Understanding the Impact of Relative Humidity and Coexisting Soluble

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Environmental Processes

Understanding the Impact of Relative Humidity and Coexisting Soluble Iron on the OH-Initiated Heterogeneous Oxidation of Organophosphate Flame Retardants Qifan Liu, John Liggio, Kun Li, Patrick Lee, and Shao-Meng Li Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 30, 2019

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Environmental Science & Technology

Understanding the Impact of Relative Humidity and Coexisting

Soluble

Iron

on

the

OH-Initiated

Heterogeneous Oxidation of Organophosphate Flame Retardants Qifan Liu, John Liggio,* Kun Li, Patrick Lee, and Shao-Meng Li Atmospheric Science and Technology Directorate, Science and Technology Branch, Environment Canada, 4905 Dufferin Street, Toronto, Ontario M3H 5T4, Canada *Corresponding

author.

Phone: 1-416-739-4840; e-mail: John. [email protected].

ABSTRACT 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

The current uncertainties in the reactivity and atmospheric persistence of particle-associated chemicals present a challenge for the prediction of long-range transport and deposition of emerging chemicals such as organophosphate flame retardants, which are ubiquitous in the global environment. Here, the OH-initiated heterogeneous oxidation kinetics of organophosphate flame retardants (OPFRs) coated on inert (NH4)2SO4 and redox-active FeSO4 particles were systematically determined as a function of relative humidity (RH). The derived reaction rate constants for the heterogeneous loss of tricresyl phosphate (TCP; k,TCP) and tris(2butoxyethyl) phosphate (TBEP; k,TBEP) were in the range of (2.69−3.57)×10–12 and (3.06−5.55)×10–12 cm3 molecules–1 s–1 respectively, depending on the RH and coexisting Fe(II) content. The k,TCP (coated on (NH4)2SO4) was relatively constant over the investigated RH range while k,TBEP was enhanced by up to 19% with increasing RH. For both OPFRs, the presence of Fe(II) enhanced their k by up to 53% over inert (NH4)2SO4. These enhancement effects (RH and Fe(II)) were attributed to fundamental changes in the organic phase state (higher RH lowered particle viscosity) and Fenton-type chemistry which resulted in the formation of reactive oxygen species, respectively. Such findings serve to emphasize the importance of ambient RH, the phase state of particle-bound organics in general, and the presence of coexisting metallic species, for an accurate description of the degradation kinetics and aging of particulate OPFRs in models used to evaluate their atmospheric persistence.

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INTRODUCTION

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Organophosphate flame retardants (OPFRs) are widely used as plasticizers, antifoaming agents, and additives in a variety of household and industrial products.1 In 2005, the global annual production of OPFRs was 1.5 million tons2 and is expected to increase into the foreseeable future, particularly with the gradual phase-out of brominated flame retardants.3 Since OPFRs do not covalently bind to the materials within commercial products, they are easily released into the environment via volatilization, dissolution, and abrasion.4 Consequently, OPFRs have been frequently detected in soil,5 sediment,6 surface water,7 seawater,8 snow,8 indoor dust,9 and ambient airborne particles10 over the past decade. Currently there are numerous reports of the global persistence of OPFRs in ambient air.10-14 Such investigated compounds include tricresyl phosphate (TCP), tris(2-butoxyethyl) phosphate (TBEP), triphenyl phosphate (TPhP), tris-1,3dichloro-2-propyl phosphate (TDCPP), tris(2-chloroethyl) phosphate (TCEP), and tris(chloroisopropyl) phosphate (TCPP), with the sum of all measured OPFR concentrations ranging from 69 to 7770 pg m–3 in samples collected in urban areas and polar regions. This is at least an order of magnitude higher than the concentration of brominated flame retardants in ambient air.10,11 Given that all of the above OPFRs have the potential to induce adverse human health effects4,15,16 and some of them (e.g. TCPP) may degrade ecosystem resilience,15 it is essential to assess their environmental fates in order to understand the risks posed by these compounds, particularly for those transported through and deposited from the atmosphere.

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In the atmosphere, the fate of OPFRs present in the gas-phase is determined by OH-initiated oxidation reactions, with well-known measured and modelled chemical kinetics.17-21 However, many atmospheric OPFRs such as TCP, TBEP, TPhP, TDCPP, TCEP, and TCPP are predominantly associated with airborne particles,22-24 where their fate is not only dependent on the OH-initiated heterogeneous degradation kinetics, but also other components of the complex matrix in particles with which particle-bound OPFRs are associated. During their residence time in the atmosphere, the particle-bound OPFR degradation is likely affected by factors such as ambient relative humidity (RH), particle physical properties (e.g. phase state), and particle chemical composition, all of which have been demonstrated to be important in the transformation and transport of atmospheric organic aerosols (OA) in general.25-31 For example, organic aerosols may exist in different phase states, ranging from solid to semisolid and liquid particles in response to changes in ambient RH,30,31 thus affecting the heterogeneous reaction rates of aerosol components with trace gaseous species.28,29 In addition, the coexistence of metallic and organic species in ambient aerosols is also known to occur.32-34 The presence of metallic species such as iron, one of the most abundant transition metals observed in atmospheric aerosols (up to 5.22 µg m−3),32-36 may increase the overall oxidation rates of particle-bound organics via Fenton-type reactions that produce reactive oxygen species (ROS) such as particle-phase OH.37,38


