Multiphase reactivity of polycyclic aromatic hydrocarbons is driven by phase separation and diffusion limitations Shouming Zhoua,1, Brian C. H. Hwangb,1, Pascale S. J. Lakeyb, Andreas Zuendc, Jonathan P. D. Abbatta,2, and Manabu Shiraiwab,2 aDepartment of Chemistry, University of Toronto, Toronto, M5S 3H6, Canada; bDepartment of Chemistry, University of California, Irvine, CA 92697; and cDepartment of Atmospheric and Oceanic Sciences, McGill University, Montreal, H3A 0B9, Canada Edited by Yinon Rudich, Weizmann Institute of Science, Rehovot, Israel, and accepted by Editorial Board Member A. R. Ravishankara May 12, 2019 (received for review February 12, 2019) Benzo[a]pyrene (BaP), a key polycyclic aromatic hydrocarbon (PAH) transport of BaP can be affected by the temperature and humidity often associated with soot particles coated by organic compounds, dependence of diffusivity and reactivity of BaP-containing parti- is a known carcinogen and mutagen. When mixed with organics, cles (10, 11). These modeling results are consistent with obser- the kinetics and mechanisms of chemical transformations of BaP by vations of BaP at remote terrestrial and marine sites, even in the ozone in indoor and outdoor environments are still not fully polar regions (2, 12). elucidated. Using direct analysis in real-time mass spectrometry Atmospheric aerosol particles are often mixtures of organics, (DART-MS), kinetics studies of the ozonolysis of BaP in thin films inorganics, and water, which are subject to complex and nonideal exhibited fast initial loss of BaP followed by a slower decay at long behavior including liquid–liquid and/or liquid–solid phase sepa- exposure times. Kinetic multilayer modeling demonstrates that the slow decay of BaP over long times can be simulated if there is slow ration, as predicted by thermodynamic models (13, 14). Experi- diffusion of BaP from the film interior to the surface, resolving long- mental studies have demonstrated the existence of multiple standing unresolved observations of incomplete PAH decay upon phases in model mixtures or laboratory-generated secondary prolonged ozone exposure. Phase separation drives the slow organic aerosol (SOA) particles as well as in field-collected or- diffusion time scales in multicomponent systems. Specifically, ganic aerosol particles (15–17). Liquid–liquid phase separation thermodynamic modeling predicts that BaP phase separates from wasalsoobservedinSOAparticles free of inorganic salts secondary organic aerosol material so that the BaP-rich layer at the (18–20). The interplay of phase state and nonideality can affect surface shields the inner BaP from ozone. Also, BaP is miscible with aerosol mass concentration and the characteristic time scale of organic oils such as squalane, linoleic acid, and cooking oil, but its gas-particle mass transfer (21). Changes in viscosity upon chemical oxidation products are virtually immiscible, resulting in the forma- aging of organic particles were observed (22), but it is not yet clear tion of a viscous surface crust that hinders diffusion of BaP from the how the effects and interplay of nonideality and phase state evolve film interior to the surface. These findings imply that phase upon multiphase chemical interactions to affect fates and atmo- separation and slow diffusion significantly prolong the chemical spheric long-range transport of organic compounds. lifetime of PAHs, affecting long-range transport of PAHs in the atmosphere and their fates in indoor environments. Significance phase state | ozone | indoor chemistry | bulk diffusion | kinetic modeling Polycyclic aromatic hydrocarbons (PAHs) are among the most olycyclic aromatic hydrocarbons (PAHs), including benzo[a] prominent toxic compounds in the air. Heterogeneous reac- Ppyrene (BaP), are among the most prominent toxic air pol- tions involving O3 can change the toxicity of PAHs, but the lutants, posing a threat to human health because their metabo- reaction mechanism and kinetics remain to be elucidated. lites and oxidation products are carcinogenic and mutagenic (1). Based on new experiments combined with state-of-the-art ki- PAHs are emitted into the atmosphere from incomplete com- netic and thermodynamic models, we show that phase sepa- bustion and biomass burning and by smoking and cooking in ration plays a critical role in the ozonolysis of PAHs mixed with indoor environments. Due to its low vapor pressure, BaP re- secondary organic aerosols and organic oils. Ozonolysis prod- sides mostly in the condensed phase, and heterogeneous oxi- ucts of PAHs phase separate to form viscous surface crusts, which protect underlying PAHs from ozonolysis to prolong dation of BaP by oxidants such as OH and O3 is a major atmospheric loss pathway (2). Laboratory measurements show their chemical lifetime. These results have significant implica- rapid degradation of BaP against ozone when adsorbed to a tions for outdoor