric acid with and without ammonium sulfate).32 Other experiments focus on

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the (photo)oxidation of different volatile organic species such as alpha-pinene ozonolysis, isoprene photooxidation, etc.29–31 When aldehydes react on acidic aerosol, high molecular weight species result due to polymerization and aldol condensation.32 These reactions often occur through the formation of an acetal or hemiacetal.34 The rates of reaction are accelerated and result in higher aerosol yields when an acidic seed is used.34 Both aldehydes and carbonyls can undergo acid-catalyzed reactions, but aldehydes are more reactive.35 Conjugation of the organic compound also results in higher aerosol yields.35 Potential acid-catalyzed reactions of aldehydes from Jang et al. are shown in Fig. 2.35 SCHEME1. Possible Acid-Catalyzed Reactions of an r,β-Unsaturated Aldehyde in the Particle Phase

n ation ability on th e oxygen of th e differen t carbon yl sp ecies. th e β-p osition of 2-cycloh exen on e, th e p KBH+ drop s by abou t As sh own in Sch em e 1, p roton ation on th e oxygen of R,β0.8 u n its (41). Fu rth er detail stu dies for su bstitu en t effects u n satu rated carbon yls leads to an allylic carbocation , wh ich on organ ic aerosol yields an d th e kin etic m ech an ism s related reson ates with a β-p osition carbocation . Con ju gation with to a rate-determ in in g step in acid-catalyzed reaction s are an in tern al olefin ic bon d in creases the basicity of the carbon yl n eeded. ACS Paragon du e to th e stability of th e p roton ated carbon yl (41). Table 2Plus Environment To con firm th e organ ic aerosol form ation in a redu ced also sh ows p KBH+ of th e carbon yls u sed in th is stu dy. Th e organ ic con cen tration an d a lon ger tim e scale (m in u tes) th an in th e flow reactor (secon ds), th e organ ic aerosols were also p KBH+ valu es of R,β-u n satu rated carbon yls vary accordin g to gen erated in th e 0.5-m 3 Teflon film bag in th e p resen ce of eith er th e len gth of th e con ju gation or th e su bstitu ted grou p s

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Fig. 2. Acid catalyzed reactions of a a,b-unsaturated aldehydes in the particle phase. Reprinted with permission from Jang, M.; Carroll, B.; Chandramouli, B.; Kamens, R. M. Particle Growth by Acid-Catalyzed Heterogeneous Reactions of Organic Carbonyls on Preexisting Aerosol, Environ. Sci. Technol. 2003, 37, 3828-3837. Copyright 2003 American Chemical Society. In experiments with SOA, the low molecular weight species formed in the oxidation reactions depend on the initial compound that is used. In general, the functional groups formed include carbonyl, hydroxyl, and carboxylic acid groups.

In acidic conditions, dimers and

oligomers can be formed through aldol condensation, gem-diol reactions, dehydration, and esterification. These reactions are catalyzed by acid in the particle phase (Fig. 3).29–31 In addition, sulfate esters have been detected when sulfuric acid is used as the catalyst.30 The addition of acid can result in an increase in particle phase organics by up to 40%.29,31 In working with multiple terpenes, Northcross and Jang find that the amount of SOA yield is proportional to the acidity.36 The results for SOA generated from aromatic compounds is mixed. The enhancement of yields from the dark oxidation (by OH) of toluene and 1,3,5-trimethylbenzene is similar to the oxidation of biogenic compounds.37 A different study found that acidic seeds provided no additional yield 5190 of

38 and emerging issues M. Hallquist et al.: SOA: current SOA for toluene, benzene, and m-xylene at high and low NOx concentrations.

Fig. 7. Possible chemical reaction pathways for the formation of oligomers and other higher-MW products observed in SOA.

