Evolution in the reactivity of citric acid towards heterogeneous

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Evolution in the Reactivity of Citric Acid toward Heterogeneous Oxidation by Gas-Phase OH Radicals Man Mei Chim,† Christopher Y. Lim,‡ Jesse H. Kroll,‡,§ and Man Nin Chan*,†,⊥ Earth System Science Programme, Faculty of Science and ⊥The Institute of Environment, Energy and Sustainability, The Chinese University of Hong Kong, Hong Kong, China ‡ Department of Civil and Environmental Engineering and §Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States Downloaded via IMPERIAL COLLEGE LONDON on December 8, 2018 at 03:24:50 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

S Supporting Information *

ABSTRACT: Oxidation initiated at or near a particle surface by gas-phase oxidants can continuously change the composition and properties of organic particles, which in turn alter the heterogeneous reactivity over time. However, chemical transformation of organic particles by heterogeneous oxidation is typically described by a single kinetic parameter (effective OH uptake coefficient, γeff), which implicitly assumes the reactivity does not change significantly over their atmospheric lifetimes. Using time-resolved particle composition and size data measured in an environmental chamber, it is shown that the heterogeneous reactivity of citric acid toward gas-phase OH radicals continuously decreases over reaction time and slows down by 16% from an initial γeff of 1.61 ± 0.16 to 1.35 ± 0.14 after oxidation equivalent to about 2 days of OH exposure. The decrease in the γeff over time can be explained by consumption of citric acid, and its concentration at the particle surface drops due to OH oxidation and the formation of reaction products during oxidation. This lowers the reactive collision probability between citric acid and gas-phase OH radicals at the gas−particle interface, leading to a smaller overall reactivity. The results suggest that the use of a single kinetic parameter could overpredict the heterogeneous OH oxidative loss rate of citric acid and other organic compounds over their atmospheric time scales. This study highlights the importance to consider the changes in particle composition upon oxidation when evaluating the evolution of the heterogeneous reactivity of organic compounds. KEYWORDS: effective uptake coefficient, heterogeneous oxidation, citric acid, hydroxyl radical, oxygenated organic aerosols

1. INTRODUCTION Organic particles can chemically transform throughout their atmospheric lifetimes by colliding with gas-phase oxidants, such as hydroxyl (OH) radicals, ozone, and nitrate radicals.1−3 Oxidation of organic compounds with gas-phase OH radicals at the gas−particle interface is an efficient chemical aging process, which can alter the size, optical properties, volatility, and hygroscopicity of the particles.4−7 Understanding the dynamics associated with heterogeneous oxidation is thus important to understand the evolution of composition and physicochemical properties of atmospheric organic particles, yet this aging process remains poorly understood. The heterogeneous reaction rates can be described by a reactive uptake coefficient (γ), which reflects the fraction of OH surface collisions that leads to a reaction.1−3 The OH uptake coefficient (γOH) describes the loss of OH in the gas phase to the particle surface. At the maximum rate, the OH radical can be removed upon each collision, and γOH ranges from 0 to 1. Alternatively, the heterogeneous reaction can be determined by measuring the decay of a target species to obtain an effective OH uptake coefficient (γeff). γeff takes all processes that consume the target species into account. For instance, when secondary radical chain reactions occur in the © XXXX American Chemical Society

particle phase, more than one target species can be consumed per OH collision, and thus γeff can be larger than one.3 During heterogeneous oxidative processes, organic compounds can be oxidized to form a variety of reaction products, which exhibit a range of polarity, volatility, and hygroscopicity through functionalization and fragmentation pathways.8 The amount of particle-phase water can also vary in response to the change in the particle composition upon oxidation.9,10 As the size, composition, and properties of organic particles continuously evolve upon oxidation, heterogeneous kinetics and chemistry can change over time. However, the reactivity of organic compounds toward gas-phase oxidants upon heterogeneous oxidation has been commonly described by a single quantity of rate constant (e.g., γeff) over time. This implicitly assumes that the reactivity does not significantly change during oxidation. Furthermore, the initial particle state (e.g., size and composition) prior to oxidation is always used to calculate the reactivity (i.e., γeff), which may not well represent the Received: Revised: Accepted: Published: A

August 25, 2018 November 1, 2018 November 15, 2018 November 15, 2018 DOI: 10.1021/acsearthspacechem.8b00118 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

