Organic Surfactants Protect Dissolved Aerosol Components against

Article Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX

pubs.acs.org/JPCA

Organic Surfactants Protect Dissolved Aerosol Components against Heterogeneous Oxidation Jennifer A. Faust* and Jonathan P. D. Abbatt Department of Chemistry, University of Toronto, Toronto, ON M5S 3H6, Canada

J. Phys. Chem. A Downloaded from pubs.acs.org by WASHINGTON UNIV on 03/01/19. For personal use only.

S Supporting Information *

ABSTRACT: Oxidative aging alters the composition of organic aerosols over time, in turn affecting the ability of aerosols to seed cloud formation and scatter solar radiation. Here we explore the heterogeneous photooxidation of model organic particles with and without a soluble surfactant coating. Tricarballylic acid (TCA), a proxy for α-pinene oxidation products, serves as a representative small organic solute. Sodium dodecyl sulfate (SDS) was selected as the representative soluble surfactant because its surface properties have been extensively characterized. A flow reactor and aerosol mass spectrometer were used to determine the second-order reaction rate constant (k = (1.9 ± 0.1) × 10−11 cm3 molecule−1 s−1) and reactive uptake coefficient (γ = 3.0) for the heterogeneous photooxidation of uncoated TCA particles by gas-phase OH radicals; such a high uptake coefficient implicates radical chain reactions in the oxidation mechanism. SDS dramatically slows the disappearance of TCA: when the SDS concentration approaches monolayer coverage, the rate of reaction of TCA with OH decreases by ∼60% relative to the rate in the absence of SDS. These results indicate that small concentrations of surface-active molecules on atmospheric particles can protect organic solutes in the bulk from oxidative aging. This effect extends the environmental lifetime of dissolved pollutants.

1. INTRODUCTION Organic compounds are a major component of atmospheric aerosol: they comprise 20−90% of a typical particle by mass.1−3 Organic aerosol particles enter the atmosphere directly from natural or anthropogenic activities, or they form in the atmosphere through chemical reactions and phase partitioning. Further multiphase reactions alter the chemical composition and structure of organic aerosols over time through oxidative aging. The resulting changes in hygroscopicity, optical depth, and other properties affect the ability of particles to seed cloud formation, to scatter solar radiation, and to deposit and react on biological surfaces such as skin and lung tissue.4,3,5−9 Multiphase reactions with gas-phase oxidants are a critical aging mechanism for atmospheric aerosols because the particles can reside in the atmosphere for many days.10−12 Here we explore the heterogeneous photooxidation of model organic aerosol particles by OH radicals, the most important daytime oxidant.5,13 Specifically, we focus on how surfactants control the reaction kinetics. Surfactants are organic ions and molecules containing hydrophobic and hydrophilic moieties that reside at the air/water interface of aqueous particles. Common examples of soluble and insoluble surfactants found on atmospheric particles include lipids, polysaccharides, fatty acids, dicarboxylic acids, and humic-like substances.14,15 Because the first species a gas-phase OH radical encounters when it collides with an atmospheric © XXXX American Chemical Society

particle is often a surfactant, surface-active ions and molecules exert a disproportionately large influence over the reaction pathways in comparison to more concentrated solutes in the bulk. Several studies have investigated surfactant control over heterogeneous reactions of gas-phase species with aqueous inorganic ions, such as N2O5 with chloride16−18 and O3 with iodide,19−21 and of gases with organic molecules coadsorbed at the air/water interface.22−25 To the best of our knowledge, despite the hypothesis that surfactants could protect dissolved constituents from OH oxidation being made decades ago, this work is the first test of how a soluble surfactant affects the heterogeneous oxidation of a small organic solute in organic aerosol.26,27 We believe that the results obtained in this work pertain to the impacts of surfactants on the uptake of radicals to cloud droplets as well. The model surfactant selected for the experiments is sodium dodecyl sulfate (SDS, Figure 1a) because it is perhaps the most well-characterized long-chain soluble surfactant.16,28 The model organic solute is tricarballylic acid (TCA, Figure 1b), which is a proxy for 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA, Figure 1c), a later generation oxidation product of α-pinene.29−31 The monoterpene α-pinene is one of the most prevalent biogenic volatile organic compounds emitted to the Received: January 7, 2019 Revised: February 16, 2019

A

DOI: 10.1021/acs.jpca.9b00167 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

Figure 2. Experimental setup and flow rates. TPOT = Toronto Photo-Oxidation Tube, AMS = aerosol mass spectrometer, SMPS = scanning mobility particle sizer, and ccm = cubic centimeters per minute. Figure 1. Structures of (a) sodium dodecyl sulfate (SDS), (b) tricarballylic acid (TCA), and (c) 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA).