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Very few studies have investigated the heterogeneous reactions between OH radicals and particle-phase OPFRs such as TBEP, TPhP, and TDCPP.39,40 The heterogeneous oxidation kinetics and thus atmospheric persistence of other commonly used OPFRs such as TCP, an important type of aryl OPFR often detected in indoor and ambient air,12,24,41 remain lacking. In addition, the two previous studies were conducted under constant RH conditions, and with chemically inert pre-existing seed particles ((NH4)2SO4).39,40 As noted above, ambient RH and coexisting particle-phase metallic species may alter particle-bound organics reactivities, thus neglecting the impact of such ambient conditions may bias the actual lifetimes of particlebound OPFRs in the atmosphere. This highlights a need to assess the effect of RH and coexisting particle-phase metallic components on the heterogeneous OH oxidation kinetics of particle-bound OPFRs. Such information is currently unavailable but is of critical importance for more accurately assessing and modelling the persistence and environmental risk of these emerging chemicals.

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In the current work, the OH-initiated heterogeneous oxidation kinetics of TCP and TBEP, serving as surrogates for hydrophobic and hydrophilic OPFRs respectively, were examined. These OPFRs were coated on inert (NH4)2SO4 and redox-active FeSO4 particles, as a function of RH from 35% to 68% to systematically investigate the impact of RH and particle composition (soluble iron) on the degradation kinetics of these OPFRs. Based upon the measured heterogeneous rate constants (k) of these two compounds with respect to OH under differing reaction conditions, more atmospherically relevant lifetimes for these compounds were determined.

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EXPERIMENTAL METHODS

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Heterogeneous OH Oxidation Experiments

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The heterogeneous reactions between OH radicals and TCP and TBEP coated onto (NH4)2SO4 and FeSO4 particles were performed in a photochemical oxidation flow tube reactor, using an experimental approach described previously39 and in the Supporting Information (SI; Figure S1). The (NH4)2SO4 and FeSO4 particles were generated via atomization (TSI, model 3706), dried through a diffusion drier (TSI, model 3062) and size-selected with a differential mobility analyzer (TSI, model 3081) to have a mode mobility diameter of approximately 95 nm. These monodispersed particles then passed through the headspace of a temperature-controlled Pyrex tube (333–343 K) containing either pure liquid TCP or TBEP, to generate coated particles with a calculated coating thickness of approximately 15 nm (calculated based upon the shift in the peak of the particle size distribution). The estimated iron concentration inside the flow tube reactor (~0.7 µg m–3) is comparable to the total mass concentration of iron in ambient aerosols,33,34 but is higher than the water-soluble iron concentration in ambient aerosols.35,36 The coated particles were introduced into a mixing vessel after having passed through an

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activated carbon denuder to remove volatile organics vapors from the flow. Based upon an evaporation model described previously,42 the evaporation of the OPFRs studied here was estimated to contribute less than 0.3% to the particle-phase loss of OPFRs within the residence time of the mixing vessel and flow tube reactor (see SI). This implies that the evaporation of particulate OPFR (TCP and TBEP) via the establishment of a new gas-particle equilibrium, would have little influence on the decrease of particulate OPFR concentration observed as a function of OH exposure.

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Upon achieving a steady-state concentration of OPFR and a stable RH under 254 nm UV light irradiation (Jelight, #82-3309-9) at 298 K in each experiment, O3 (Ozone Solutions, model TG10; 0−2 ppm) was introduced into the flow tube reactor. The RH in the reactor was constantly maintained at selected values in the range of 35−68% by varying the ratio of wet to dry air used as an air source (Figure S1). OH radicals were generated by the UV photolysis of O3 at 254 nm in the presence of this water vapor. The O3 concentration and RH were measured using an O3 analyzer (2B Technologies, model 202) and an RH sensor (Vaisala Inc., model HMP60), respectively. The size distribution of particles exiting the reactor was measured using a scanning mobility particle sizer (TSI, model 3936). The OPFR concentration and the overall O/C ratio43 of particulate organics were measured using an Aerodyne high-resolution time-offlight aerosol mass spectrometer (HR-ToF-AMS).44 In offline calibrations, the OH radical concentration was determined using CO as a tracer compound. The OH exposure was quantified by measuring the loss of CO via its reaction with OH at 298 K (1.54×10−13 cm3 molecules–1 s–1),45 and was in the range of 1.0×1011–1.5×1012 molecules cm–3 s. Control experiments demonstrated that O3 had no effect on the degradation of OPFRs (Figure S2). Further details regarding the photochemical oxidation flow tube reactor and OH exposure measurements are provided in SI. The chemical structures and physicochemical properties of TCP and TBEP are given in Table S1.

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Positive Matrix Factor (PMF) Analysis

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PMF analysis was performed to differentiate the HR-ToF-AMS signals of the parent OPFR from its corresponding oxidation products, to improve the accuracy of the rate constant determination (see below). The application of PMF has been described in general previously46 and in the context of these experiments in the SI. This approach has been widely used for source apportionment of ambient particles in field measurements47,48 as well as heterogeneous kinetics studies of laboratory generated OA40,49 due to its ability to separate the signals of a multicomponent matrix.