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ACS Earth and Space Chemistry

reaction time) of 2.2 × 1011 molecules cm−3 s, which is equivalent to about 2 days of OH exposure assuming a 24 h averaged OH concentration of 1.5 × 106 molecules cm−3. To account for the wall-loss effect, first-order decay rate constants were determined for the particle organic mass and particle size in separate experiments under similar conditions but with the UV lights remaining off (i.e., without oxidation). The wall-loss rate constant for the particle organic mass was determined from the normalized decay in particle organic mass loading measured by the AMS.14 The wall-loss effect of particle size was determined by fitting a first-order wall-loss rate constant for each particle size bin in the SMPS data. The wallloss effect for the oxidation experiment was then corrected for both particle organic mass and particle size using the fitted decay rate constants (Figure S2). The bulk particle elemental composition (e.g., O/C ratio) was determined from AMS data using the parametrization developed by Canagaratna et al.15 The phase state of the particles can play a key role in controlling the reactivity.16−18 Citric acid particles have been shown to absorb and desorb water reversibly without phase transition,19 suggesting that the particles are present as liquid droplets at 40% RH. Citric acid droplets are known to be viscous at low RH.20 High particle viscosity slows down the diffusion of species within the particle, which can lead to an inhomogeneous distribution of the species.21−23 To examine the viscosity effect on the reactivity, the characteristic time for diffusion of citric acid (τD) is determined:24,25

composition of the particles after oxidation. This leads to the question of whether a single kinetic parameter could accurately describe the heterogeneous reactivity of organic compounds toward gas-phase OH radicals. This study investigates the evolution in reactivity of citric acid particles toward gas-phase OH radicals upon heterogeneous oxidation using a small environmental chamber. Citric acid (C6H8O7) is chosen for its low volatility and for its importance as a surrogate for highly oxygenated organic compounds.11,12 An Aerodyne aerosol mass spectrometer (AMS) and a scanning mobility particle sizer (SMPS) are employed to obtain the real-time information about the composition and size of citric acid particles, respectively, during oxidation. With the time-resolved particle size and composition data, the change in the heterogeneous reactivity of citric acid toward gas-phase OH radicals over the course of reaction is determined.

2. EXPERIMENTAL METHOD 2.1. Chamber Experiment. Experiments were conducted in a 150 L PFA environmental chamber, held at 20 °C and a relative humidity (RH) of 40%. Prior to the experiment, the chamber was first flushed with purified air passing through silica gel, activated carbon, and HEPA filters to remove water, volatile organic compounds, and particles, respectively. Citric acid particles were generated by atomizing an aqueous solution at 0.1 wt % through a constant-output atomizer (TSI) for 2 min. Ozone was produced using a mercury pen-ray lamp (Jelight) at 185 nm and was flowed into the chamber for 1 min. Ozone concentration, which controls the level of OH concentration inside the chamber, was measured to be about 150 ppb using an ozone monitor (2B Technologies). Before oxidation, citric acid particles, two tracer gases (hexafluorobenzene used as a dilution tracer and isopentane used as an OH tracer), and ozone were introduced into the chamber and allowed to be well-mixed for 15 min. The oxidation was initiated by the photolysis of ozone using UV light at 254 nm from two mercury lamps (UVP, LLC) positioned outside the chamber. O(1D) generated by ozone photolysis reacted with water vapor to form a pair of OH radicals. Kessler et al.11 has shown that citric acid is not directly photolyzed by UV light and that generation of particle-phase oxidants is negligible. Particle size and number distributions were measured with a SMPS (TSI) with time resolution of 2 min. Nonrefractory particle chemical composition was monitored with an AMS (Aerodyne Research Inc.) with a time resolution of 1 min. As air was continually drawn from the chamber for particlephase and gas-phase measurements, a fresh purified air flow was introduced to the chamber to balance the flow. The air dilution rate was determined by the measurement of hexafluorobenzene, which has a very slow reaction rate against OH and ozone oxidation and assumed to be chemically inert. After correcting for chamber dilution, the decay of isopentane was used to quantify OH concentration, using a rate coefficient of 3.65 × 10−12 cm3 molecule−1.13 The concentration of the two tracer gases was measured by a gas chromatograph coupled with a flame ionization detector (SRI GC 8610). In this work, the OH concentration was measured to be 1.9 × 108 molecules cm−3 at the beginning of the experiment and decreased to 1.1 × 108 molecules cm−3 near the end of the experiment (Figure S1). The time-averaged OH concentration was 1.4 × 108 molecules cm−3. The experiments lasted for 34 min with a maximum OH exposure (= OH concentration ×