2.1. Aerosol Generation. Polydisperse aerosols were generated by atomizing solutions of ammonium sulfate (MP Biomedicals, ACS reagent grade), tricarballylic acid (SigmaAldrich, ≥99%), and/or sodium dodecyl sulfate (SigmaAldrich, ≥98.5%) in Milli-Q water (18.2 MΩ cm) with a home-built atomizer. “TCA” solutions for atomization contained 50 μM AS and 150 μM TCA. “SDS” solutions for atomization contained 50 μM AS and 37 μM SDS. “Mixed” solutions for atomization contained 50 μM AS, 150 μM TCA, and varying concentrations of SDS (7.6, 14, 24, or 37 μM). The pH values of the bulk solutions were not adjusted. The surface-weighted and volume-weighted mode diameters of the resulting wet size distributions at 80% relative humidity were approximately 90 and 100 nm for TCA and 130 and 145 nm for SDS and ranged from 95−120 to 110−135 nm for mixed particles, respectively. Particles were not dried prior to oxidation. Table S1 summarizes average particle properties, including composition, density, and pH. 2.2. Photooxidation. The TPOT has been described in detail elsewhere.41,43,44 Briefly, it is a 3.2 L stainless steel flow tube that houses two ultraviolet (UV) lamps, each at a wavelength of 254 nm. The reactor was operated in the laminar flow regime with a Reynolds number of 25. The residence time in the flow tube was ∼100 s when the total flow rate was 2000 ccm (cm3 min−1). High relative humidity (RH ≈ 80%) inside the TPOT was achieved by bubbling N2 through Milli-Q water at a rate of 650 ccm and mixing the resulting N2/H2O flow with the 1230 ccm flow from the atomizer. The relative humidity was monitored at the exit of the flow tube with a Vaisala hygrometer and corrected for the increased temperature inside the flow tube (T ≈ 300 K with UV lamps on). OH radicals were generated in the flow tube from the photolysis of ozone in the presence of water vapor. Ozone was generated by passing O2 diluted in N2 through a Jelight ozone generator (Model 600) at a total flow rate of 120 ccm. The O2/N2 ratio was varied to adjust the ozone concentration from 0.08 to 7 ppm, leading to OH exposures ranging from 8 × 109 to 5 × 1011 molecules cm−3 s. These experimental conditions are equivalent to between 2 h and 6 days of exposure to ambient OH levels of 106 molecules cm−3. The UV lamps remained on for the duration of the kinetics measurements to maintain a constant temperature in the flow tube of 300 K. The O2/N2 flow was routed either through the ozone generator or through a bypass line to monitor the particle composition in the presence and absence of OH, respectively. Control experiments confirmed that TCA, SDS, and mixed particles did not react when exposed to UV light in the absence of oxidants or when exposed to ozone in the absence of UV light. 2.3. OH Quantification. OH concentrations in the flow tube were determined using the standard approach employed in aerosol oxidation flow tubes, whereby the decay of a trace reactant with OH is monitored. In particular, we calculated the OH concentrations using a numerical kinetics model based on

atmosphere, and it is a precursor to secondary organic aerosol (SOA).32 Thus, the particles in our experiments model highly oxygenated SOA. Consider the approach of a gas-phase OH radical to a TCA particle, serving as model SOA. When OH adsorbs to the surface of the particle, it can react directly in the interfacial region, diffuse into the bulk and react within the particle, or desorb without reacting.33 The observed uptake coefficient γ characterizes the fraction of OH collisions that leads to reaction of a TCA molecule. When the disappearance of particle-phase TCA is monitored rather than the disappearance of gas-phase OH, it is possible for γ to exceed unity if one OH collision contributes to the loss of more than one TCA molecule through secondary radical reactions. Systems with γ > 1 include the reactions of OH with squalane,34 monosaccharides,35 and various carboxylic acids, such as oleic acid, linoleic acid, linolenic acid, succinic acid, methylsuccinic acid, and 2-methylglutaric acid.34,36−39 The observed uptake coefficient provides a single number that incorporates the effects of many different molecular-level processes. For example, what happens when a gas-phase OH radical collides with the surface of a salty TCA particle? Does the OH radical first strike TCA, a water molecule, or an inorganic ion? How viscous are the particles? Can OH and TCA diffuse freely through the interfacial region? Do firstgeneration oxidation products remain trapped at the surface, or do they diffuse into the bulk? Based on prior studies, the first reaction step likely involves thermal accommodation at the surface according to the Langmuir−Hinshelwood mechanism, but the subsequent steps depend on structure and phase.11,40 The situation becomes even more complex in a particle that also contains SDS. How are the SDS surfactant chains oriented at the surface? How tightly packed are the hydrocarbon tails? What fraction of impinging OH penetrates through the SDS surface layer? To address these questions, we employ an aerosol oxidation flow reactor coupled to an aerosol mass spectrometer. Our two major findings are (1) that secondary chemistry causes the observed uptake coefficient to be >1 for the heterogeneous reaction of OH with TCA and (2) that even submonolayer coverages of SDS protect TCA against heterogeneous oxidation, with additional protection provided by the SDS oxidation products.

