Uptake of Pyrene onto Fatty Acid Coated NaCl Aerosol Particles - The

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Uptake of Pyrene onto Fatty Acid Coated NaCl Aerosol Particles Ephraim Woods, Alexander W. Hull, and Megan L. Tigue J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b11023 • Publication Date (Web): 07 Dec 2016 Downloaded from http://pubs.acs.org on December 8, 2016

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The Journal of Physical Chemistry

Uptake of Pyrene onto Fatty Acid Coated NaCl Aerosol Particles Ephraim Woods III*, Alexander W. Hull, and Megan L. Tigue Department of Chemistry, Colgate University, Hamilton, NY 13346 [email protected]

Abstract

Photoelectric charging experiments monitor the uptake of pyrene onto NaCl aerosol particles coated with either oleic acid or myristic acid. In both cases, thin coatings produce a small net decrease in pyrene uptake. In the larger coverage limit, the uptake of the myristic acid coated particles remains nearly constant while the oleic acid coated particles exhibit greater uptake rates than the bare NaCl particles. Fitting the results with a multilayer kinetic model yields uptake rate coefficients as well as parameters that describe the distribution of organic molecules on the aerosol particle surface. The model accounts for the decrease in uptake associated with thin coatings of oleic acid through a concomitant reduction in surface area. The adsorption rate constants for the myristic and oleic acid coated surfaces are 50 and 80 times faster, respectively, than for NaCl. The desorption rates for pyrene on the fatty acid surfaces are faster, as well. For myristic acid coatings, the fast desorption (over 400 times the rate of desorption from NaCl) results in slower net adsorption, while for oleic acid (approximately 12 times the desorption rate 1 ACS Paragon Plus Environment


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from NaCl), the net uptake rate increases with coverage. The results also suggest that both myristic acid and oleic acid spread incompletely on the aerosol surfaces under the conditions of these experiments. In the optimized kinetic model, the fatty acids cover approximately 50% of the surface when the nominal coating thickness is approximately 6 nm. The surface is over 90% covered with a nominal coating thickness of 20 nm, which is approximately 10% of particle diameter in these experiments. Very thin oleic acid coatings reduce the surface area of particles consistent with the preferential coverage of highly corrugated or porous regions.

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1. Introduction

Field studies show that a large fraction of tropospheric aerosol particles are heterogeneous, with both inorganic and organic constituents.1-3 While the sources of these mixed particles are disparate, two common sources are sea spray aerosol and secondary organic aerosol (SOA) generated from inorganic seed particles. The morphology of these particles is fairly consistent for values of relative humidity (RH) below the efflorescence point: inorganic salts form the core of the particle and organic species, often fatty acids, are distributed on the exterior.2 At higher values of RH, where an aqueous phase may be present, the morphology of the organic coating depends on a number of factors, including the solubility of the organic fraction, the mixing ratio, and the surface and interfacial tensions.4-5 Because a high fraction of the organic content of aerosol is not water soluble,6 multiphase particles should be common in a wide variety of conditions. The consequences of this morphology for heterogeneous chemistry in the troposphere and the aerosol contribution to radiation balance are an active area of research. Aerosol particles with this core-shell structure differ in their behavior from the bare inorganic core particles in several ways. For example, the hygroscopic properties and activity as cloud condensation nuclei (CCN) of inorganic particles are altered by organic coatings.7-13 Some low solubility organics shift the deliquescence relative humidity of inorganic salt particles.12, 14 The optical properties may be complex, as well. The real refractive index of internally mixed ammonium sulfate and succinic acid particles is higher than for either of the two pure substances.15 SOA particles with black carbon cores show enhancement of light absorption.16-19 Mie core shell theory describes the optical properties of mixed phase particles well when there is a well-defined core-shell morphology.20 Organic coatings on particles also affect the uptake of reactive gases. Monolayer surfactant coatings on aqueous salt particles inhibit the reactive uptake of N2O5 by as much as a factor of four21-22 and decrease the reaction rate with ozone by nearly a factor of two.23 In some cases, the organic films or coatings themselves are susceptible to oxidation.24-25 Heterogeneous chemistry in the atmosphere can change the morphology of these coatings, leading to changes in hygroscopicity and reactivity. Reid and coworkers4 illustrate the morphology changes associated with the ozonolysis of oleic acid – sodium chloride mixtures, a similar composition to particles in this study.



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The research described here concerns the uptake of the polycyclic aromatic hydrocarbon (PAH), pyrene, onto NaCl particles coated with either oleic or myristic acid. For these experiments, we consider low RH conditions where the inorganic core is a solid. It builds upon our previous work, which considered only inorganic particles.26 These systems serve as models for non-reactive uptake. NaCl is the predominant component of sea spray aerosol, which is among the most abundant tropospheric aerosol. Fatty acids are similarly common constituents of aerosol, including sea spray27 and biomass burning28, and are popular in model systems for heterogeneous atmospheric chemistry4, 29-34 Further, PAHs are important pollutants whose gasparticle partitioning has health consequences.35 Finally, the low ionization potential of PAHs makes them convenient targets for our experimental methodology, which uses photoionization to detect adsorbed molecules. The ability to detect sub-monolayer concentrations of adsorbed PAH allows us to follow the initial uptake kinetics, where both the particle composition and surface morphology play a key role in the dynamics. The results address very basic questions about the factors that control uptake of semivolatile species by mixed phase particles, and they provide insight into the gas-particle partitioning of an important class of pollutant molecules.