τD =

d p2 4Dorg π 2

(1)

where dp is the diameter of the citric acid particle and Dorg is the diffusion coefficient of citric acid in the particle, which is estimated to range from 5 × 10−12 to 6 × 10−12 m2 s−1 for 40% RH.26 The time scale for diffusive mixing of citric acid is estimated to be 0.16−0.19 ms. The time between collision events for the citric acid and gas-phase OH radicals can be estimated from the collision frequency of gas-phase OH radicals with the particle surface: Jcoll ≅

[OH]cOHA 4

(2)

where cOH is the mean thermal velocity of OH radicals and A is the surface area of the citric acid particle. The time between each collision event (1/Jcoll) is estimated to be about 0.24− 0.52 ms. As the diffusive mixing time and collisional lifetime are on the same order of magnitude, as a first approximation, citric acid could be assumed to be reasonably well mixed within the particles for the experimental conditions in this study. It also acknowledges that when the diffusion time scale is much larger than the collision frequency, the heterogeneous reactivity characterized by the decay of the citric acid could be reduced due to the slow diffusion of the citric acid to the particle surface for oxidation (i.e., the overall oxidation is diffusion-controlled). Furthermore, this would lead to an inhomogeneous distribution of the citric acid and the oxidation products within the particles. For instance, citric acid near the particle surface would be more subjected to OH oxidation. The surface concentration of citric acid would be lower than that in the bulk phase, whereas the oxidation products would be accumulated at the surface once they form upon oxidation. B

DOI: 10.1021/acsearthspacechem.8b00118 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

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3. RESULTS AND DISCUSSION 3.1. Oxidation Kinetics. The time-resolved reactive loss of citric acid due to heterogeneous OH oxidation can be quantified by the measurement of a marker ion, C4H4O+ (m/z = 68), from the high-resolution AMS mass spectrum (Figure 1a). Kessler et al.11 has shown that the fragment

ln

It = − k[OH]Δt I0

(4)

where I0 is initial mass concentration of citric acid before oxidation, It is the mass concentration of citric acid at a reaction time t, and [OH] is the concentration of gas-phase OH radicals. We would like to note that eq 4 can also be used to describe the reactions between OH radicals and citric acid occurring in the gas phase. In our study, the k value derived from eq 4 should be considered as an effective heterogeneous OH rate constant because the reactions occur at the particle/ gas interface (a heterogeneous system) but do not occur in the gas phase (a homogeneous system). Figure 1c shows the normalized decay of the parent citric acid particles against OH exposure. The k value is determined to be (1.58 ± 0.01) × 10−12 cm3 molecule−1 s−1. Although the OH decay rate of citric acid did not change significantly and can be described by a single k value over the course of the experiment, the determination of k can be affected by the particle size and composition,28 which continuously change over reaction time (Figure 2).

Figure 1. Experimental results showing the time evolution of the (a) normalized mass of marker ion C4H4O+, (b) normalized wall-lossand dilution-corrected particle organic mass, and (c) normalized parent decay of citric acid particle as a function of OH exposure upon heterogeneous OH oxidation.

C4H4O+ ion is a good tracer for citric acid as it does not constitute a significant portion of the individual mass spectra of the oxidation products. The mass concentration of citric acid (Ij) can be determined by the fractional contribution of the marker ion to the total organic ions.11 Ij =

ij i total

mOA

Figure 2. Experimental results from AMS and SMPS showing the time evolution of (a) O/C ratio and (b) mean surface-weighted diameter of citric acid particles upon heterogeneous OH oxidation.