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Extended Aerosol Inorganics Model (E-AIM)

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To gain insight into the water content of OPFR particles investigated here, the E-AIM model (available at http://www.aim.env.uea.ac.uk/aim/aim.php)50 was used to predict the

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hygroscopicity of pure TCP and TBEP particles under differing RH conditions, as given in Figure S3. The measured hygroscopic growth of pure (NH4)2SO4 and FeSO4 particles are also provided in Figure S3.

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RESULTS AND DISCUSSION

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Degradation of OPFRs

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The chemical degradation of the particulate OPFRs can be observed from the HR-ToF-AMS measurements during a given experiment. The aerosol mass spectra of the unreacted, reacted, and the difference spectrum (reacted−unreacted) for TCP and TBEP coated on (NH4)2SO4 particles (denoted TCP@AS and TBEP@AS respectively) are shown in Figure 1 A−D. The fragmentation patterns measured with the HR-ToF-AMS for unreacted TCP and TBEP (black lines in Figure 1A and Figure 1C respectively) are comparable to those reported in previous studies (measured with GC-MS).51,52 However, the fragments of these two OPFRs, from both unreacted and oxidized products, are distinctly different from each other since they contain distinctly different functional groups (i.e. tolyl and alkyl groups for TCP and TBEP respectively; see Table S1).

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As shown in Figure 1A, significant signals at m/z channels 368, 261, 91, 77, and 65 for TCP are observable, which correspond to (C7H7O)3PO+, (C7H7O)2PO+, C7H7+, C6H5+, and C5H5+ respectively, while m/z 368 represents the molecular ion of TCP. When particulate TCP was exposed to OH radicals, the normalized intensities of the above fragments decreased substantially (Figure 1B), indicating that the degradation of TCP occurred. Conversely, fragments at m/z 384, 382, 275, 94, and 44 increased due to the presence of oxidation products and their associated fragmentation (i.e. (C7H7O)2(OC7H7O)PO+ (m/z 384), (C7H7O)2(OC7H5O)PO+ (m/z 382), (C7H7O)(OC7H5O)PO+ (m/z 275), C6H6O+ (m/z 94), and CO2+ (m/z 44)). The possible chemical structures of fragments at m/z 384, 382, and 275 are shown in Figure S4. Clearly, these three fragments are indicative of OH-addition to the phenyl ring and H-abstraction from methyl group by OH during the TCP oxidation process, consistent with the chemical mechanism of OH oxidation of toluene.53 In the case of TBEP, while its molecular-ion at m/z 398 cannot be detected with the HR-ToFAMS (Figure 1C) due to the high energy of electron impact ionization (70 eV), a decrease in the signals of other fragments including m/z 299, 227, 199, 57, 41, and 29 was observed with OH exposure (Figure 1D). These fragments correspond to + + (C4H9OC2H4O)2PO(OH2) ,(C2H5O)PO(OC2H4OC4H9)(OH2) ,(HO)PO(OC2H4OC4H9)(OH2)+, C4H9+, C3H5+, and C2H5+ respectively. Concurrently, oxidation of TBEP resulted in the appearance of a series of oxygenated fragments at m/z 115, 99, 73, 55, and 43, corresponding to C6H11O2+, C5H7O2+, C3H5O2+, C3H3O+, and C2H3O+ respectively.



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Derivation of Kinetics

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The rate constant of heterogeneous oxidation is quantified by measuring the loss of the particulate OPFR, as estimated by a selected tracer ion.39 For TCP, the molecular-ion peak at m/z 368 was selected as a tracer, while the largest detectable fragment at m/z 299 was selected for TBEP. The observed rate constant (kobs; cm3 molecule−1 s−1) for the heterogeneous OH oxidation reaction can be determined via:29

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ln

[OPFR] 𝐼 = ln = ― 𝑘obs[OH]𝑡 [OPFR]0 𝐼0

where [OPFR]0, [OPFR], [OH], and t represents the initial concentration of OPFR (molecules cm−3), the measured OPFR concentration (molecules cm−3) at a given OH exposure, the gasphase concentration of OH radicals (molecules cm−3), and the residence time (s) of OPFR respectively. The terms I and I0 are signal intensities for the above-mentioned selected tracer fragments (proportional to the OPFR concentration) in the presence and absence of OH radicals. The changes in I/I0 as a function of OH exposure from TCP and TBEP coated on (NH4)2SO4 experiments at 298 K and 35% RH are shown in Figure S5, and is fit to an exponential function such that kobs can be calculated. The measured kobs was further corrected for gas-phase diffusion using a previously developed empirical formula54,55 to obtain a true rate constant (kt) (see Table 1).