2. Experimental Section

Figure 1 shows a schematic diagram of the experiment, which is similar to that of our previous study involving uncoated particles.26 An atomizer generates a 2.0 slpm flow of aerosol particles from an aqueous NaCl solution, and a 0.5 slpm portion of this flow enters a diffusion dryer, where the relative humidity (RH) drops below 20%, followed by a Po-210 static elimination device, which brings the flow into charge equilibrium. A differential mobility analyzer (DMA) operating with a sheath flow of 3.0 slpm size selects this low-RH flow to a diameter of approximately 200 nm. The particles that emerge from the DMA then pass through a jacketed flask, which serves as a pick up cell for the fatty acid coating. The bottom of the flask contains either a bed of myristic acid or a pool of oleic acid. Controlling the temperature of the flask in the range of 70 – 90 °C controls the thickness of the organic layer. A scanning mobility particle sizer (SMPS) samples a small portion of these particles. Comparing the mobility diameter of the coated and uncoated particles yields the nominal coating thickness. These coated particles then enter the sliding injector inlet of a cylindrical, stainless steel flow tube.


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Figure 1. Diagram of the experimental apparatus. The arrows show the direction of flow.

A separate 2.00-slpm flow passes through a jacketed flask containing pyrene before entering the rear of the flow tube. For the uptake experiments, the temperature of the pyrene bed in the pickup cell is 318 K to ensure that 295 K air flow is saturated with pyrene. The pyrene-saturated air is a few degrees warmer than the aerosol carrier gas when it enters the sliding injector flow tube, but the slow linear velocity in the flow tube (approximately 1 cm/s) allows ample time for equilibration. The aerosol flow and pyrene flow combine in the sliding injector flow tube, configured identically to the tube in the previous study.26 We measure the pyrene gas-particle interaction times directly by switching the control voltage on the DMA and timing the arrival of particles using an aerosol electrometer as the detector. The flow time from the DMA inlet to the sliding injector is a constant and may be subtracted from the total flow time to yield the interaction time. There is some spread in the residence time of particles in the flow tube for a particular injector position, as much as 15% difference for flow times near 60 seconds. We use the midpoint in time between the arrival of the first particles and the steady-state concentration as the nominal interaction time. The values measured this way are within a few seconds of values calculated using the volume flow rate.



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After interacting with pyrene vapor in the flow tube, the particles enter the ionization cell where the third harmonic of a Nd:YAG laser (355 nm), multi-photon ionizes surface adsorbed pyrene. The energy for the laser is 2 mJ/pulse, and the unfocussed spot size is approximately 4 mm in diameter in the cell. Although the aerosol continues to interact with gas phase pyrene between the ionization cell and the electrometer, any further adsorption or desorption of pyrene is irrelevant, because as it does not affect the aerosol charge state. The ratio of the electrometer signal after ionization to that with the laser blocked, which reflects only the number density of singly-charged aerosol particles exiting the DMA, is proportional to the number of pyrene molecules adsorbed to each particle. This number is the photoelectric charging efficiency (φ). These experiments focus on regimes where the final average coverage is less than one monolayer. The lone exception is the signal calibration experiment. In a calibration experiment, we expose particles to a higher temperature pyrene pickup cell similar to ones used for the organic coating. The greater adsorption of pyrene leads to a measureable increase in the mobility diameter of the particles, which, in turn, allows for an independent calculation of the number of pyrene molecules. The previous paper outlines the procedure.26 For the experimental parameters described here, the calibration coefficient is 2.1×10! pyrene molecules per unit of photoelectric charge. The limit of detection for our experiment is roughly 500 adsorbed pyrene molecules per particle.

3. Multilayer kinetic model

As we have in our previous uptake kinetics work,26 we use the notation of Vinokurov and Kankare,36 who provided analytic solutions for the full system of differential equations for an arbitrary number of layers with Langmuir-type uptake rules.

The changing pyrene vapor

pressure in the kinetic flow tube and the heterogeneous surface in our experiment prevent us from using the analytical solution provided in their work. As a result, we restrict our model to a fixed number of layers and numerically integrate the resulting equations. In our model, the aerosol particle surface consists of either NaCl or fatty acid. Let xi represent the fraction of the exposed NaCl surface covered by i layers of adsorbed pyrene and yi be the analogous quantity for pyrene adsorbed to the organic coating. Using this definition, the surface in the absence of pyrene would have x0 = y0 = 1 and the remaining fractions would be zero. A


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surface with a uniform monolayer of pyrene would have x1 = y1 = 1 with the other fractions zero. The total number of adsorbed pyrene molecules per aerosol particle is 𝑁 = 𝑁!" 𝑓

𝑖 ∙ 𝑦! + (1 − 𝑓)

𝑖 ∙ 𝑥!