2. METHODS The experimental setup shown in Figure 2 was adapted from earlier work.41,42 Particles containing ammonium sulfate (AS), tricarballylic acid (TCA), and/or sodium dodecyl sulfate (SDS) were exposed to OH radicals in the Toronto PhotoOxidation Tube (TPOT). Aerosol size distributions and composition were monitored at the exit of the flow tube. B


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The Journal of Physical Chemistry A Scheme 1. Possible Pathways for TCA Oxidation by OH (Adapted from Ref 29)a

a

Molecules shown in color have been detected previously in bulk aqueous-phase experiments29 and could contribute to C4H4O+ (blue, red) and C6H4O2+ (red) signals in the mass spectrometer.

the work of George et al.41,45 The model consisted of reactions involved in the photochemical production of OH from water vapor and ozone, along with the major reactive sinks for OH, including wall loss, which is the dominant sink for OH in the TPOT. OH loss on aerosol particles is relatively minor in comparison to the other sinks. The required inputs to the model were the concentrations of water vapor and ozone, which was measured in the absence of UV light with a Model 202 ozone analyzer from 2B Technologies. The full set of reactions and rate constants is listed in the Supporting Information.46 Model predictions were validated by reacting mesitylene or o-xylene with OH radicals in the TPOT under normal experimental conditions. Milli-Q water was atomized instead of the aqueous salt and organic solutions. The calibration gases were prepared by bubbling N2 through either liquid mesitylene (Sigma-Aldrich, 99%) or o-xylene (Sigma-Aldrich, ≥99.0%) and alternately sending the flow to the TPOT or through a bypass line. The gas-phase concentrations, which ranged from 20 to 150 ppb, were controlled by varying the temperature of the liquid and by diluting the gas flow at the bubbler outlet. Decay of mesitylene or o-xylene was monitored with a quadrupole proton transfer reaction−mass spectrometer (PTR-MS) from IONICON. OH concentrations were calculated from the experimental data by using second-order rate constants of 6.0 × 10−11 and 1.5 × 10−11 cm3 molecule−1 s−1 for the gas-phase reactions of OH with mesitylene and oxylene, respectively.47−50 These reactions were also implemented in the kinetics numerical model. On the basis of this comparison, we conservatively estimate the errors in the reported OH concentrations to be ±50%. Note that mesitylene and o-xylene experiments were performed separately, and neither calibration gas was present in the flow tube during aerosol experiments.

2.4. Aerosol Detection. The size distribution of the particles exiting the TPOT was continuously monitored by a scanning mobility particle sizer (SMPS, TSI Model 3034), with the relative humidity of the sheath flow at ∼90%. Hygroscopic growth factors for ammonium sulfate particles were used to adjust the diameters measured by the SMPS to match experimental conditions in the flow tube.51 Photooxidation caused less than a 5% change in the mode diameter of the particle size distributions. The particle composition was monitored with an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HRToF-AMS), which characterizes the size and mass composition of nonrefractory aerosol.52 All organic signals were normalized to the total sulfate content of the particles to account for variations in collection efficiency and lens transmission over the course of a kinetics experiment. Prior to experiments, the TPOT was cleaned with OH radicals overnight. Particle-free filter measurements were recorded by the AMS at regular intervals throughout the experiments to correct for gas-phase artifacts in the mass spectra. TCA was monitored by the fragment C4H4O+ (m/z = 68.026), and SDS was monitored by the fragment C12H24+ (m/z = 168.188). These peaks were selected because they were unique to their respective parent compounds and because they had negligible backgrounds. Data analysis was conducted with version 1.21 of the ToFAMS data analysis software PIKA in IGOR Pro version 6.37. Relative ionization efficiencies of sulfate, TCA, and SDS were calibrated using aerosol characterized by an aerosol particle mass analyzer (Kanomax). 2.5. Modeling Particle Composition with E-AIM. EAIM Model II53,54 was used to estimate the average composition and density of the particles based on relative humidity and average particle mass concentrations (measured by AMS) as experimental inputs. SDS was assumed not to affect hygroscopicity. Customized E-AIM parameters for C