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It should be pointed out that the heterogeneous kinetics of OA may be underestimated when a non-molecular-ion is used as a tracer (e.g. m/z 299 for TBEP) to measure the particle-phase concentration of organics. This may potentially arise from an interference in the signal of a given tracer ion, caused by contributions from particle-phase oxidation products (and subsequent smaller m/z fragments), leading to a lower observed rate constant kobs. To avoid such an effect, Liu et al. proposed the utilization of PMF analysis in kinetic experiments to separate the contributions of reactants and oxidation products to the overall signal, thus improving the accuracy of the rate constant determination.49 Application of PMF to the current data (Figure S6 & S7) results in true rate constants (kt_PMF) for TCP which are very similar to those calculated using the molecular-ion tracer m/z 368 (kt_tracer_368) in all cases (kt_PMF/kt_tracer_368 = 0.96−1.03; see Table 1). In contrast, the kt_PMF values derived for TBEP are 53−61% higher than the kt calculated using an individual non-molecular-ion tracer m/z 299 (kt_tracer_299), underscoring the necessity of using the PMF approach here. For clarity, all further discussions related to rate constants refer exclusively to kt_PMF.

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Impact of RH

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The effect of RH was investigated for TCP and TBEP by adjusting the RH of the reaction system to 35%, 50%, and 68%, under a constant temperature of 298 K. The measured kt_PMF of

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TCP coated on (NH4)2SO4 was relatively constant (~(2.75±0.38)×10−12 cm3 molecule−1 s−1) throughout the entire RH range (Figure 2A), while the kt_PMF of TBEP coated on (NH4)2SO4 increased by (19±2)%, from (3.06±0.40)×10−12 to (3.63±0.44)×10−12 cm3 molecule−1 s−1 with an increase of RH from 35% to 68%, as shown in Figure 2B. These results indicate that RH has a negligible impact on the heterogeneous reactivity of TCP towards OH and a positive effect on the reactivity of TBEP. One potential mechanism explaining such a difference in RH impact between species, is that the particle water content controls the organic phase state (particle viscosity) of OPFR@AS particles,28 thus influencing the OPFR reactivity. As shown in Figure S3, the E-AIM predicted hygroscopic growth factor (GF) for TCP remains relatively unchanged at a value of ~1 over the investigated RH range, while the predicted GF for TBEP slightly increases with increasing RH and is consistently 1−3% higher than that of TCP. This suggests that while TCP is relatively non-hygroscopic, TBEP demonstrates limited hygroscopic growth across the investigated RH range. A higher hygroscopicity for TBEP can be expected since TBEP has a water solubility 3300 times greater than TCP (see Table S1).4 Given the extremely low hygroscopicity of TCP at 35−68% RH and the lower RH inside the flow tube reactor relative to the deliquescence RH of (NH4)2SO4 particle (80%),56 the organic phase state of TCP@AS (likely governed by particle water content28) is expected to remain unchanged over the investigated RH range, resulting in a relatively stable TCP reactivity (i.e. constant kt_PMF of TCP@AS; see Figure 2A). This is consistent with a previous kinetic study on dehydroabietic acid (an organic substance having low water solubility) which similarly demonstrated that the heterogeneous OH reactivity was minimally influenced by RH over the investigated RH range (20−80%).57 In the case of TBEP@AS, the observed increases in rate constant with increasing RH can be rationalized through the qualitative analysis of particle-phase reactivity described below. Based upon the measured I/I0 and the coated particle diameter, the maximum OH diffusion depth (d) was estimated to be 8.0 nm at 35% RH and 9.8×1011 molecules cm−3 s OH exposure and 9.6 nm at 68% RH and 9.7×1011 molecules cm−3 OH exposure (see SI). Under such a scenario, the heterogeneous OH oxidation process would be kinetically limited by the mass transfer of OH in TBEP. In this case, where transport is governed by OH diffusion in the particle, a characteristic mixing time for OH (tmix) can be estimated as: tmix = d2/DOH.58 Here, the water diffusion coefficient (DH2O; m2 s−1) is used as a proxy for the OH diffusion coefficient (DOH; m2 s−1) since H2O and OH possess similar transport properties.59 At the present time, DH2O in TBEP is unavailable. However, previous studies suggest that DH2O in several organics of diverse structure are in the range of 6.9×10−15−8.0×10−11 m2 s−1 and 2.0×10−11−1.1×10−9 m2 s−1 at 35% and 68% RH, respectively (Table S2).60-64 If DH2O in TBEP is within these reported ranges, then the tmix is 8×10−7−9×10−3 s and 8×10−8−5×10−6 s at 35% and 68% RH respectively. The significantly lower (10−1800 times lower) tmix for OH at 68% RH relative to 35% RH suggests that the uptake of water with increasing RH in the current TBEP@AS experiments,



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would lower the particle viscosity (a lower tmix for OH reflects a lower particle viscosity61) and promote OH diffusion in the organic phase, thereby enhancing the reactivity of TBEP (i.e. increased kt_PMF of TBEP@AS with increased RH; see Figure 2B). Such a change in phase state for organic aerosols containing water-soluble organic compounds as a result of increased RH has been demonstrated previously.28-30 For example, levoglucosan particles underwent a RHinduced phase transition from highly viscous to less viscous particles with an increase of RH from 5% to 60%.30 This led to an enhanced heterogeneous OH reactivity for levoglucosan particles at higher RH.28 The current results are also consistent with a previous study on aerosol particles consisting of succinic acid, which indicated that heterogeneous OH oxidation kinetics were increased by a factor of 41 in the liquid phase compared to the solid phase.29