!

(1)

!

where f is the fraction of the NaCl surface covered by either oleic or myristic acid and NML represents the number of pyrene molecules in one molecular layer. Both adsorption and desorption processes contribute to changes in the coverage fractions, and Figure 2 depicts these processes. For example, the differential rate law for 𝑥! is 𝑑𝑥! ! ! = 𝑘!,! 𝑥! − 𝑘!,! 𝑃 ! 𝑥! , 𝑑𝑡

(2)

! ! where 𝑘!,! is the desorption rate constant for pyrene adsorbed directly to the NaCl surface, 𝑘!,! is

the bare NaCl surface adsorption rate constant, and 𝑃

!

is the gas phase concentration of pyrene

molecules. As with our previous work,26 this approach assumes that gas phase diffusion is rapid compared to the net uptake, and 𝑃

!

is unaffected by uptake by particles. The loss of pyrene

vapor to the walls of the flow tube is not negligible, and we treat those wall losses as we have in the past. Similarly, the value of the pyrene vapor pressure is consistent with our previous work.



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Figure 2. Diagram of the scheme represented in the kinetic model. The bare NaCl and myristic acid coated surfaces include separate rate constants for single and multilayer adsorption and desorption. The oleic acid coated surface includes single layer adsorption and desorption, as well as bulk dissolution and readsorption to the surface.

The differential rate equation for 𝑥! has four contributions. It may increase by desorption from 𝑥! and adsorption by 𝑥! , and it may also decrease by adsorption and desorption processes involving 𝑥! . In total, the rate of change for 𝑥! is 𝑑𝑥! ! ! = 𝑘 ! 𝑥! + 𝑘!,! 𝑃 ! 𝑥! − 𝑘 ! 𝑃 ! 𝑥! − 𝑘!,! 𝑥! , 𝑑𝑡

(3)

where 𝑘 ! and 𝑘 ! are the adsorption and desorption rate constants, respectively, for pyrene interacting with adsorbed pyrene. The remaining fractions have the same four contributions to their rate of change, giving 𝑑𝑥! (4) = 𝑘 ! 𝑥!!! + 𝑘 ! 𝑃 ! 𝑥!!! − 𝑘 ! 𝑃 ! 𝑥! − 𝑘 ! 𝑥! . 𝑑𝑡 We use separate treatments for the myristic acid and oleic acid coatings, because myristic acid is a solid and oleic acid is a liquid. The rate equations for the solid myristic acid coating are identical to those of the uncoated NaCl surface, with y replacing x in each. We allow that pyrene 8 ACS Paragon Plus Environment

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! ! ! ! adsorption and desorption rate constants (𝑘!,! , 𝑘!,! , 𝑘!,! , 𝑘!,! ) are distinct for the two surfaces;

however, we assume that the multilayer rate constants, 𝑘 ! and 𝑘 ! are the same. Under our experimental conditions, the fractions 𝑥! through 𝑥! account for >98% of the surface, but our model includes fractions through 𝑥! . Accordingly, the differential rate law for 𝑥! is missing the terms describing the formation of the seventh layer (−𝑘 ! 𝑃 ! 𝑥! ) and desorption from the seventh layer (+𝑘 ! 𝑥! ). There are two differences in the model for the oleic acid coating. First, the model must include the possibility that pyrene dissolves in the liquid layer. The rate equations for the surface layers are, then 𝑑𝑦! ! ! = −𝑘!,! 𝑃 ! 𝑦! + 𝑘!,! 𝑦! + 𝑘!,! 𝑦! − 𝑘!,! 𝑦! 𝑃 𝑑𝑡

(5)

!

𝑑𝑦! (6) ! ! = 𝑘!,! 𝑃 ! 𝑦! − 𝑘!,! 𝑦! − 𝑘 ! 𝑃 ! 𝑦! + 𝑘 ! 𝑦! − 𝑘!,! 𝑦! + 𝑘!,! 𝑃 ! 𝑦! 𝑑𝑡 The last two terms in each expression account for motion from the surface to the bulk, with rate constant 𝑘!,! , and from the bulk to the surface, with rate constant 𝑘!,! , respectively. 𝑃

!

is the

bulk molar concentration of pyrene in the bulk oleic acid layer, calculated by dividing the number of moles of pyrene molecules that exist in the bulk by the volume of the oleic acid layer, 𝑃

!