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Figure 3. (a) Normalized decay of the C4H4O+ signal due to heterogeneous photooxidation of TCA particles. The red line is an exponential fit (with offset) to the experimental data points, shown as gray circles. (b) Magnified view at low OH exposures. The red dashed line represents the observed uptake coefficient of γ = 3.0. The green dash-dotted line (γ = 0.5) and the blue dotted line (γ = 5) mark boundaries to guide the eye.

3.1.1. TCA Kinetics. The effective second-order rate constant for the heterogeneous reaction of OH(g) with TCA(aq) was determined from the exponential fit in Figure 3a:

modeling TCA are given in the Supporting Information. As discussed later, the AMS measures ensemble-averaged composition, so the properties modeled with E-AIM reflect average values for all particles generated by the atomizer.

I /I0 = I0 + (1 − I0)e−k[OH]t

3. RESULTS AND DISCUSSION The results discussed here focus on reaction kinetics for the photooxidation of particles containing either TCA or SDS or an internal mixture of both components. The Supporting Information includes representative mass spectra for these systems. 3.1. Tricarballylic Acid Aerosols. Scheme 1 shows a proposed mechanism for the photooxidation of TCA by OH based on aqueous-phase experiments by Aljawhary et al.29 Here, loss of TCA was monitored by the decay in the C4H4O+ signal, as presented in Figure 3a. This fragment was selected because background contaminants and SDS added minimal contributions to the signal. Nevertheless, for rigorous analysis, all organic signals were corrected for residual background, which was determined by atomizing a control solution of ammonium sulfate instead of the experimental solution containing both ammonium sulfate and TCA. Before subtraction, organic signals were normalized by the measured total sulfate content (∑SO4) to account for variations in collection efficiency and lens transmission in the mass spectrometer:

where I and I0 are the final and initial C4H4O+ signals, respectively, and [OH]t is the OH exposure in the flow tube. The effective second-order rate constant k was found to be (1.9 ± 0.1) × 10−11 cm3 molecule−1 s−1. On the basis of the asymptote of the exponential fit in Figure 3a, it is possible that the reaction of OH with TCA does not reach completion at OH exposures greater than 3 × 1011 molecules cm−3 s. The residual C4H4O+ signal is unlikely to be caused by background contamination because of the correction procedure discussed above. Background checks with ammonium sulfate were performed at regular intervals during each experiment, but the asymptotic behavior was reproducible across experiment days for both C4H4O+ and C6H4O2+, an alternative tracer for TCA. Because electron impact is a hard ionization technique, the residual signals could originate from those oxidation products that form C4H4O+ and C6H4O2+ fragments in the mass spectrometer (see molecules drawn in blue and red in Scheme 1). The AMS mass spectra before and after oxidation are shown in Figure S1a. The most prominent change after reaction is an increase in CO+ and CO2+ signals, which indicates formation of ketones and carboxylic acids. Additional insight from a softer ionization technique, such as chemical ionization mass spectrometry, is needed to determine whether unreacted TCA remains at high OH exposure. If the heterogeneous reaction of OH(g) with TCA(aq) truly does not reach completion, mass transport and morphology may be the limiting factors. The reacto-diffusive length l characterizes how far OH radicals can diffuse into the particles before reacting with TCA. For the TCA particles in this study, l < 2 nm:

(∑ SO4 )AS jij Org zyz ji Org zyz ji Org zyz jj z z z × = jjj − jjj j ∑ SO zz j ∑ SO zz j ∑ SO zz (∑ SO4 )TCA 4 {corrected 4 {TCA 4 { AS k k k (1)

Here,

( ) Org ∑ SO4

represents the background-corrected

corrected

ratio of total organic to total sulfate signals in TCA particles,

( ) Org ∑ SO4

represents the observed organic-to-sulfate ratio in

TCA

TCA particles,

( ) Org ∑ SO4

l=

represents the observed organic-to-

AS

sulfate ratio in AS particles, and

(∑ SO4 )AS (∑ SO4 )TCA

(2)

is a scaling factor to

DOH kaq[TCA]

(3)