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No significant particle size change was observed for both TCP@AS and TBEP@AS particles across the entire investigated RH range. Such an observation is in agreement with the E-AIM results (Figure S3), and further confirms that the hygroscopicity of these two OPFRs is limited under the current experimental conditions. In addition, the same m/z fragments were observed for the oxidized TBEP particles at all measured RHs, indicating that the reaction mechanism is minimally influenced by RH. Consequently, the faster kinetics of TBEP@AS observed at higher RH is attributed to changes in organic phase state (viscosity) induced by RH changes, rather than changes in reaction mechanism. This is consistent with a previous study of 2methylglutaric acid which has similarly shown that RH only physically affected the heterogeneous OH reactivity.61

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Impact of Coexisting Soluble Iron (Fe(II))

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The potential impact of coexisting Fe(II) is evaluated by investigating the OH-initiated heterogeneous oxidation kinetics of TCP and TBEP coated on FeSO4 particles (denoted TCP@FS and TBEP@FS respectively) under differing humidity conditions (35–68% RH). The measured kt_PMF of TCP@FS remained relatively constant (~(3.43±0.49)×10–12 cm3 molecule−1 s−1) throughout the entire RH range (Figure 2C). This is due to an unchanged particle water content for TCP@FS over the investigated RH range, as discussed below. However, in the case of TBEP@FS (Figure 2D), the derived kt_PMF increased by (32±6)%, from (4.22±0.63)×10–12 to (5.55±0.66)×10–12 cm3 molecule−1 s−1 with an increase of RH from 35% to 68%. This relative enhancement for TBEP@FS (32%) with increased RH is more prominent than that of TBEP@AS of 19% over the same RH range. Such a difference is attributed to enhanced Fenton-type chemistry in TBEP@FS particles at higher RH, as also discussed below.

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The presence of FeSO4 resulted in faster kinetics for both OPFRs over the entire RH range, as reflected in the relative increase in the measured kt_PMF of OPFR@FS over OPFR@AS (Figure 3A & B) which varies from (22±5)% to (53±1)% depending upon the OPFR and RH conditions. A faster measured reaction rate for OPFR coated on FeSO4 can be attributed to the unique redox


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activity of Fe(II), which is known to have important catalytic effects during the oxidation of secondary organic materials generated from various reaction systems,38,65-67 while (NH4)2SO4 is chemically inert. More specifically, redox-active Fe(II) can induce Fenton-type reactions in the particle phase that leads to the formation of ROS,37,38 further accelerating the oxidation of OPFR. Consequently, the reaction rates for OPFRs (TCP and TBEP) coated on FeSO4 remain higher than those on (NH4)2SO4 across the investigated RH range (see Figure 3 A & B). The Fe(II) effect is also reflected in the HR-ToF-AMS measured oxygen content difference between OPFR@FS and OPFR@AS experiments at a given OH exposure. For example, for both OPFRs, the measured O/C ratio (often used as a measure of the oxidation state of organic particles68,69) for OPFR@FS is consistently 5−17% higher than that of OPFR@AS over the investigated RH range at an OH exposure of ~4.6 ×1011 molecules cm–3 s (Figure 3 C & D). It should be noted that for a given OPFR, the same m/z fragments were observed across all OPFR@FS and OPFR@AS experiments, suggesting that particle-phase Fenton-type reactions would yield products similar to OH oxidation reactions. This is in agreement with a previous study on the photo-Fenton oxidation of glycolaldehyde.37 Previous studies have also clearly shown that water has a significant impact on Fenton-type chemistry.37,38,70 Thus, understanding the role of water in particle-phase Fenton-type chemistry is of crucial importance in interpreting the current kinetic data for OPFR@FS under differing RH conditions. Here, water may directly participate in the iron redox cycling (i.e. the Fentontype reactions), from which ROS are formed (e.g. Fe(Ⅲ) + H2O + hv→Fe(II) + OH + H+).70 Water may also indirectly affect Fenton-type chemistry by acting as a reaction medium for these reactions.37 Therefore, a higher particle water content would lead to an acceleration of particle-phase iron redox cycling process via these two pathways (direct and indirect), generating more ROS and resulting in faster oxidation kinetics for OPFR. In the current study, for both OPFRs no obvious changes in the particle size of OPFR@FS were observed over the investigated RH range. This suggests that while FeSO4 exhibits obvious hygroscopic growth at 35−68% RH (Figure S3), its hygroscopicity is strongly suppressed by OPFR coatings (TCP and TBEP) which have limited hygroscopicity (Figure S3). The hygroscopicity suppression for FeSO4 is consistent with a previously reported suppression effect for the hygroscopicity of hygroscopic particles coated with relatively non-hydroscopic compounds (similar to the case of OPFR@FS).71 Under such a scenario, the overall water uptake of OPFR@FS is likely dominated by OPFR coating. As a result, the particle water content of OPFR@FS would be similar to that of OPFR@AS ((NH4)2SO4 is non-hygroscopic below 80% RH56) at a given experimental RH, which has important implications for the oxidation kinetics of OPFR (see below). For TCP@FS, the particle water content (a very small amount of water may be present in the particle even though TCP has a very limited hygroscopicity over the investigated RH range) is