3.2. Effective OH Uptake Coefficient. To take into account the impact of variation in particle size and composition on the heterogeneous reactivity upon oxidation, the effective OH uptake coefficient, γeff, is a more useful parameter to evaluate the change in kinetics. With the timeresolved particle size and composition data, the change in the γeff over reaction time can be determined:26

(3)

where ij is the peak signal of the fragment marker ion, itotal is the sum of all organic peak signals, and mOA is the wall-lossand dilution-corrected particle organic mass, which has a decreasing trend, as shown in Figure 1b. This suggests that the particle organic mass loss by the formation and volatilization of fragmentation products outweighs the particle mass gain by the functionalization processes during oxidation. The oxidation kinetics over the course of the experiment can be described by the effective heterogeneous OH rate constant (k), which can be fitted with an exponential function:11,27

γeff =

2d pρ mfsNAk 3M cOH

(5)

where dp is the mean surface-weighted diameter, ρ is the density of the particle, mfs is the mass fraction of unreacted citric acid to the total particle mass, NA is Avogadro’s number, C

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Figure 3. Change in (a) mass and (b) mass fraction of citric acid, reaction products and particle-phase water upon heterogeneous OH oxidation.

and M is the molecular weight of citric acid. It is noted that dp, ρ, and mfs are variables which change as a function of reaction time (and OH exposure). During oxidation, the change in particle diameter is provided by the SMPS; it decreases slightly from 217 to 194 nm with a geometric standard deviation of 1.3−1.4 (Figure 2). The composition of the particle (i.e., mfs) continuously changes upon oxidation (Figure 3). Using the particle organic mass (i.e., mass of citric acid and oxidation products) (Figure S2), the mass of unreacted citric acid (Figure 3) can be calculated using eq 4, assuming that the AMS collection efficiency does not change significantly during oxidation. It is acknowledged that we do not have evidence for this assumption. Further work is needed to verify this hypothesis. The total particle mass consists of particle organic mass and particle-phase water. As the AMS does not provide information on the aerosol water content, the amount of particle-phase water is estimated using the parametrization developed by Rickards et al.,29 which considers the correlation between the hygroscopicity parameter (κ, which describes the degree of hygroscopic growth of the particles) and the O/C ratio of organic particles: κ v = (0.190 ± 0.017)O/C − (0.0048 ± 0.0139)

Figure 2 shows the O/C ratio is relatively constant over time, with the measured value (∼1.07) before oxidation and is lower than the exact O/C value of citric acid (∼1.17). This underestimation has been reported in pure organic compounds in previous AMS studies.11 More recently, Canagaratna et al.15 have evaluated the accuracy of the AMS elemental analysis approach and have proposed that the elemental composition obtained from AMS mass spectra could be biased by the vaporization and ion fragmentation processes. In the AMS, organic molecules can decompose during the vaporization process to form molecules with elemental compositions that differ from the parent organic molecule. We would also like to note that the overall trend observed would remain unchanged, regardless of the correction factor used. As the O/C ratio changes slightly upon oxidation, the aerosol water content varies insignificantly and is estimated to be about 3.3−4.6 μg m−3 (Figure 3). The small decrease in the amount of particlephase water is likely attributed to the evaporative loss of water associated with the formation and volatilization of fragmentation products during oxidation. Figure 4 shows that the γeff decreases from an initial value of 1.61 ± 0.16 to 1.35 ± 0.14 at the maximum OH exposure. γeff > 1 suggests that each OH collision leads to a reaction that depletes more than one citric acid molecule, and secondary chemistry likely occurs in the particle phase. One possibility is that for the OH reactions with citric acid, alkoxy radicals resulting from the self-reactions of two peroxy radicals can react with neighboring unreacted citric acid molecules by intermolecular hydrogen abstraction and eventually produce the peroxy radicals. These peroxy radicals can react again with other peroxy radicals to regenerate alkoxy radicals. This propagates the radical chain reactions and increases the γeff by the secondary chemistry.31 As the reactions between the OH radical and citric acid are very fast, this suggests that the reactions likely happen quickly at the particle surface without diffusion of OH into the bulk. If OH reaction occurs only on the surface of the particles in our study, eq 5 is valid for describing the OH oxidation of citric acid without the need to evoke bulk phase parameters such as the diffusion coefficients of OH in the organic matrix. The γeff obtained in this study is about 3.6−4.4 times greater than the value (γeff = 0.37 ± 0.08) reported by Kessler et al.,11 in which the experiment was carried out under similar conditions (particle diameter of 200 nm and 30% RH) at a

(6)

where κv denotes the volume-based hygroscopicity parameter, which can be converted into the mass-based hygroscopicity parameter (κm) using the conversion from Mikhailov et al.:30 κm = κv