Here, DOH = 2.2 × 10−5 cm2 s−1 is the aqueous-phase diffusion coefficient of OH,55 and kaq = 3.1 × 108 M−1 s−1 is the rate constant of the aqueous-phase reaction of OH with TCA.29 The concentration of TCA is given by [TCA] = (mfs)(ρ)/ MW, where mfs is the average mass fraction of solute, ρ is the

account for variations in collection efficiency and lens transmission. This correction was performed for all control and oxidation experiments and for all particle types (AS, SDS, and mixed). D


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6.1 × 104 cm s−1 is the mean gas-phase speed of OH. Because the aerosol population is polydisperse, D was calculated as a weighted average across all size bins of the SMPS area distribution. The average particle density (ρ = 1.33 g mL−1) and average mass fraction of TCA (mfs = 0.56) were determined from a combination of AMS measurements and E-AIM calculations. The initial rate constant k0 is given by

average particle density modeled with E-AIM, and MW is the molecular weight of the solute. Although the reacto-diffusive length is short in relation to the size of the particles, TCA molecules should require no more than a microsecond to diffuse from deep in the bulk to replenish the top 2 nm of a liquid particle. The characteristic time scale for diffusive mixing (τD) is given by36 τD =

r2 DTCA π

2

k0 = −

(4)

2DρNA(mfs)k 0 3MW cOH ̅

(6)

As shown in Figure 3b, γ = 3.0 ± 1.5 for the uptake of OH on TCA particles. The observed uptake coefficient depends not only on reaction and partitioning but also on mass accommodation and gas-phase diffusion.33 We assume that mass accommodation does not limit uptake.64 After applying the Fuchs−Sutugin correction for gas-phase diffusion,65 we find that the reactive uptake coefficient is 3.5. The 16% correction for gas-phase diffusion is a relatively minor contribution to the overall uncertainty in γ. Indeed, AMS measurements typically characterize organic mass concentrations in ambient aerosol to within ±38%,66 though the uncertainty in our flow tube experiments is lower because normalization accounts for variations in collection efficiency and lens transmission and because the relative ionization efficiency of TCA was independently verified. Other systematic errors in our calculation of γ stem from SMPS and RH measurements: conditions in the flow tube are above the deliquescence point of the particles, so uncertainties in relative humidity affect water uptake, which in turn affects particle composition modeled by E-AIM. For example, changing the relative humidity in the model from 80% to 90% causes a 30% decrease in γ. Overall, we conservatively estimate an uncertainty of ±50% in the calculated uptake coefficient. We can now interpret the reactive uptake coefficient in the context of the questions raised in the Introduction. In particular, the uptake coefficient exceeds unity. This finding implicates secondary chemistry, likely radical chain reactions. Indeed, the proposed mechanism for the aqueous-phase reaction of TCA with OH proceeds via peroxy radical intermediates,29 which compete with OH to react with TCA. As a result, each collision of OH consumes more than one TCA molecule; in other words, when one OH radical strikes the particle, three TCA molecules are lost. Our results align with earlier experiments from Kolesar et al., who found uptake coefficients as high as 2.8 for heterogeneous reactions of OH on well-mixed particles containing squalane and α-pinene oxidation products, for which TCA is a proxy.67 The reactive uptake coefficient offers further insight into the competing time scales that control the lifetime of TCA molecules in organic particles. The characteristic lifetime due to bulk-phase reaction is estimated to be between 0.9 and 900 h for steady-state dissolved OH concentrations of 10−12−10−15 M.29 In comparison, only 32 h is required for a TCA molecule in the particle to react with OH radicals from gas-phase collisions. This time scale is given by