=

𝑧 𝑁! 𝑉!"#

%$(7)

Here, 𝑧 is the population of pyrene in the bulk liquid (in units of monolayers), and its time dependence is 𝑑𝑧 = 𝑘!,! (1 − 𝑦! ) − 𝑘!,! 𝑧 𝑦! . 𝑑𝑡

(8)

It is analogous to the layer variables, 𝑥! and 𝑦! , but its value is not constrained to be between 0 !"

and 1. The product, 𝑁! !" , then, is the net flux of molecules into the bulk. We constrain the bulk concentration to be zero for thin oleic acid layers where there is no bulk phase. We use 1.0 nm for this threshold thickness, but the choice is arbitrary and has little effect on the calculations for any thickness below 2.0 nm. The model assumes that dissolved pyrene molecules go undetected in the experiment, which is sensitive primarily to the outermost layer. The second difference is that we do not consider pyrene multilayer formation for the oleic acid coated



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portion of the particle. There are two reasons for this omission. First, the more dynamic liquid surface should not promote multilayer formation as efficiently as the more static solid layer. Second, including multilayer formation worsens the fit to the experimental data, especially for thick coatings. The independent variable in the experiment is the nominal coating thickness, but there is no direct experimental control for the fraction of the surface coated by the fatty acid. For relatively large mass fractions of oleic acid, these mixtures are essentially oleic acid droplets with a completely enveloped core of solid NaCl.4 The thin coatings considered here, ranging from 0% to 10% of the total diameter, are not necessarily uniform. The model, then, must include a function that transforms the nominal coating thickness into fractional coverage. We introduce a single-parameter exponential function to describe the coverage, (9)

f δ = 1 − e!!/!!

In this expression, δ is the nominal coating thickness and 𝛿! is a parameter that describes the spreading of the fatty acid. The surface area of particles also changes as the nominal coating thickness increases. In its simplest form, the model calculates 𝑁!" as the surface area of a sphere divided by the molecular footprint of pyrene. We estimate26 the footprint of a single pyrene molecule to be 0.53 𝑛𝑚! . The model defined above is unable to capture some features of the data, described in Section 4. In particular, it cannot reproduce the behavior of oleic acid, which shows an initial decrease in uptake rate with increasing coating thickness and a sharply increasing rate for thicker coatings. One possible explanation for this change in trend is that, while pyrene adsorbs more efficiently onto the oleic acid surface, the first oleic acid deposited on the NaCl surface effectively reduces the active surface area by covering corrugated regions. To model this effect, we assume that the uncoated particles have a greater number of adsorption sites than would a sphere of the same mobility diameter owing to both its non-spherical shape and number of surface defects. Further, we assume that these extra sites disappear as the oleic acid covers the surface, making the particles more smooth and spherical. Accordingly, we rewrite 𝑁!" as !"!!"! 𝑁!" 𝛿 = 𝑁!" (𝛿)(1 + 𝐴 𝑒

!

! !! )

(10)


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where, analogous to 𝛿! , 𝛿! is the parameter that controls the conversion of irregular to smooth surface, and 𝐴 is the fractional excess in surface area of the bare NaCl particles over the hypothetical spherical case. An alternative explanation for the decrease in uptake for thin coatings is that some NaCl surface sites produce faster uptake than others and oleic acid preferentially covers these “sticky” NaCl sites. To model this scenario, we introduce another kind of surface, which represents these sticky sites. The fraction of these sites covered by 𝑖 layers of pyrene is 𝑠! , and we add the corresponding differential rate expressions to the overall system described by equations 2-4. We assume that the sticky sites are distributed uniformly on the particle surface, and we assign unique uptake and desorption rate constants to it. We also consider the possibility that surface diffusion of pyrene results in adsorption to sticky sites and that oleic acid may preferentially coat the sticky sites, as well as pyrene. A separate morphology parameter, analogous to 𝛿! in equation 9, controls the distribution of oleic acid on the sticky sites. This approach introduces much more complexity than the surface area function, but produces generally poorer fits to the data. As a result, we present only the surface area reduction approach to avoid distraction from the main focus of this work. A final caveat is that the model calculations use the mean mobility diameter from our experiment, and not the full size distribution. The geometric standard deviation in the particle size distribution is small, typically near 1.08. Comparing model results using the full distribution with those using only the average size shows negligible differences. In any case, it is not the limiting source of uncertainty in the model fits. The models for both oleic acid and myristic acid coated particles contain a large number of kinetic parameters, and our data is not sufficient to make unique determinations of them. Fortunately, we can constrain many of the values. For the rate constants that govern the bare NaCl surface, we have prior measurements from that simpler system.26 In other cases, we constrain values using physically reasonable estimates. Even more sophisticated models would be necessary to fully describe this system. For example, the KM-SUB model,37 which describes the surface to bulk transport in finer detail, would provide a more realistic picture. Other features of our system that we do not include in the model are the differential experimental sensitivities to pyrene adsorbed to different sites and the explicit probe depth of the photoelectric charging detection. The model we present is the simplest one that allows us to reach conclusions from our 11 ACS Paragon Plus Environment


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Figure 3. Size distribution of uncoated particles (○) and particles coated with nominally 4 nm (□) and 10 nm (∆) coatings of oleic acid.