ρw ρd

(7)

where ρw and ρd are the density of water and the density of the dry particle, respectively. As a first approximation, the densities of oxidation products are assumed to be the same as that of citric acid and remain constant upon oxidation under 40% RH.26 The particle density is estimated using the additivity rule with the particle composition data (i.e., mfs) and is found to change insignificantly over time (Figure S3). At a given RH (water activity, aw = RH/100), the variation in the mass of particle-phase water (mw) in response to the change in particle composition can then be determined as follows: m 1 = 1 + κ m OA aw mw

(8) D

DOI: 10.1021/acsearthspacechem.8b00118 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

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acid particles become well-mixed under lower gas-phase OH concentrations. Citric acid is known to be viscous at low humidity.12 High aerosol viscosity can potentially reduce the diffusion of the species within the particle, which in turn influences the uptake and reaction rate of gas-phase oxidants such as OH radicals and ozone.21−23 Davies and Wilson26 have observed that the effective uptake coefficient of OH onto citric acid particles was correlated with a combined effect of particle viscosity and gas-phase OH concentration. As discussed in the experimental method (section 2.1), citric acid molecules are assumed to be reasonably well-mixed within the particles under our experimental conditions (OH = 108 molecules cm−3) because the diffusive mixing time and OH particle collisional time scale are estimated to be on the same order of magnitude. However, when the experiments were carried out at higher OH concentrations in flow-tube studies (with typical gas-phase OH concentrations on the order of 1010−1011 molecules cm−3), the time scale between collision events for gas-phase OH radicals and citric acid becomes much smaller than the diffusive mixing time. This would suggest that citric acid particles were not well-mixed and the OH oxidation was primarily limited at the particle surface.26 The results of this simple analysis indicate that, at lower OH concentrations, the loss of citric acid is expected to be faster due to the faster diffusion of citric acid from the bulk to the surface for oxidation within the time scale of each successive OH collision event. Overall, the larger γeff measured under lower OH concentrations reveals that heterogeneous oxidation of citric acid particles and possibly other organic compounds is expected to be faster than previously predicted. The effects of gas-phase oxidant concentrations on the heterogeneous reactivity of organic particles certainly deserve further study. It can be seen that the reactivity of citric acid toward gasphase OH radicals changes during aging. The decrease in the γ eff over time could be explained by the decreasing concentration of citric acid at or near the particle surface due to oxidation and the formation of reaction products during oxidation. This lowers the reactive collision probability between citric acid and gas-phase OH radicals at the gas− particle interface and thus reduces the overall reactivity. It is worthwhile to note that, in this study, the O/C ratio changes very slightly, and the aerosol water content predicted by the parametrization varies insignificantly during oxidation. The change in the particle composition upon oxidation is thus primarily attributed to the OH oxidation of citric acid and the formation of oxidation products over time. This also suggests that there is small change in hygroscopicity and cloud formation ability of the citric acid particles after oxidation. Although the contribution of particle-phase water to the total particle mass is small (Figure 3) and the particle hygroscopicity effect is not significant in this study, it would likely contribute to a greater effect in the change in heterogeneous kinetics if the experiments were carried out under higher RH conditions and longer oxidation times.5 The results also suggest that using the initial particle state may not be appropriate when quantifying the heterogeneous reactivity of organic compounds after oxidation. For instance, after about 2 days of OH exposure, the γeff could be overestimated by ∼16% when the initial particle size and composition data are used for calculating the kinetics (i.e., eq 5). Assuming a constant rate of decrease in γeff with increasing OH exposure, this overestimate in γeff could be as high as 50% after a week of heterogeneous oxidation. The result of this

Figure 4. Evolution of effective OH uptake coefficient (γeff) of citric acid against OH exposure upon heterogeneous oxidation.