where r = 50 nm is the mode radius in the volume-weighted particle size distribution, and DTCA = 5 × 10−6 cm2 s−1 is the aqueous-phase diffusion coefficient of TCA (approximated from that of citric acid).56 Based on the submicrosecond value of τD, the diffusion of “fresh” TCA molecules from the bulk to the surface should occur fast enough for OH to react with all TCA molecules in the particle, especially when OH exposure is high. It is possible that particle viscosity reduces diffusion rates, but aerosol mass spectrometry does not directly probe particle morphology. If the particle-phase TCA concentration exceeds the solubility limit, then TCA will not be evenly distributed throughout the particles. The estimated experimental concentration of TCA is 520 g L−1, which is ∼60% greater than the measured solubility of TCA in pure water at 291 K.57 No experimental measurements are available at higher temperatures, but EPI Suite predicts that the solubility of TCA in pure water at 298 K should be ∼1000 g L−1, roughly double the concentration of TCA in our experiments.58 However, the “salting out” effect may limit the solubility of TCA in the presence of ammonium sulfate. The TCA particles measured in this work contain 2.8 m (NH4)2SO4 at pH = 2.2 (averages determined from AMS measurements and E-AIM calculations). The highly acidic conditions further reduce the solubility of TCA, which should be uncharged at such a low pH given its pKa values of 3.47, 4.54, and 5.89.59 Overall, we conclude that the TCA particles could be phase-separated, or they could exist as supersaturated metastable solutions. We note here that the AMS data do not resolve single particles, so any discussion of particle composition and properties, including pH, must be restricted to ensemble averages. For example, particle size affects acidity,60 but the modeled pH values listed in Table S1 do not take into account the distribution of particle sizes produced by the atomizer. Furthermore, the aerosol generated is polydisperse not only in size but also in composition: Single-particle measurements have shown that atomizing multicomponent solutions creates a range of particle compositions.61−63 Given instrumental limitations, we rely on ensemble averages as simplified metrics of aerosol properties across the entire distribution of particles. 3.1.2. Reactive Uptake. Apart from the long-time behavior of the TCA + OH reaction, the chemical kinetics can be usefully characterized in a quantitative manner by evaluating the kinetics of the initial uptake process, as shown in Figure 3b. At low OH exposure, the observed uptake coefficient γ is given by36,42 γ=

d(I /I0) d([OH]t )

τ= (5)

NTCA 1 γAcOH ̅ [OH] 4

(7)

where NTCA is the total number of TCA molecules in the particle, A is the area of the particle, and [OH] = 1 × 106 molecules cm−3 is the typical ambient concentration of OH radicals. Thus, heterogeneous reaction with OH is an

where D is the mode diameter of the particles, ρ is the average density of the particles, NA is Avogadro’s number, k0 is the initial first-order rate constant at low OH exposure, and cO̅ H = E


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The Journal of Physical Chemistry A important loss pathway for TCA in organic particles, potentially larger than the pathway via bulk reaction with dissolved OH radicals formed photolytically or via Fenton chemistry. 3.2. SDS Aerosols. For comparison with the TCA reaction system, particles containing SDS and AS were oxidized in the TPOT according to the same experimental procedure. Surface tension measurements for SDS solutions saturated with ammonium sulfate have shown that a single monolayer (ML) of SDS contains ∼2.5 × 1014 molecules cm−2.68 To develop a meaningful definition of SDS surface coverage in a polydisperse aerosol population, the overall fractional surface coverage (Θ) was calculated as a weighted average across all size bins in the SMPS distribution:16,24 ∑i θiNi

(9)

Figure 4. Normalized decay of the C12H24+ signal due to heterogeneous photooxidation of SDS particles. The effective second-order rate constant k was determined from the exponential fit (solid black line) to the experimental data (gray diamonds). The observed uptake coefficient γ was determined from the linear fit (dashed red line) to the data at low OH exposures.

where θi is the fractional surface coverage of the particles in size bin i and Ni is the number of particles in size bin i. The assumption that particles of all sizes contain the same mass fraction of SDS is an oversimplification; however, Θ provides a single convenient number that is useful to characterize the aerosol distribution as a whole. The SDS particles in these experiments contain a relatively high mass fraction of SDS (average mfs = 0.24), which is the maximum concentration that could be achieved without excessive frothing in the atomizer. We estimate that this mass fraction corresponds to an overall surface coverage Θ of ∼3.4 monolayers. Values of surface coverage greater than one indicate that SDS could form multiple layers at the surface of the particles, or perhaps even micelles. Again, overall surface coverage Θ is a single parameter that does not capture the complexity of diverse particle sizes and compositions. The calculated surface coverages θ range from extremes of θ = 0.5 (for the 0.1% of the particle population with D ≈ 10 nm) to θ = 13 (for the 0.1% of the particle population with D ≈ 230 nm). Figures S3 and S4 display the distribution of fractional surface coverages for all particles based on size measurements by SMPS, under the assumption that particle composition is relatively consistent across the population. SDS reacts rapidly with OH with a second-order rate constant of (4.4 ± 0.4) × 10−11 cm3 molecule−1 s−1, as shown in Figure 4 for the decay of the C12H24+ signal. C12H24+ was selected as the tracer ion for SDS because the organic background was negligible at this m/z, and there was no interference with TCA. Just as with the tracer signals for TCA, the SDS tracer signal may be artificially high as the reaction progresses if higher molecular weight products fragment into C12H24+ in the mass spectrometer. In fact, Huang et al. predominantly observed functionalization products during the heterogeneous photooxidation of SDS with OH exposures ranging from 6 × 1010 to 2 × 1011 molecules cm−3 s.22 We detect many small oxygenated fragments (see Figures S1c and S2c) and an overall increase in the average oxidation state of carbon as OH exposure increases, but we do not detect appreciable products with m/z > 168 (Figure S2c). In Figure 4, the C12H24+ signal almost completely disappears for [OH]t > 2 × 1011 molecules cm−3 s; this observation suggests that few functionalization products contribute to the C12H24+ signal.