data without introducing parameters we can neither estimate with confidence nor resolve with fitting procedures. Our goal is to identify the features of coated particles that control uptake. 4. Results

A key element of the experiment is the generation of coated particles from atomized NaCl aerosol particles. Figure 3 shows representative mobility diameter number distributions for the stream of seed NaCl particles and oleic acid coated particles with two different average thicknesses: 4 and 10 nm. Though we do not know the orientation of adsorbed molecules in this experiment, the nearly 2-nm thickness of a well-ordered monolayer of oleic acid is a useful point of comparison. The solid lines in the figure are fits to a log normal distribution. The distributions show only a small increase in the geometric standard deviation, indicating that all the particles grow a similar amount. Figure 4 shows the pyrene coverage (average number of pyrene molecules per aerosol particle) plotted against the nominal coating thickness for a) myristic acid and b) oleic acid. In both cases, the figure shows uptake data for three different gas-particle interaction times: 17, 29, and 35 seconds. The average coverage comes directly from the calibrated photoelectric charging data. The solid lines in these figures represent model fits to the data, and the fits are described later in this section. The nominal coating thickness is half of the difference in mobility diameter for the coated and uncoated particles, (

!!!! !

). We record these data by monitoring continuously


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Figure 4. a) The average number of pyrene molecules adsorbed to myristic acid coated particles as a function of the nominal coating thickness for three different flow tube interaction times: 17 sec., 29 sec., and 35 sec. b) The average number of pyrene molecules adsorbed to oleic acid coated particles as a function of the nominal coating thickness for three different flow tube interaction times: 17 sec., 29 sec., and 35 sec.

both the photoelectric charging efficiency and particle size while slowly increasing the temperature of the pick-up cell. In both cases, the uptake for a given interaction time initially decreases with increasing coating thickness, reaches a minimum, and eventually turns upward forming a U-shape. There are two coarse-grained differences between the two: the net uptake rate for oleic acid is higher for most coverages, and the minimum in the data lies at a larger value of the nominal coating thickness for myristic acid than it does for oleic acid. The value of 𝑑! , which we control with the DMA, is higher for myristic acid (235 nm) than for oleic acid (187.5 nm), leading to the difference in the y-intercepts of these plots. We select these larger particles for the myristic acid experiment to increase the pyrene signal and improve the quality of the data.



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As described in the previous section, the model we use to describe the myristic acid coated particles is simpler than for oleic acid, because it is a solid. To model these data, we constrain the ! ! values of 𝑘!,! , 𝑘!,! , 𝑘 ! , and 𝑘 ! to be consistent with our results for uncoated particles. (Because

the prior analysis for bare NaCl assumed spherical particles, we first reanalyzed the uncoated NaCl data using the excess surface area assumption with 1.5 times the surface area of a sphere. That is, the value of A in equation 10 is 0.5. This change leads to a slight decrease in the adsorption rate constants compared to those in Ref. 26. These revised parameters for the bare ! ! NaCl surface apply to both the myristic acid and oleic acid data.) The parameters, 𝑘!,! , 𝑘!,! , and

𝛿! are adjustable, and we optimize them using the nonlinear model fit functionality in Mathematica.38 The fits reflect a global optimization using all three data sets. We gauge the sensitivity of the model parameters by fixing one of the three and re-optimizing the others. Table 1 shows the optimized parameters with uncertainties estimated from the sensitivity analysis. Table 1. Model parameters for myristic and oleic acid coated NaCl particles parameter

myristic acid/NaCl

oleic acid/NaCl

note

! 𝑘!,! / 10!!" 𝑐𝑚 ! 𝑠 !!

1.7 ± 0.6

1.7 ± 0.6

1

! 𝑘!,! /𝑠 !!

0.022 ± 0.002

0.022 ± 0.002

1

𝑘 ! / 10!!" 𝑐𝑚 ! 𝑠 !!

58 ± 10

58 ± 10

1

𝑘 ! /𝑠 !!

0.15 ± 0.01

0.15 ± 0.01

1

! 𝑘!,! / 10!!" 𝑐𝑚 ! 𝑠 !!

85 ± 30

136 ± 20

2

! 𝑘!,! /𝑠 !!

10 ± 1

0.26 ± 0.3

2

𝛿! /𝑛𝑚

9.0 ± 0.5

9.0 ± 0.5

2

𝛿! /𝑛𝑚

--

1 ± 0.3

2

𝐴

--

0.5 ± 0.2

2

𝑘!,! /𝑠 !!

--

0.13

3

𝑘!,! /10!!" 𝑐𝑚 ! 𝑠 !!