higher OH radical concentration (1 × 109 to 3 × 1011 molecules cm−3) in a flow-tube reactor. Davies and Wilson26 have also reported a small γeff (0.055−0.092) for the OH reaction with citric acid when a higher OH radical concentration (1010−1011 molecules cm−3) was used in their flow-tube reactor coupled with direct analysis in real time (DART) mass spectrometry at RH less than 40%. (Figure S4). The larger γeff measured in this study could be attributed to the effect of gas-phase oxidant concentration on the kinetics. Such an effect has been observed in previous studies, with γeff generally found to increase with decreasing absolute gasphase OH concentration.31−34 This may result from the slower reactive OH uptake than the rate of adsorption of gas-phase OH radicals on the particle surface (i.e., Langmuir−Hinshelwood mechanism), the shielding effect of precursor gas (i.e., ozone) on the particle surface,31 the diffusion limitation of species to the surface for OH oxidation, 26 or some combination of these factors. For instance, Renbaum and Smith31 have suggested that high concentration of radical precursors may saturate the particle surface and thus affect the heterogeneous reaction rate. For OH oxidation, ozone is commonly chosen as the radical precursor. The presence of ozone may inhibit the rate of OH reaction by (1) the formation of an ozone adsorptive layer on the particle surface, which acts as a physical barrier and hinders the collision of OH radicals with the organic species at the particle surface, and (2) the reaction of ozone with OH radicals at the interface, which could consume the OH radicals. In this study, the maximum ozone concentration was 150 ppb at the beginning of the experiment; for the flow-tube studies, a much higher ozone concentration (hundreds of ppb or a few ppm) was commonly used. Whereas the atmospheric ozone concentrations rarely exceed 250 ppb, the higher ozone concentrations used in flowtube studies might significantly hinder the heterogeneous reaction rate, as suggested by Renbaum and Smith.31 Further experimental and modeling studies would be needed to investigate the effect of radical precursor concentration (e.g., ozone) on the heterogeneous reactivity of organic particles. Another possibility for the dependence of γeff on the gasphase OH concentration is the enhanced diffusion rate of organic species to the particle surface for oxidation as citric E

DOI: 10.1021/acsearthspacechem.8b00118 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

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ACS Earth and Space Chemistry simple analysis implies that the heterogeneous reactivity of citric acid toward gas-phase OH radicals in oxidized particles could be substantially slower than that in fresh particles, suggesting that the chemical lifetime of citric acid due to OH oxidation is expected to increase when the particles are chemically aged. The fact that γeff decreases with time could have important implications for the heterogeneous chemical loss of chemical species of interest (e.g., individual pollutants or chemical tracers, etc.). Assuming a constant value of γeff would lead to overpredictions in the extent of heterogeneous aging of such species and hence underpredictions in their particle-phase concentrations in chemical transport models.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Man Nin Chan: 0000-0002-2384-2695 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.M.C. and M.N.C. are supported by HKRGC Project ID 2191111 (ref 24300516). C.Y.L. and J.H.K. are supported by NOAA Grant NA16OAR4310112 and NSF Grant AGS1638672, and C.Y.L. is supported by the NSF Graduate Research Fellowship Program.

4. CONCLUSION This study provides experimental evidence that the heterogeneous reactivity of citric acid toward gas-phase OH radicals continuously decreases over the reaction time. This could be explained by the decrease in the concentration of citric acid at the particle surface due to OH oxidation and formation of reaction products during oxidation. These suggest that a constant value of kinetic parameter, such as γeff, may not always accurately represent the heterogeneous OH reactivity of citric acid and likely other organic compounds during atmospheric aging and would lead to overpredictions in the extent of heterogeneous aging of organic compounds in chemical transport models. Instead of the initial particle state, the variations in particle composition and size upon oxidation have to be considered in order to more accurately quantify the heterogeneous kinetics. Considering the evolution of the heterogeneous reactivity of organic compounds toward gasphase oxidants (e.g., OH, ozone, and nitrate radicals) in the models could bring a better prediction of particle-phase concentration of organic compounds and determine the chemical stability of organic compounds against heterogeneous oxidation in the atmosphere. The strong dependence of γeff on the gas-phase OH concentration observed in this study and in the literature highlights the importance of using ambient gas-phase oxidant concentration for heterogeneous oxidation experiments. Future research using larger environmental chambers that allow oxidation experiments to be carried out over longer reaction times (e.g., hours or a day), ideally under ambient OH concentrations, is warranted to better understand the evolution in reactivity of organic compounds toward gas-phase OH radicals upon heterogeneous oxidation. Further, chamber experiments with ambient OH concentration can prevent immediate gas-phase oxidation of volatile products by gasphase oxidants, thus allowing better quantification of the volatilization and fragmentation processes during heterogeneous oxidation.