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likely to remain unchanged over the investigated RH range due to the extremely low hygroscopicity of TCP (Figure S3), resulting in an unchanged organic phase state (likely governed by particle water content28) and an unchanged particle-phase Fenton-type reaction rate. In this case, the kt_PMF for TCP@FS is expected to remain relatively stable over the investigated RH range (Figure 2C). Further, considering that RH has a negligible effect on the OH reactivity of TCP@AS (Figure 2A), the relative difference in kt_PMF between TCP@FS and TCP@AS is also expected to remain relatively stable ((22±5)−(28±4)%; see Figure 3A). This is in contrast to Figure 3B (TBEP experiments), where the kt_PMF difference for TBEP is not uniform over the entire RH range. At 35% RH, TBEP@FS has a kt_PMF which is (38±2)% higher than that of TBEP@AS. This relative enhancement becomes more prominent ((53±1)%) when RH increases to 68%. As mentioned above, it is postulated that there is no significant difference in particle water content between TBEP@FS and TBEP@AS at a given experimental RH. Thus, the results of Figure 3B are likely caused mainly by changes in Fentontype reaction rate rather than changes in organic phase state. Based on the E-AIM predicted hygroscopic growth factors for TBEP (Figure S3), the estimated particle water content for TBEP increased by 8% with an increase of RH from 35% to 68%. Given the essential role of water in Fenton-type chemistry (see above), a higher particle water content at 68% RH may lead to an acceleration of iron redox cycling process (i.e. increased Fenton-type reaction rate), producing more ROS and accelerating the oxidation rate of TBEP. This was manifested as the increase in the kt_PMF difference between TBEP@FS and TBEP@AS with increasing RH (Figure 3B).

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Atmospheric Fate and Implications

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In assessing the impact of RH and coexisting Fe(II) on the atmospheric persistence of OPFRs

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studied here, their effects on the atmospheric lifetimes 𝜏 (𝜏 =1/kt_PMF[OH]GM) of TCP (𝜏TCP) and TBEP (𝜏TBEP) should be considered, as shown in Figure 4A & B, which are of vital importance in the environmental risk assessment for these compounds. Based on the true reaction rate constant derived using PMF analysis (kt_PMF) and a global mean OH concentration ([OH]GM) which ranges from 6.5×105 to 1.6×106 molecules cm–3 (i.e. an atmospherically

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relevant range in Figure 4),72-74 the calculated 𝜏TCP and 𝜏TBEP are in the range of 2.3–6.5 and 1.3–5.8 days respectively, depending upon the [OH]GM, RH, and coexisting species ((NH4)2SO4 and FeSO4). Currently, due to a lack of experimental kinetics data for the majority of organic compounds in commercial use, the structure–activity relationship (SAR) method is widely used to predict the rate constants for atmospheric gas-phase reactions between OH radicals and organic molecules,75-77 which has also been implemented in the AOPWIN model created by US Environmental Protection Agency.78 The AOPWIN modeled kinetics data are in turn used to estimate the overall atmospheric lifetimes of OPFRs.79,80 The atmospheric lifetimes of TCP and TBEP in the gas phase based upon the AOPWIN derived rate constants are calculated for


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comparison (blue line in Figure 4) and would clearly underestimate the overall 𝜏 for these two OPFRs by at least a factor of four if used in assessing their atmospheric fates, compared with the 𝜏 estimated using the present results. This is consistent with previous results for other particle-phase OPFRs such as TPhP and TDCPP which demonstrated an increased atmospheric persistence due to significant differences between measured heterogeneous rate constants and those modelled for the gas-phase.39 The atmospheric lifetimes derived for TCP and TBEP currently can also be placed in context with other studied OPFRs. Assuming a global mean [OH] of 9.4×105 molecules cm−3,74 the estimated 𝜏TCP based upon TCP@AS kinetic data at 35% RH is 4.5 days (see Table 1), slightly longer than the estimated 𝜏TBEP (4.0 days) based upon TBEP@AS kinetic data at 35% RH, but is slightly shorter than the 𝜏 of TPhP (5.9 days) based upon TPhP@AS kinetic data at 38% RH, and is significantly shorter than the 𝜏 of TDCPP (13.4 days) based upon TDCPP@AS kinetic data at 38% RH (see Table S3).39 The extended lifetime of TDCPP (i.e. lower reactivity) relative to TCP, TBEP, and TPhP is consistent with its lower reactivity in water relative to other aryl and alkyl phosphates.81 Regardless, the estimates of atmospheric lifetime for TCP and TBEP here, suggest that these compounds are less persistent than halogenated OPFRs, but are sufficiently long to undergo medium to long-range transport in the atmosphere.13,14 While the AOPWIN predicted k values agree well with the experimental data for several OPFRs in gas phase in general (Figure S8),17-19 the current and previous results39,40 clearly suggest that there is a large deviation between predicted and measured k for these compounds in the particle phase. Hence, the AOPWIN model may not provide reliable kinetic data for particulate OPFRs, which is often used to assess the atmospheric fate of these compounds.79,80 However, it remains unclear whether the relative trend in measured particle-phase k is consistent with the expected trend in the gas-phase OH reactivity based upon chemical structure. This possibility is investigated by comparing the AOPWIN predicted k with measured k for five OPFRs (coated on (NH4)2SO4 at ~35% RH) in the currently available literature. As can be observed in Figure S9, with the exception of TDCPP, the relative trend in measured k for the other four particulate OPFRs is consistent with the predicted gas-phase OH reactivity in general, from which an empirical relationship can be obtained: measured k (10–12 cm3 molecules–1 s–1) = 0.05×predicted k (10–11 cm3 molecules–1 s–1) + 2.34. This suggests that the atmospheric fate of non-halogenated OPFRs in the particle phase may potentially be predicted under certain conditions (at a given RH), but requires further heterogeneous kinetics measurement data for a large set of OPFRs of diverse structure, to improve such an empirical relationship. Similar empirical relationships are also obtained for several OPFRs in the aqueous phase, as given in the SI and Figure S10. Despite the potential for an empirical relationship between heterogeneous OH reactivity and structure, the current results indicate that other environmental factors which are not easily