--

0.70

3

1 – Constrained using previously published data for uncoated particles.26 2 – Optimized from fits to data in Figure 4 3 – Estimated using experimental data from similar systems


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In the case of myristic acid, the U-shape of the model data arises from two competing factors. One factor is that the net uptake rate onto myristic acid surface is slightly slower than for the bare NaCl surface, causing the small negative slope to the data for small coating thicknesses. On the other hand, the particles grow in surface area, leading to a greater frequency of pyreneparticle collisions. For thick coatings, the increase in surface area dominates and reverses the initial downward slope. While the adsorption rate constant is approximately 50 times larger for ! ! the myristic acid surface than for the NaCl surface (𝑘!,! ≅ 50 𝑘!,! ), the desorption rate constant ! ! is 450 times larger (𝑘!,! ≅ 450 𝑘!,! ). Fast desorption of pyrene from the myristic acid surface

limits the net uptake rate for coated particles, making the average pyrene coverage of the coated particles smaller than bare NaCl particles for the interaction times in this experiment. For the oleic acid coating, the increase in uptake rate for thick coatings is greater than can be explained by the increasing surface area of the larger coated particles; it must be the case that the oleic acid coated surface has a greater uptake rate than the uncoated NaCl surface. This observation leads to a problem in fitting the data with the model used for myristic acid. A surface where the dynamics are a simple admixture of uptake occurring on bare NaCl and faster uptake on oleic acid coated NaCl could never produce the dip in uptake evident in these data. Instead, the net uptake for a given interaction time would increase monotonically from the uncoated case to one that potentially reflects a fully coated particle. As discussed in Section 3, we suggest that the particle transitions from a rough to smooth surface, losing surface area compared to the uncoated particle. We further suggest that oleic acid is much more likely to exhibit this behavior, because bulk oleic acid can flow on the surface while myristic acid cannot. The expression for 𝑁!" given by Equation 10 governs this transition with two additional parameters: the fractional excess of surface area over the spherical case (𝐴) and the characteristic thickness for the transition (𝛿! ). The excess surface area for atomized NaCl aerosol particles could be quite high.39 We do not include these parameters into the fitting procedure, because the system is already underdetermined. Instead, to demonstrate that this approach can reproduce our data, we choose 𝐴 = 0.5 and 𝛿! = 1 𝑛𝑚. That is, the bare NaCl particles have 50% more surface sites than a sphere having the equivalent mobility diameter and oleic acid smoothens the surface such that a nominally 1 nm coating reduces the number of excess sites by a factor of 1/e. 15 ACS Paragon Plus Environment


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! ! As with the myristic acid data, we constrain the values of 𝑘!,! , 𝑘!,! , 𝑘 ! , and 𝑘 ! to be consistent

with our result for uncoated particles. In addition, the oleic acid model contains constants to describe transport between the surface and the bulk, 𝑘!,! and 𝑘!,! . We estimate these values using measured diffusion constants for analogous systems. The surface to bulk transport rate constant, 𝑘!,! , is a pseudo first order constant equal to the reciprocal of the lifetime of surface adsorbed molecules with respect to dissolution. We estimate it using the characteristic time !

associated with diffusion in one direction, 𝜏 = !

!,!

=

! !! !!

. Using 𝐷 = 1×10!!" 𝑐𝑚! /𝑠, which is

consistent with pyrene diffusion through combustion aerosol,40 and an approximate layer thickness of 1 nm, the lifetime is roughly 8 seconds, or 𝑘!,! ≅ 0.13 𝑠 !! . Using similar considerations, we estimate that 𝑘!,! ≅ 0.7 𝑐𝑚! 𝑠 !! . The fit program includes the remainder of the constants as adjustable parameters. Figure 4 shows the results of the model fit, and Table 1 lists the optimized parameters. Rather than varying the coating thickness for fixed interaction times, we can fix the coating thickness and vary the interaction time. From a practical point of view, we prefer the former, because maintaining a fixed coating thickness for long periods of time is difficult. Nevertheless, as a check on our experimental method, we can compare our model results to the interaction time dependent data. Figure 5 compares the model-calculated result to the experimental result for two different coating thicknesses of oleic acid. Although the model parameters are not optimized using these data, the agreement is still excellent. These data more clearly show that the pyrene ! surface concentration rises rapidly owing to the large 𝑘!,! value, then levels off as desorption and

dissolution become appreciable.


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Figure 5. The average number of pyrene molecules adsorbed to oleic acid coated NaCl aerosol particles as a function of the interaction time for two nominal coating thicknesses (∆ 4 nm, ☐ 13 nm). The solid lines represent the model calculation using fitted parameters from the data in Figure 4.