Figure 3. Acid-catalyzed reaction pathways in the particle phase that can lead to the formation of molecular Reprinted from Hallquist, M.; et al. oxidation The formation, properties, Ithigh is reasonable to weight assume products. that the range of proposed formed from isoprene under high-NO x conditions

oligomeric species and reaction pathways listed in Table 8 (Szmigielski et al., 2007a). Obviously, the explicit identiare a direct result of the wide range of VOC oxidation profication of the other suggested products of acid-catalyzed ducts generated from the different SOA precursors. How- 7 chemistry (e.g., aldol condensation products) would be exACS Paragon Plus Environment ever, the experimental conditions also play a major role in tremely valuable for the final evaluation of these SOA forinfluencing the chemical pathways. For example, Surratt et mation pathways but again appropriate reference compounds al. (2006) showed that SOA generated from the photooxidaare lacking. Another group of SOA related accretion reaction


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and impact of secondary organic aerosol: current and emerging issues, Atmos. Chem. Phys. 2009, 9, 5155-5236 under the Creative Commons Attribution 3.0 License. Acidic reactions can also affect inorganic atmospheric compounds. For example, the deposition of transported mineral dust is thought to deliver iron to ocean organisms, and thus be important for biogeochemistry (Fig. 4). Iron is leached from minerals during their exposure to acid, but the amount of iron, its oxidation state, and the rate of dissolution depends on the minerology of the dust, the acidity, and the identity and amount of the acid.39,40 For example, iron is dissolved more rapidly when it is present in the form of a poorly crystallized oxyhydroxide rather than in a crystal lattice such as hematite or goethite.40

In contrast to sulfuric and

hydrochloric acid, nitric acid suppresses the leaching of iron due to the formation of nitrates on the mineral surface.39 The presence of minerals that do not contain iron, such as TiO2, mixed with iron containing minerals can increase or decrease the dissolution of the iron depending on the type of mineral and environmental conditions such as the amount of light.41 Another feature of iron containing aerosol that is important is the origin of the aerosol because different regions have different sources of iron and different aerosol pH values.42 Regional and aging information is being added to models to better understand dust aerosols and iron solubility.43,44

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Figure 4. Heterogeneous chemistry that occurs during the atmospheric transport of mineral dust can impact the amount of bioavailable iron in the oceans.

Phosphorous is an important nutrient for both land and water ecosystems that works synergistically with nitrogen.45 Phosphorous can be found in mineral dust, but its availability to act as a nutrient is impacted by its solubility. Phosphorous in Saharan dust can be solubilized by atmospheric acidification, with other factors such as SO2 and NOx influencing the solubility.46 Halogens in the atmosphere influence methane and ozone cycles as well as aerosol particle chemistry because they are good oxidizers for tropospheric organic and inorganic compounds.47 Marine aerosol often contains a mixture of chlorine, bromine, and iodine, and the formation of Cl2, Br2, I2, and combinations of the halogens (e.g. BrCl), oxidized forms (e.g. ClO), and radicals are impacted by a combination of factors such as precursors ions, pH, radiation, ozone, and oxidants.48,49 Chlorine radicals in the atmosphere are more common in acidic particles because they can be released from sea salt through acid displacement.50,51 Cl2 can be produced via a reaction 9



pathway with N2O5 in liquid aerosol particles, which requires a pH less than 2.52 In particles with a pH less than 3, gas phase Br2 can form through a reaction of the Br in sea salt with OH in the atmosphere.53 I2 is often produced before Br2 and Cl2 but the addition of ozone can prevent I2 formation while enhancing that of Br2.49 While Br2 must have a low pH for formation, I2 can be formed at higher pH values.49 In addition, the presence of weak acids in particles can enhance Ireactive species over an increased pH range, while the presence of compounds such as phenols can inhibit their production even at ideal pH.54,55 Another pathway for I- formation is known where the presence of Fe2+ greatly enhances formation in aqueous aerosols.56 The primary source of SO42- in the atmosphere is the oxidation of SO2, and this process is impacted by acidity. There are multiple pathways for the oxidation of SO2. It can occur in cloud water containing mineral dust, which has a lower pH leading to more dissolved SO2 and higher rates of oxidation.57 It can also occur by oxidation of NO2 on fine aerosols, but this reaction works best when the pH is around 7 and RH > 60 % which are conditions that can be found in fog/cloud aerosols.58 Another pathway is the interfacial surface reaction of SO2 oxidation that occurs when the pH is less than 4.59,60