Figure 4 presents the linear fit to the initial rate of reaction, which was used to calculate γ according to eq 5. The observed uptake coefficient of OH into the SDS particles exceeds unity, and the reactive uptake coefficient was found to be 3.9 ± 2.0, or 4.5 with the correction for gas-phase diffusion. These high values implicate secondary chemistry in the heterogeneous reaction mechanism, as in the case of TCA. 3.3. Mixed Aerosols. In mixed particles, TCA and SDS compete to react with OH. SDS is more surface active than TCA and reacts with OH approximately twice as fast as TCA does (see Figures 3a and 4); thus, even small amounts of SDS can protect TCA from oxidative aging by scavenging OH radicals at the surface of the particles. In the absence of SDS, ∼25% of TCA molecules appear not to react even at high OH exposures, as discussed above. This fraction increases with increasing SDS coverage, as shown by the asymptotes of the exponential fits in Figure 5a. In the case of Θ = 0.86, nearly 50% of TCA molecules remain unreacted. This change could have dramatic implications for the concentration of organic molecules in surfactant-coated atmospheric aerosols and for organic-coated cloud droplets as well. The rate of C4H4O+ consumption decreases with increasing SDS coverage over the tested range from Θ = 0 to Θ = 1.3. Once the SDS concentration reaches monolayer coverage, the rate constant for the reaction of TCA with OH has decreased by about 60% relative to that in the absence of SDS. The effect is so strong that SDS slows TCA oxidation by 40% even when the fractional surface coverage is only 0.34 and the mole ratio of dodecyl sulfate to tricarballylic acid is only 0.04 (see Figure 5b). Whereas the C4H4O+ signal in Figure 3a continues to slowly decrease at high OH exposures, SDS reacts so rapidly in the mixed aerosol that the C12H24+ signal in Figure 6 is depleted before the OH exposure exceeds 1 × 1011 molecules cm−3 s. The reaction rate of SDS exhibits a slight dependence on surface coverage (see the inset in Figure 6). This finding agrees with earlier studies showing that thinner films are oxidized more rapidly than thicker films for squalene-coated ammonium sulfate particles.69 The ability of SDS to protect TCA from OH at OH exposures that lead to complete SDS decay is presumably due to oxidation products that are also surfactants.

Θ=

θ=

∑i θi (mfs)ρDNA 6 × ML × MW

(8)

F


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Figure 5. (a) Normalized decay of C4H4O+ signals due to heterogeneous photooxidation of TCA in particles containing varying fractional surface coverages (Θ) of SDS. The filled circles are experimental data points for particles containing TCA and SDS, and the solid lines are exponential fits to these data. The dashed line is the exponential fit from Figure 3a for the reaction of TCA in the absence of SDS. (b) Rate constants k determined from the exponential fits in (a) normalized by the rate constant k0 in the absence of SDS.

Figure 6. Normalized decay of C12H24+ signals due to heterogeneous photooxidation of mixed particles containing varying fractional surface coverages (Θ) of SDS. The filled, colored diamonds are experimental data for particles containing SDS and TCA. The open, gray diamonds are experimental data for particles containing SDS but not TCA. The lines are exponential fits to the measured data points. The inset shows how the rate constant k, determined from these fits, varies with SDS surface coverage.

react with TCA. Once organoperoxy radicals and firstgeneration oxidation products form, the reaction pathways expand. Figure 7B shows how reactive intermediates can (d) react directly with OH at the surface, (e) react with OH after diffusion at the surface, (f) react with OH after diffusion into the bulk, (g) react with TCA, or (h) react with other intermediates. These additional pathways in panel B demonstrate how the reactive uptake coefficient can exceed one in the OH + TCA system. The reaction of OH with TCA in the mixed particles (Figure 7C−E) is predominantly controlled by the structure of surfactants at the air/water interface. At low surface coverages (panel C), SDS molecules form a loosely packed, disordered surface layer. Some incident OH radicals react with the exposed hydrocarbon tails, whereas other OH radicals penetrate into the particle to react with TCA. Alkyl radicals that form from the reaction of OH with SDS could also oxidize TCA molecules below the surface. As SDS surface coverage increases (panel D), SDS molecules form a more tightly