5. Discussion

The parameters in this model fit fall into two general categories: those describing rates and those describing the morphology of the particles. We first consider the adsorption rate constants ! ! for the organic coated surface, 𝑘!,! , 𝑘!,! . The model results in Table 1 show that the uptake rate

for the organic coated surface is substantially higher than for the bare NaCl surface. The corresponding initial uptake coefficients, 𝛾! , are 5×10!! for myristic acid, are 9×10!! for oleic acid, and ~1×10!! for NaCl. As we argued previously,26 purely kinematic concerns predict near unity sticking probabilities for thermal, room temperature pyrene; therefore, from that point of view, all three of the uptake coefficients are surprisingly low. We rationalize these small numbers by imagining that newly adsorbed pyrene molecules form a weakly bonded system that can either quickly desorb or go on to make stronger bonds with the surface. The timescale of the experiment cannot resolve these dynamics, and the measurements reflect the net uptake rate. Treating the initial, weakly bound state as an activated complex, a simple transition state theory representation of the initial uptake yields a free energy barrier to adsorption of approximately 27 kJ/mol for the bare NaCl surface. (The calculation assumes that the free energy ‡ change in desorption is 55 kJ/mol,41 and the free energy barrier to desorption is 𝛥𝐺!"# =



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−𝑘! 𝑇 ln

! !!,! !" ! !

!! !

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. The barrier to adsorption is the difference between the two.) A similar

calculation for the myristic acid and oleic acid yields barriers of 12 kJ/mol and 21 kJ/mol, respectively. A possible reason for the decrease in this barrier for the organic surface is the likely less stringent requirements for the orientation of pyrene molecules adsorbed to the softer, more porous organic surface. Additionally, impinging pyrene molecules may interact with the organic coated surface longer than for the rigid NaCl surface. The desorption rate from both the organic coated surface is also higher than for the bare NaCl surfaces, with myristic acid having the highest of the three. It seems that, while there is a higher barrier associated with forming stable, adsorbed pyrene on bare NaCl, there is also a deeper well. The ion – induced dipole forces that likely dominate the pyrene adsorption on NaCl are relatively strong compared to intermolecular forces between pyrene and the fatty acids. The slower rate of desorption from oleic acid than myristic acid can be attributed to direct and indirect effects of the unsaturated site in oleic acid. π donor-acceptor interactions may enhance intermolecular bonding between pyrene and oleic acid. There is similar evidence that these effects can enhance the sorption of aromatic molecules in soil organic matter.42 We also expect that the greater fluidity of oleic acid, a consequence of its hairpin geometry, allows pyrene to access stronger bonding sites. This slower desorption from the oleic acid surface is the primary reason for the greater net uptake rate under the conditions in this experiment. The parameters that describe morphology give some insight into the nature of these heterogeneous mixed particles. Our model suggests that neither acid spreads uniformly on the NaCl particle surfaces on the timescale of the experiment. The 𝛿! parameters are similar for the acids, in the range of 7.5-10 nm for approximately 200 nm particles. As shown in Figure 6, the model particles become 50% coated when the thickness is around 6 nm. This result is consistent with the observation of island formation for oleic acid coated, solid inorganic particles using microscopy.43


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In constrast, the 𝛿! parameter for oleic acid is very small, indicating that the excess surface area becomes inaccessible with only a few nanometers of nominal coating thickness. This result suggests that the coating nucleates preferentially on corrugated regions of the surface. The large disparity between 𝛿! and 𝛿! evokes a picture of a particle with intermediate coverage (e.g. 5 nm) that has appreciable areas of uncoated NaCl and domains of oleic acid covering areas that deviate strongly from the spherical case and produce additional surface area. Thicker coatings resemble solid NaCl particles completely or nearly completely enveloped by the fatty acid shell.

Figure 6. The solid red line shows 𝒇(𝜹), the fraction of the surface covered by oleic acid (right-hand scale) as a function of the nominal coating thickness. The dotted blue line refers to the left-hand scale and shows the ratio of the monolayer population in the oleic acid model, 𝑵𝑴𝑳 (𝜹), to that of a sphere. It reflects the loss of surface area with increasing oleic acid coverage.

The drawings in Figure 6 illustrate this progression. The notion is consistent with previous work by Ewing et al.,39 wherein IR spectroscopy of atomized NaCl aerosol particles showed significant water content at low RH. The structures consistent with their spectroscopic characterization are those with either internal cavities or pores, such as in Figure 6. Though our model produces a quantitative representation of the fraction of these sites available for a given coverage, there are two important limitations in interpreting the results. First, the mobility diameter of these mixed particles is not solely a function of their size. In particular, the density and shape of the particles also contribute. As a result, we cannot equate these morphological parameters strictly with the thickness of the coating. Second, it is likely that the values of these parameters partially compensate for incompleteness or deficiencies in the model, described in section 3. The robust conclusion is that both parameters are necessary to fit the data 19 ACS Paragon Plus Environment


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and that they have appreciably different values. That is, for oleic acid, the changing of the morphology of the particle and the evolution of the surface composition are clearly separate effects. In the case of myristic acid, a single spreading parameter (𝛿! ) is able to describe the experimental data. As a result, we cannot determine whether the surface area changes independently of the surface composition. 6. Conclusions

These results illustrate some of the factors that control non-reactive uptake of gases by internally mixed, multiphase particles. For the case presented here, an inorganic salt core (NaCl) with a fatty acid coating (myristic or oleic acid), the organic coating can either enhance or hinder the net uptake rate. Comparing the experimental results to a relatively simple kinetic and morphological model reveals several important features of the uptake dynamics. Both the adsorption rate and desorption rate for the fatty acid coatings are faster than for the salt surface, with the faster desorption rate limiting the overall coverage for the myristic acid coated particles. The site of unsaturation in oleic acid is responsible for the disparity in desorption rates either directly, though intermolecular forces, or indirectly, through changes in the fluidity of the organic layer. The two acids occupy a similar fraction of the particle surface for a given coating thickness. The surface area of particles coated with oleic acid first decreases with increasing coating thickness before increasing in the thick coating limit.