D. Effect of pH on Climate To assess the impact of acidity on the climate effects of aerosol particles, we need to determine how acidity affects the light scattering properties of particles as well as their interactions with clouds. Water soluble organic carbon influences aerosol optical properties and varying pH and light exposure can change the absorption spectra of particles containing water soluble organic carbon causing both enhancement and reduction of absorption under different conditions.61 Water soluble organic carbon can also undergo changes due to long-range transport, including exposure to acids, which can influence their abilities to absorb and scatter light.62 pH and phase can change

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aerosol optical properties due to deprotonation/protonation of different organic species in the atmosphere.63 One example is the absorption spectra of brown carbon samples, which tend to shift towards longer wavelengths and stronger absorbances as the pH of the sample increases.64 The ability of particles to become or influence clouds can also be impacted by pH. Because a larger number of higher molecular weight species are formed when acidic seeds are used, the cloud condensation nuclei activity may be altered. For example, recent studies have shown that secondary organic aerosol without salt can undergo phase separation at high relative humidities (> 90%), where less soluble components partition to the air-aerosol interface.65–67 This phase separation has been hypothesized to result in a reduction of surface tension of the activating droplets. As a result, these droplets activate more readily, leading to a larger number of cloud condensation nuclei.68,69 The formation of more high molecular weight species in acid-catalyzed reactions would result in additional material partitioning to the surface, which may result in the reduction of surface tension and increased activation. Particle acidity can also change the species of the sulfate salt in aerosol particles, and hygroscopicity depends sensitively on the salt (sulfate species) which can impact cloud condensation nuclei formation.13,70 In addition, the gas-particle partitioning of water-soluble ionizable compounds depend on pH because low pH tends to push gases towards their protonated and more volatile states.13,71 Increased partitioning can cause a decrease in refractive index when particles are composed solely of organic or when composed of organic with an inorganic seed.72

E. Phase Transitions and pH Further complicating aerosol pH is the fact that aerosol particles are exposed to a wide range of relative humidities (RHs) in the course of a diurnal cycle. As aerosol particles take up and release water, their pH changes. They may additionally go through phase transitions. Four

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different phase transitions are of interest for aerosol particles composed of organic compounds and salts: efflorescence, deliquescence, phase separation and mixing.73,74 Efflorescence refers to the crystallization of aqueous aerosol, deliquescence refers to water uptake of a solid aerosol, phase separation refers to demixing of a particle into two liquid phases, and mixing refers to the transition from two liquid states to one liquid state. Not all of these phase transitions are observed for all systems. Note also that phase separation of complex organic mixtures without salt can also take place at high relative humidities due to the different solubilities of the components.65,66 pH has a strong effect on liquid-liquid phase separation and other phase transitions. Using optical microscopy, we explored the effect of pH in droplets with diameters of tens to hundreds of micrometers.75,76 Droplets were composed of an organic compound (2-methylglutaric acid, 3methylglutaric

acid,

2,2-bis(hydroxymethyl)butyric

acid,

3,3-dimethylglutaric

acid,

diethylmalonic acid, or 1,2,6-hexanetriol) mixed with ammonium sulfate. To raise the pH, sodium hydroxide was added to the system; to lower it, sulfuric acid was added. Droplets were ramped slowly (< 1 % RH/min) through different relative humidity conditions to determine the position of the following phase transitions: efflorescence, deliquescence, phase separation, and mixing. Control studies were run to investigate the effect of the changes in particle composition and ionic strength. As the pH is raised, the protonation state of the organic acid changes. The pH at which the organic compound is deprotonated depends on its pKa. The amount of the organic compound in each protonation state depends on the difference between the pH of the solution and the pKa. In studies in which the pH was raised, we used 3-methylglutaric acid, which has pKa1 = 4.24 and pKa2 = 5.41.