In particular, functionalization of SDS may occur, forming more oxidized products. However, those products are likely surfactants themselves, able to coat the particles and react with incoming OH radicals (or with TCA deeper below the surface). The heterogeneous reaction of gaseous OH with organic particles containing both TCA and SDS constitutes a highly complex system. Reactive intermediates from SDS and TCA oxidation can contribute to a complicated web of secondary reactions, dependent in part upon the distribution of molecules in the particles. Our most significant finding is that low concentrations of surfactants effectively shield inner molecules from reaction when the reacto-diffusive length is short. 3.4. Mechanistic Interpretation. Figure 7 presents a stylized interpretation of OH uptake and reaction in TCA and in mixed particles. Figure 7A depicts initial collisions of gasphase OH with TCA particles: OH radicals can (a) directly collide and react with TCA at the surface, (b) diffuse and react with TCA near the surface, or (c) diffuse into the bulk and G


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Figure 7. Possible collision partners with OH at the surface of TCA particles (panels A and B) and mixed particles containing TCA and SDS (panels C−E). SDS oxidation products can also react with OH, TCA, SDS, and other reactive intermediates in the interfacial region. SDS oxidation products, water molecules, and ammonium, sulfate, and sodium ions are omitted for clarity.

packed, ordered monolayer coating the surface of the particle. However, TCA is not completely blocked from reacting (as seen in Figure 5a), indicating that some OH radicals penetrate through patches at the surface and encounter OH. In addition, some TCA molecules may be consumed by reaction with intermediates of the OH + SDS reaction. Finally, at very high SDS concentrations (panel E), SDS molecules may begin aggregating into micelles that exclude TCA. The critical micelle concentration for SDS in pure water is 8 mM, but that value drops to micromolar levels in saturated AS solutions.68,70 For comparison, the estimated analytical concentration of SDS is 0.6 mM in the particles with Θ = 1.3 in these experiments. Indeed, micelle formation could explain why the OH + TCA reaction is not completely blocked for particles with Θ > 1.

oxidation products also sitting at the gas−liquid interface, able to react with OH. TCA and SDS are representative compounds, but the results reported here likely have far-ranging implications because surfactants are so common on organic particles and cloud droplets.26 Field measurements of fine particulate matter at isolated locations around the world suggest that typical concentrations of surfactants are ∼100 pmol m−3,71−73 which should be more than sufficient to alter heterogeneous oxidation. It will be interesting to see if such surfactants have comparable effects to the well-ordered surface layers that SDS can form. The potential for organic surfactants to impede the uptake of OH radicals from the gas phase to aerosol particles and cloud droplets has been proposed for decades.26,27 To our knowledge, this is the first quantitative demonstration that such shielding proceeds. The overall results are that rapid oxidation of the surfactants occurs, but highly soluble species that do not reside at the particle/droplet interface have a longer oxidative lifetime than in the absence of surfactants. This effect is likely to lengthen the atmospheric lifetime of soluble environmental pollutants.

4. CONCLUSIONS We have shown that particle-phase tricarballylic acid readily reacts with gas-phase OH radicals, with an uptake coefficient >1 on a time scale competitive with and probably faster than aqueous-phase oxidation via dissolved OH. Most importantly, adding soluble surfactant to the aerosol dramatically protects TCA from heterogeneous oxidation. When the SDS:TCA mole ratio was only 0.04:1, corresponding to a fractional surface coverage of ∼0.34, the rate of TCA oxidation was already 40% slower than in the absence of SDS. A full monolayer of SDS reduced the loss rate of TCA by 60% relative to the loss rate in the absence of surfactant. Moreover, when a monolayer of SDS was present, ∼50% of the original TCA signal measured by aerosol mass spectrometry remained unreacted even at OH exposures corresponding to 6 days under ambient conditions. SDS was particularly effective at protecting TCA because SDS reacts more rapidly with OH than TCA does. As well, the protective effect from SDS is present even for OH exposures that lead to full SDS loss. This is likely due to the SDS

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.9b00167.

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Average particle properties; E-AIM parametrizations for TCA; estimation of gas-phase diffusion coefficient of OH; Acuchem model input; normalized mass spectra; distribution of surface coverages (PDF)

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Jennifer A. Faust: 0000-0002-2574-7579 Jonathan P. D. Abbatt: 0000-0002-3372-334X Present Address

J.A.F.: Department of Chemistry, College of Wooster, Wooster, OH 44691. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the Natural Sciences and Engineering Research Council of Canada. REFERENCES

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Organic Surfactants Protect Dissolved Aerosol Components against

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