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Acknowledgements This work was funded by the National Science Foundation through grant number CHE-1012224.

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Acoustic Technique for Measuring the Enhancement. Aerosol Sci. Technol. 2009, 43, 10061012. 19. Schnaiter, M.; Linke, C.; Mohler, O.; Naumann, K. H.; Saathoff, H.; Wagner, R.; Schurath, U.; Wehner, B., Absorption Amplification of Black Carbon Internally Mixed with Secondary Organic Aerosol. J. Geophys. Res. Atmos. 2005, 110, D19204. 20. Li, Y.; Xue, R.; Ezell, M. J.; Finlayson-Pitts, B. J., Experimental and Theoretical Investigation of Aerosol Optical Properties. Procedia Eng. 2015, 102, 1204-1211. 21. McNeill, V. F.; Patterson, J.; Wolfe, G. M.; Thornton, J. A., The Effect of Varying Levels of Surfactant on the Reactive Uptake of N2o5 to Aqueous Aerosol. Atmos. Chem. Phys. 2006, 6, 1635-1644. 22. Thornton, J.; Abbatt, J., N2o5 Reaction on Submicron Sea Salt Aerosol: Kinetics, Products, and the Effect of Surface Active Organics. J. Phys. Chem. A 2005, 109, 10004-10012. 23. Rouviere, A.; Ammann, M., The Effect of Fatty Acid Surfactants on the Uptake of Ozone to Aqueous Halogenide Particles. Atmos. Chem. Phys. 2010, 10, 11489-11500. 24. Ellison, G. B.; Tuck, A. F.; Vaida, V., Atmospheric Processing of Organic Aerosols. J. Geophys. Res. Atmos. 1999, 104, 11633-11641. 25. Katrib, Y.; Martin, S. T.; Hung, H. M.; Rudich, Y.; Zhang, H. Z.; Slowik, J. G.; Davidovits, P.; Jayne, J. T.; Worsnop, D. R., Products and Mechanisms of Ozone Reactions with Oleic Acid for Aerosol Particles Having Core-Shell Morphologies. J. Phys. Chem. A 2004, 108, 6686-6695. 26. Woods, E.; Yi, C.; Gerson, J. R.; Zaman, R. A., Uptake of Pyrene by Nacl, Nano3, and Mgcl2 Aerosol Particles. J. Phys. Chem. A 2012, 116, 4137-4143. 27. Tervahattu, H.; Hartonen, K.; Kerminen, V.; Kupiainen, K.; Aarnio, P.; Koskentalo, T.; Tuck, A.; Vaida, V., New Evidence of an Organic Layer on Marine Aerosols. J. Geophys. Res. Atmos. 2002, 107, 4053. 28. Silva, P. J.; Liu, D. Y.; Noble, C. A.; Prather, K. A., Size and Chemical Characterization of Individual Particles Resulting from Biomass Burning of Local Southern California Species. Environ. Sci. Technol. 1999, 33, 3068-3076. 23 ACS Paragon Plus Environment


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TOC graphic:


pyrene pick up cell

DMA

coating pick up cell

exhaust

Nd:YAG

3x flow tube

diffusion dryer

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Po-210

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ionization cell

atomizer SMPS

electrometer power meter

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k

+

k

k

k

+

k1,x

-

k1,x

k

1,y

+ 1,y

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+

k

-

myristic acid

NaCl +

k1,y k

+

k

+

k1,x

-

k1,x

ks,b kb,s oleic acid

NaCl


-

k1,y

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4.5 4.0

δ = 0 nm δ = 4 nm δ =10 nm

3.5

-3

3.0 2.5

3

dN / 10 cm

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2.0 1.5 1.0 0.5 0.0 150

200

250

mobility diameter / nm


300


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a)

17 sec 29 sec 35 sec

b)

17 sec 29 sec 35 sec


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9 8 7

4

6

Npyrene/10

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5 4 3 2

δ = 4 nm δ = 13 nm

1 0 0

5

10

interaction time / s

15


20


1.0

1.5

0.9 0.8

1.4

0.6

1.3

0.5

f(δ)

sphere

0.7

NML/NML

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.4

1.2

0.3 0.2

1.1

0.1 1.0

0

5

10

15

0.0 20

δ / nm


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Uptake of Pyrene onto Fatty Acid Coated NaCl Aerosol Particles - The

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