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heterogeneous OH oxidation; dependence of effective OH uptake coefficient of citric acid particles on the gasphase OH concentration (PDF)

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REFERENCES

(1) George, I. J.; Abbatt, J. P. D. Heterogeneous Oxidation of Atmospheric Aerosol Particles by Gas-Phase Radials. Nat. Chem. 2010, 2, 713−722. (2) Kroll, J. H.; Lim, C. Y.; Kessler, S. H.; Wilson, K. R. Heterogeneous Oxidation of Atmospheric Organic Aerosol: Kinetics of Changes to the Amount and Oxidation State of Particle-Phase Organic Carbon. J. Phys. Chem. A 2015, 119, 10767−10783. (3) Chapleski, R. C., Jr.; Zhang, Y.; Troya, D.; Morris, J. R. Heterogeneous Chemistry and Reaction Dynamics of the Atmospheric Oxidants, O3, NO3, and OH, on Organic Surfaces. Chem. Soc. Rev. 2016, 45, 3731−3746. (4) Enami, S.; Hoffmann, M. R.; Colussi, A. J. Acidity Enhances the Formation of a Persistent Ozonide at Aqueous Ascorbate/Ozone Gas Interfaces. Proc. Natl. Acad. Sci. U. S. A. 2008, 105 (21), 7365−7369. (5) Cappa, C. D.; Che, D. L.; Kessler, S. H.; Kroll, J. H.; Wilson, K. R. Variations in Organic Aerosol Optical and Hygroscopic Properties upon Heterogeneous OH Oxidation. J. Geophys. Res. 2011, DOI: 10.1029/2011JD015918. (6) Lambe, A. T.; Ahern, A. T.; Williams, L. R.; Slowik, J. G.; Wong, J. P. S.; Abbatt, J. P. D.; Brune, W. H.; Ng, N. L.; Wright, J. P.; Croasdale, D. R.; et al. Characterization of Aerosol Photooxidation Flow Reactors: Heterogeneous Oxidation, Secondary Organic Aerosol Formation and Cloud Condensation Nuclei Activity Measurements. Atmos. Meas. Tech. 2011, 4 (3), 445−461. (7) Harmon, C. W.; Ruehl, C. R.; Cappa, C. D.; Wilson, K. R. A Statistical Description of the Evolution of Cloud Condensation Nuclei Activity during the Heterogeneous Oxidation of Squalane and Bis (2ethylhexyl) Sebacate Aerosol by Hydroxyl Radicals. Phys. Chem. Chem. Phys. 2013, 15, 9679−9693. (8) Kroll, J. H.; Smith, J. D.; Che, D. L.; Kessler, S. H.; Worsnop, D. R.; Wilson, K. R. Measurement of Fragmentation and Functionalization Pathways in the Heterogeneous Oxidation of Oxidized Organic Aerosol. Phys. Chem. Chem. Phys. 2009, 11 (36), 8005−8014. (9) Chim, M. M.; Chow, C. Y.; Davies, J. F.; Chan, M. N. Effects of Relative Humidity and Particle Phase Water on the Heterogeneous OH Oxidation of 2-Methylglutaric Acid Aqueous Droplets. J. Phys. Chem. A 2017, 121 (8), 1666−1674. (10) Chim, M. M.; Cheng, C. T.; Davies, J. F.; Berkemeier, T.; Shiraiwa, M.; Zuend, A.; Chan, M. N. Compositional Evolution of Particle-phase Reaction Products and Water in the Heterogeneous OH Oxidation of Model Aqueous Organic Aerosols. Atmos. Chem. Phys. 2017, 17 (23), 14415−14431. (11) Kessler, S. H.; Nah, T.; Daumit, K. E.; Smith, J. D.; Leone, S. R.; Kolb, C. E.; Worsnop, D. R.; Wilson, K. R.; Kroll, J. H. OHinitiated Heterogeneous Aging of Highly Oxidized Organic Aerosol. J. Phys. Chem. A 2012, 116 (24), 6358−6365.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsearthspacechem.8b00118. Gas-phase OH concentration in the environment chamber during heterogeneous OH oxidation of citric acid particles; change in particle organic mass measured by the AMS with and without particle wall-loss correction upon heterogeneous OH oxidation of citric acid particles; change in particle density upon F

DOI: 10.1021/acsearthspacechem.8b00118 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acsearthspacechem.8b00118 ACS Earth Space Chem. XXXX, XXX, XXX−XXX