Therefore, this compound is predominately doubly protonated at pH 3.65, singly

protonated at pH 5.17, and doubly deprotonated at pH 6.45, the pH values used in the study. Deprotonated organic acids have increased solubility in salt solutions due to increased ion-ion

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interactions. As a result, they are not as readily salted out of solution, which results in a lower separation relative humidity (SRH; Fig. 5).75 Between a pH of 3.65 and a pH of 6.45, the SRH decreases 14.8 % RH.75 As the pH is lowered, the ammonium:sulfate ratio in solution changes. Over the pH range used in our experiments, the ammonium:sulfate ratio was 2:1, 1.5:1, 1:1, and < 1:1. As the pH is lowered, the dominant sulfate species changes from sulfate to bisulfate. Bisulfate is not as effective of a salting out agent as sulfate, and as a result, SRH decreases with decreasing pH (Fig. 5).76

For organic compounds that have an SRH with ammonium sulfate of > 90% (3,3-

dimethylglutaric acid and diethylmalonic acid), the percentage decrease as the ammonium:sulfate ratio is changed is less than for organic compounds with lower SRH. For 3-methylglutaric acid, no phase separation is observed at a pH of 0.35, and 2,2,-bis(hydroxymethyl)butyric acid and 1,2,6-hexanetriol have SRH at < 5 %.76

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a)

b)

Figure 5. The dependence of the separation relative humidity (SRH) on pH. a) SRH for 3methylglutaric acid and ammonium sulfate at pH 2.68 and for 3-methylglutaric acid/ammonium sulfate/sulfuric acid at lower pH and 3-methylglutaric acid/ammonium sulfate/sodium hydroxide at higher pH. At pH = 0.35, no phase separation is observed. b) SRH for several organic compounds mixed with ammonium sulfate. To reach lower pH, sulfuric acid is added. The bounds of pH for different ammonium to sulfate ratios (ASR) are indicated by the dashed lines. Adapted with permission from Losey, D. J.; Ott, E.-J. E.; Freedman, M. A. Effects of High Acidity on Phase Transitions of Organic Aerosol, J. Phys. Chem. A 2018, 122, 3819-3828. Copyright 2018 American Chemical Society.

pH also affects the position of other aerosol phase transitions. Most notably, with the addition of sodium hydroxide to the 3-methylglutaric acid/ammonium sulfate system, the mixing

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relative humidity (MRH) is found to differ from SRH, resulting in a hysteresis between these two phase transitions.75 Such a hysteresis is expected because SRH generally requires the formation of a nucleus, whereas MRH does not. This reasoning also explains the hysteresis observed between the efflorescence relative humidity (ERH) and the deliquescence relative humidity (DRH).

As a result, SRH is kinetically controlled and MRH is determined solely by

thermodynamics. Generally, however, SRH is found to be the same as MRH. 3-Methylglutaric acid/ammonium sulfate/sodium hydroxide is the only system in which a hysteresis has been observed.75 pH affects the positions of ERH and DRH as well, but the effect is not as well understood.75,76 Based on the values at which the phase transitions occur, ERH is sometimes driven by the crystallization of the inorganic salt and sometimes hypothesized to be controlled by the initial crystallization of the organic acid.77

For the 3-methylglutaric acid/ammonium

sulfate/sodium hydroxide system, ERH and DRH both increase with increasing pH.75 For the systems at low pH, ERH increases with decreasing pH for 3,3-dimethylglutaric acid and diethylmalonic acid, and decreases for the other systems. For 2-methylglutaric acid and 3methylglutaric acid, ERH is not present at a pH of 0.35. In the systems in which DRH can be measured (2-methylglutaric acid, 3-methylglutaric acid, 2,2-bis(hydroxymethyl)butyric acid, and 1,2,6-hexanetriol), it decreases with decreasing pH.76 This behavior and the underlying reasons for it are unclear and should be explored in more detail. F. Measurement of pH Many techniques have been used to measure the pH of ambient aerosol indirectly, some of which were recently detailed in a review by Hennigan et al.78

The best methods use

thermodynamic models, such as E-AIM, ISORROPIA-II, and the Aerosol Inorganic-Organic Functional

groups

Activity

Coefficients

(AIOMFAC)

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model,79,80

which

incorporate


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concentrations of molecules and ions present in the gas and particle in gas, liquid, and solid phases. Each model minimizes the Gibbs energy of the system, but each in a different way. ISORROPIAII includes no organic compounds, E-AIM includes a few organic compounds (though users can add their own), and AIOMFAC considers many organic compounds.