and indoor air quality by affecting PAH long- range transport and fate in indoor environments. variety of substrates such as water, ammonium sulfate, soot, and organic compounds (3–6). Author contributions: J.P.D.A. and M.S. designed research; S.Z., B.C.H.H., P.S.J.L., A.Z., Upon chemical aging in the atmosphere, PAH-containing J.P.D.A., and M.S. performed research; S.Z., B.C.H.H., P.S.J.L., A.Z., J.P.D.A., and M.S. ana- particles are likely coated by semi- or low-volatile organic com- lyzed data; and S.Z., B.C.H.H., P.S.J.L., A.Z., J.P.D.A., and M.S. wrote the paper. pounds, which are formed by multigenerational gas-phase oxi- The authors declare no conflict of interest. dation of volatile organic compounds. Laboratory experiments This article is a PNAS Direct Submission. Y.R. is a guest editor invited by the Editorial Board. have shown that organic coatings can retard multiphase reactions This open access article is distributed under Creative Commons Attribution-NonCommercial- of ozone with PAHs due to kinetic limitations of bulk diffusion NoDerivatives License 4.0 (CC BY-NC-ND). (3, 7, 8). The extent of coating effects depends on the phase state 1S.Z. and B.C.H.H. contributed equally to this work. of organic coatings, which can be liquid, amorphous semisolid, or 2To whom correspondence may be addressed. Email: [email protected] or m. glassy solid, depending on chemical composition, relative humidity [email protected]. (RH), and temperature (9). This shielding effect of PAHs from This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. oxidation was recently implemented into regional and global air- 1073/pnas.1902517116/-/DCSupplemental. quality models showing how regional and global distributions and Published online May 29, 2019. 11658–11663 | PNAS | June 11, 2019 | vol. 116 | no. 24 www.pnas.org/cgi/doi/10.1073/pnas.1902517116 Downloaded by guest on September 27, 2021 Although extensive laboratory measurements and modeling of 1.4 PAHs have been conducted under atmospherically relevant conditions, chemical transformation of PAHs in indoor envi- 1.2 ronments, where we spend ∼90% of the time, is poorly un- 15 ppb O3 derstood (23). PAHs can be transported from outside air and can 100 ppb O3 also be emitted by indoor activities such as smoking, cooking, 1.0 500 ppb O3 and burning of solid fuels (24). Recent findings show that BaP 0 1000 ppb O diol-epoxide products, which are highly carcinogenic, can be 3 formed in indoor environments (25), and chronic exposure to 0.8 these compounds can be detrimental to human health. Thus, both the chemical transformation of PAHs with gaseous ozone 0.6 under indoor-relevant conditions and the PAH reactivity in [BaP]/[BaP] mixtures of chemicals emitted from indoor sources need to be elucidated. A long-standing, unexplained issue in the field of 0.4 environmental chemistry and health is the observation that although ozonolysis experiments show a rapid initial loss of condensed-phase 0.2 PAHs, prolonged exposure to ozone does not necessarily lead to complete PAH decay (26–29). 0.0 In this study, the decay kinetics of BaP upon exposure to 0 10 20 30 40 50 60 70 ozone were measured using direct analysis in real-time mass spectrometry (DART-MS). Three different sets of experimental Time (minutes) i and modeling scenarios were explored: ( ) the ozonolysis kinetics Fig. 1. Decay of BaP concentration in BaP/BES films exposed to different of pure BaP films (containing a small amount of an internal mixing ratios of ozone (15 to 1,000 ppb) at 296 K and RH <5%. Filled circles standard); (ii) the effects of the reaction substrate by mixing BaP show the experimental data, with error bars representing the SD of 10 with α-pinene SOA material, a mixture of different oxidized measurements. The solid lines are KM-SUB simulation results with a single organic compounds; and (iii) the effects of mixing BaP with kinetic parameter set shown in SI Appendix, Table S1. liquid substrates such as squalane, linoleic acid, and cooking oil. The experimental conditions were simulated using the kinetic the key aspect of the model prediction is that the oxidation of EARTH, ATMOSPHERIC, multilayer model of aerosol surface and bulk chemistry (KM- AND PLANETARY SCIENCES SUB), which resolves mass transport and chemical reactions at BaP becomes limited by bulk diffusion of BaP from the film the surface and in the condensed phase explicitly (30), combined interior to the surface at longer reaction times. This explains the ∼ with the Aerosol Inorganic-Organic Mixtures Functional Groups subsequent slow BaP decay after 10 min, as well as the saturation Activity Coefficients (AIOMFAC) thermodynamic model (31–33) of BaP decay rates at higher O3 mixing ratios. Bulk diffusivity of ∼ −17 −19 2· −1 at the core of a thermodynamic equilibrium framework that pre- BaP was predicted to be in the range of 10 to 10 cm s , dicts chemical composition of different coexisting phases.