382

modelled, such as RH and pre-existing particle composition will also play a significant role in

383

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the degradation and lifetimes of OPFRs. For example, 𝜏TBEP was reduced from 2.4–5.8 days at 35% RH to 2.0–4.9 days at 68% RH (based upon TBEP@AS kinetic data), as can be observed in Figure 4B (solid vs. dashed black line), and is likely to be reduced further at the higher RH which is often observed in the atmosphere.82 It is expected that other hydrophilic OPFRs such as TCEP and TCPP, the two most important chlorinated OPFRs measured at high concentrations (up to 2 and 11 ng m–3 respectively) in ambient air,10-14 will possess a similar RH–reactivity relationship. It should be noted that in the current study, TBEP@AS likely has a core-shell particle structure over the investigated RH range (35−68% RH). However, TBEP@AS may change to a homogeneous liquid particle once the (NH4)2SO4 core is deliquesced upon humidification (RH>80%), thus influencing the TBEP reactivity. Further study is warranted to assess the impact of particle mixing state on the OH reactivity of particulate OPFR (e.g. TBEP).

395

The presence of Fe(II) also resulted in a reduction of the estimated atmospheric lifetimes for

396

both OPFRs, as reflected in a shorter 𝜏TCP (by ~25%) and 𝜏TBEP (by ~53%) at 68% RH relative to those derived using OPFR@AS kinetics data at 68% RH (see dashed black vs. red line in Figure 4A & B). This implies that other redox-active metals in ambient aerosols such as Mn and Cu may similarly enhance the OH-initiated degradation of particle-bound OPFRs, especially in urban areas where both metallic and OPFR species are more prevalent.11,12,35 It should also be noted that the particle lifetimes based upon OPFR@FS kinetic data at 68% RH (i.e. 2.2–5.2 days and 1.3–3.2 days for TCP and TBEP respectively; red line in Figure 4) should be regarded as lower limits given the complex mixing state (i.e. the effect of other organic coatings) of ambient particles which has been observed to reduce OH reaction kinetics (i.e. extended particle lifetime).39 Ambient temperature variations may also impact the OH reactivity of atmospheric aerosol since lower temperature has been demonstrated to increase organic aerosol viscosity,31 thus reducing particle-phase OH radical diffusion, and increasing particle degradation lifetimes.

384 385 386 387 388 389 390 391 392 393

397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417

The present results, while providing new kinetic data for several species, also demonstrate the complexity of factors that have the potential to impact the degradation of OPFRs and other emerging particle-phase chemicals in the atmosphere. Consequently, a full understanding of the atmospheric fate of OPFRs will only be achieved through the inclusion of these external/internal factors in the modelling of fates and subsequent risk assessment for these species. This highlights a need for obtaining further data from experiments which systematically investigate the impact of RH, temperature, and the physical and chemical nature of pre-existing particles, for a larger suite of OPFRs and other chemicals deemed a priority risk.


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418

ASSOCIATED CONTENT

419

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website.

421

425

Details of the photochemical oxidation flow tube reactor; OH exposure measurements; other experimental details; Evaporation of particulate OPFRs; PMF analysis; E-AIM predicted hygroscopic growth factors for OPFRs, Fragments analysis for oxidized TCP; Estimation of OH diffusion depth; Comparison of AOPWIN predicted gas-phase k and measured gas-phase and aqueous-phase k for several OPFRs.

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AUTHOR INFORMATION

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Corresponding Author

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*Phone: 1-416-739-4840; e-mail: [email protected].

429

Notes

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The authors declare no competing financial interest.

431

ACKNOWLEDGMENTS

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We thank Dr. Tom Harner for helpful discussions. This research was financially supported by the Chemicals Management Plan (CMP) of Canada.