An ongoing current

discussion in the literature is the comparison of the models for the same data sets. The reader is referred to Murphy et al. for a discussion of the comparison of E-AIM to ISORROPIA-II,13 Pye et al. for a comparison of ISORROPIA-II and AIOMFAC,81 and Jia et al. for a comparison of the three models.82 Measurements of aerosol pH that have used thermodynamic models (most commonly, ISORROPIA-II and E-AIM) and have been employed internationally. In these studies, aerosol particles are found to be highly acidic with pH from -2.5 to 2.8.71,83–86 A size dependence is sometimes associated with pH measurements, though this can either decrease pH or increase pH, depending on the regional influences on the aerosol composition.71,84 Diurnal variations are found due to water content, with the lower daytime water content having the effect of lowering pH.85 Trends can also change in the same location between winter and summer due to regional aerosol sources, changes in composition, and changes in water content.85 Even when levels of NH4 are very high such as in Beijing and Xian, aerosol pH remains acidic (pH=4.5-5).86

G. Direct Measurement of pH The only direct measurements of pH have focused on idealized systems. Dallemagne et al. used a fluorescent probe in particles composed of poly(ethylene glycol) 400 and ammonium sulfate.87 Using the ratio of the fluorescence intensity at two different wavelengths, pH changes over a two-unit range can be sensitively measured. The fluorescent probe partitioned to the organic phase. For this system, the pH differed by less than 0.5 between the well-mixed system at high

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relative humidity and the organic-rich phase upon liquid-liquid phase separation. Changes in relative humidity for the phase separated system resulted in small changes of pH.87 Rindelaub et al. used the approach of measuring the integrated intensity of Raman features due to sulfate and bisulfate for MgSO4 – H2SO4 aerosol particles, combined with thermodynamic calculations to obtain pH.88 Because this system can only be used in a narrow pH range, Craig et al. expanded the number of model systems to include model aerosol particles composed of additional combinations of salts and acids (NaNO3/HNO3, (NH4)2C2O4/HCl, NaCH3COO/HCl, Na2CO3/HCl, (NH4)2SO4/(NH4)2C2O4/H2SO4) to investigate pH over a wider range of systems and values using the same method.89 Craig et al. used pH paper to determine the pH of model aerosol composed of ammonium sulfate and sulfuric acid in the range of 0-4.5 (Fig. 6).90 This work built on previous studies that have used pH paper to measure the pH of aerosol particles. By using Raman spectroscopy, they were able to calibrate the pH paper to the pH of the aerosol particles.90 The particles were maintained at ~90% RH to ensure sufficient water content. Ambient samples were also used on days with relative humidities at or above 60%, but here the results are not as precise due to the heterogeneity of the samples collected.90 Finally, Wei et al. used surface enhanced Raman spectroscopy (SERS) to measure the pH of 20 µm droplets with phosphate buffer.91 They functionalized Au nanoparticle dimers with a 4mercaptobenzoic acid pH indicator. The junction between the nanoparticles provides the enhanced Raman signal.

Because many of these nanoprobes were in each droplet, confocal Raman

microscopy was used to map the pH within the particle at pH values around the pKa of the indicator. Wei et al. found that the droplets are more acidic at the air-interface and hydrophobic substrate compared with the interior by approximately 3 pH units.91 This difference in pH across the particle was attributed to the migration of protons to the interface of the droplet.91

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Figure 6. Schematic of the measurement of aerosol pH using pH paper. Reprinted with permission from Craig, R. L.; Peterson, P. K.; Nandy, L.; Lei, Z.; Hossain, M. A.; Camerena, S.; Dodson, R. A.; Cook, R. D.; Dutcher, C. S.; Ault, A. P. Direct Determination of Aerosol pH: Size-Resolved Measurements of Submicrometer and Supermicrometer Aqueous Particles, Anal. Chem. 2018, 90, 11232-11239. Copyright 2018 American Chemical Society.

G. Conclusion and Future Directions In the atmosphere, pH controls which reactions occur in aqueous particles, which determines the composition of these particles. Properties of aerosol particles such as phase transitions are highly sensitive to pH. Composition and physical properties of aerosol particles determine their optical properties and interactions with water vapor, and as a result, impact climate. In addition, acidity impacts the toxicity of the particles. In particular, highly acidic particles result in more irritation of the lung tissue, resulting in a higher incidence of disease. Challenges that remain include: 1) Understanding the detailed chemistry that is affected when aerosol reactions occur in the 0-3 pH range and determining what effect this difference in composition will have on aerosol volatility and viscosity. 2) Characterizing the mechanisms behind the differences observed in deliquescence and efflorescence at low pH. A wider variety of systems should be used to investigate the full O:C

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range for liquid-liquid phase separation at low pH. The consequences of low pH on the phase transitions and phase state of aerosol particles composed of complex organic mixtures with and without salt should be characterized. pH is among the most fundamental of chemical properties of aqueous solutions, taught to schoolchildren by the changes in the color of solutions made from purple cabbage leaves. Who could imagine the complexity of its measurement in aerosol particles? Currently, pH is calculated for ambient particles based on the composition of the gas and aerosol phase using thermodynamic models. Direct measurements have been performed only on model systems using methods that would be difficult to transfer to ambient particles (with the possible exception of the use of pH paper). Several challenges compound the direct measurement of aerosol pH: 1) In small particles, only a limited number of protons are available to be measured. For example, at pH 3, there are only 3.2 x 102 H+ in a 100 nm particle. By pH 0, this number rises to 3.2 x 105 H+. Any measurement technique will rely on having some baseline measure of the number of H+ ions. Developing a method to measure the pH of individual particles at these small sizes will be challenging. 2) Aerosol particles continually take up and release water as the ambient relative humidity changes. Measuring the pH of aerosol particles with limited water content has similar challenges to measurement of pH in small particles. 3) Measured aerosol pH is likely to be affected by complex mixtures and the high ionic strengths found in ambient particles. It is unclear how much of a measurement challenge this will pose.

Because of the importance of pH for aerosol processes in the environment and the challenges posed to such a measurement, this area is one that warrants the attention of the physical chemistry

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community. This community may have innovative methods for tackling these problems both from experimental and theoretical perspectives.

Author Information Corresponding Author *E-mail: [email protected] *Phone: 814-867-4267 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. Acknowledgement The authors are grateful for support from the US DOE (DE SC0018032) and NSF (AGS1723290). Specifically, E.-J. E. O. was funded through this DOE grant and K. E. M. was funded through this NSF grant during the preparation of the manuscript.

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TOC Graphic: Models: ISORROPIA-II E-AIM Direct Measurement AIOMFAC pH Measurement

Fluorescence SERS

Climate

Acidic Aerosol

Health

Reactivity

Phase Separation

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BIOS

Katherine E. Marak is a second year Ph. D. student at The Pennsylvania State University in the Department of Chemistry. She received her B.A. in Chemistry from Drew University in Madison, NJ. Her research interests are the physical properties of aerosol particles and her current work focuses on factors affecting the ice nucleation activity of model aerosols.

Emily-Jean E. Ott is a third year Ph. D student at the Pennsylvania State University in the Department of Chemistry. She received a B.S. in Chemistry and Mathematics at Andrews University in Berrien Springs, MI. Her research focuses on characterization of aerosol phase transitions by microscopy.

Miriam Arak Freedman is an Associate Professor of Chemistry at the Pennsylvania State University. She obtained a B. A. at Swarthmore College, an M. S. in Mathematics from the University of Minnesota, and an M. S. and Ph. D. in Chemistry from the University of Chicago. She performed postdoctoral research at the University of Colorado. Her research focuses on problems at the interface of physical and atmospheric chemistry. http://research.chem.psu.edu/mafgroup

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The Role of pH in Aerosol Processes and Measurement Challenges

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