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REFERENCES

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(81) Li, C.; Wei, G.; Chen, J.; Zhao, Y.; Zhang, Y. N.; Su, L.; Qin, W. Aqueous OH radical reaction rate constants for organophosphorus flame retardants and plasticizers: experimental and modeling studies. Environ. Sci. Technol. 2018, 52 (5), 2790-2799. (82) Wu, Z.; Wang, Y.; Tan, T.; Zhu, Y.; Li, M.; Shang, D.; Wang, H.; Lu, K.; Guo, S.; Zeng, L.; Zhang Y. Aerosol liquid water driven by anthropogenic inorganic salts: implying its key role in haze formation over the North China Plain. Environ. Sci. Technol. Lett. 2018, 5 (3), 160-166. (83) Evaluation of measurement data — Guide to the expression of uncertainty in measurement, Joint Committee for Guides in Metrology (JCGM), 2008, 8.



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Table 1. Reaction kinetics and estimated atmospheric lifetimes of TCP and TBEP coated on (NH4)2SO4 and FeSO4 at 298 K and 35–68% RH. k (×10–12 cm3 molecule−1 s−1) Particle type a

RH (%) kobs_tracerb

TCP@AS

TCP@FS

TBEP@AS

TBEP@FS

kt_tracer

kt_PMFc

average kt_PMF

Atmospheric lifetime (days)d Experimental AOPWIN

35 50

2.58±0.44 2.89±0.55 2.77±0.59 2.75±0.38 2.45±0.36 2.72±0.45 2.80±0.54

4.5 (3.7–5.6) 4.4 (3.7–5.4)

68

2.50±0.23 2.80±0.30 2.69±0.39

4.6 (4.0–5.3)

35

3.05±0.45 3.49±0.59 3.46±0.68 3.43±0.49

3.6 (3.0–4.4)

50

3.03±0.29 3.46±0.38 3.57±0.53

3.5 (3.0–4.0)

68

2.94±0.48 3.34±0.63 3.27±0.70

3.8 (3.1–4.8)

35

1.79±0.14 1.91±0.16 3.06±0.40

4.0 (3.6–4.6)

50

1.94±0.16 2.09±0.19 3.22±0.43

3.8 (3.4–4.4)

68

2.13±0.14 2.30±0.16 3.63±0.44

3.4 (3.1–3.8)

35

2.42±0.27 2.67±0.30 4.22±0.63

2.9 (2.6–3.4)

50

2.68±0.30 2.97±0.33 4.54±0.68

2.7 (2.4–3.2)

68

3.07±0.17 3.45±0.22 5.55±0.66

2.2 (2.0–2.5)

a

0.9

0.1

TCP: tricresyl phosphate; TBEP: tris(2-butoxyethyl) phosphate; AS: ammonium sulfate ((NH4)2SO4); FS: iron (Fe(II)) sulfate (FeSO4); OPFR@AS: OPFR coated on (NH4)2SO4 particle; OPFR@FS: OPFR coated on FeSO4 particle. b The molecular-ion at m/z 368 was selected as a tracer for TCP while a non-molecular-ion at m/z 299 was selected for TBEP. All values are presented as average kobs_tracer ± standard error (n=5 experiments). c The k_t_PMF uncertainty is calculated using the GUM method83 which combines the uncertainty from replicate experiments and the uncertainty from PMF analysis procedure. d Assuming a global mean OH concentration of 9.4×105 molecules cm−3.74


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Figure

Figure 1. (A) Mass spectra of unreacted (black lines) and oxidized (red lines) TCP@AS at an OH exposure of 8.7×1011 molecules cm−3 s; (B) difference mass spectra between the oxidized and unreacted TCP@AS (oxidized−unreacted); (C) mass spectra of unreacted and oxidized TBEP@AS at an OH exposure of 9.8×1011 molecules cm−3 s; (D) difference mass spectra between the oxidized and unreacted TBEP@AS (oxidized−unreacted). Positive and negative values in B & D indicate the mass fragments that are enhanced and reduced for OPFRs@AS upon OH exposure respectively. The inset graphs in B & C show the corresponding magnified area.



Figure 2. Rate constants based upon PMF analysis (kt_PMF) for (A) TCP and (B) TBEP coated on (NH4)2SO4 particles, and (C) TCP and (D) TBEP coated on FeSO4 particles at 35−68% RH and 298 K.

Figure 3. The relative percentage difference in kt_PMF (OPFR@FeSO4 vs. OPFR@(NH4)2SO4) for (A) TCP and (B) TBEP. Measured O/C ratios for OPFR (TCP and TBEP) coated on (NH4)2SO4 and FeSO4 at (C) 35% RH and (D) 68% RH and at ~4.6 ×1011 molecules cm–3 s OH exposure. Relative percentage difference = ((kt_PMF_OPFR@FS/kt_PMF_OPFR@AS) −1) × 100%.


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Figure 4. Estimated atmospheric lifetime of (A) TCP, and (B) TBEP as a function of OH concentration. The atmospheric lifetimes of particle-phase and gas-phase OPFRs were calculated based upon the measured kt_PMF in Table 1 and the AOPWIN model78 respectively. The color series indicate different experimental conditions as illustrated in the legends. Atmospherically relevant range: global mean OH concentration 6.5×105–1.6×106 molecules cm–3.72-74



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Understanding the Impact of Relative Humidity and Coexisting Soluble

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