Investigation into Photo-induced Auto-oxidation of Polycyclic Aromatic

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Environmental Processes

Investigation into Photo-induced Auto-oxidation of Polycyclic Aromatic Hydrocarbons Resulting in Brown Carbon Production John Haynes, Keith E. Miller, and Brian J Majestic Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b05704 • Publication Date (Web): 27 Dec 2018 Downloaded from http://pubs.acs.org on January 2, 2019

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Environmental Science & Technology

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Investigation into Photo-induced Auto-oxidation of Polycyclic Aromatic Hydrocarbons

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Resulting in Brown Carbon Production

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John P. Haynes1, Keith E. Miller1, Brian J. Majestic1*

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12190

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CO 80208, USA

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*corresponding author, [email protected]

E Iliff Ave, Olin Hall, Department of Chemistry and Biochemistry, University of Denver, Denver,

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Abstract

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Brown carbon (BrC) is a collection of oxidized atmospheric aromatic compounds detected worldwide

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with broad functionality. This multifunctional nature allows BrC to be water soluble, bioavailable, and to

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demonstrate light absorption at multiple wavelengths. Polycyclic aromatic hydrocarbons (PAH) are major

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primary products of combustion emissions and have long been known to oxidize in the environment as a

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component of secondary organic aerosols. In this study we have exposed aqueous PAH suspensions to

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simulated sunlight to investigate oxidized PAH as BrC precursors. Illuminated samples of naphthalene

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and anthracene demonstrated growth of several new products with absorptions and oxidation consistent

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with humic-like substances (HULIS). Reactions of aqueous naphthalene, anthracene, and their oxidized

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derivatives were found to produce chromatographic and spectroscopic evidence of HULIS formation

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when exposed to sunlight. The association of oxyradicals with HULIS has implications on human health

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via lung tissue damage; and its absorption character may add to radiative forcing processes in the

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atmosphere. Overall product character from naphthalene and anthracene indicate reaction mechanism

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pathways using oxidized alcohol and quinone as intermediate species.

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INTRODUCTION

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The atmospheric formation of light absorbing carbonaceous aerosols, collectively known as brown carbon

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(BrC), is a highly varied and robust subject of many recent studies1-2. The accumulation of these

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compounds of highly varying composition and absorption3 results in a translucent haze producing a

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yellow to dark orange color, differentiating it from opaque black carbon species4-6. BrC is an atmospheric

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phenomenon typically observed in urban and wildfire regions and, as such, is found to evolve from

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carbon combustion emissions7-8 and materialize within cloud water environments1, 9. The absorbance

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character of these compounds implicates them with global radiative forcing10-11 and they are associated

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with respiratory health effects12 due to their significant aromatic composition, similar to polycyclic

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aromatic hydrocarbons12-14.

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Polycyclic aromatic hydrocarbons (PAH) are persistent pollutants that can affect natural ecosystems

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thousands of kilometers from their source14-17. Aerosolized PAH pose a health hazard as their intake is

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associated with respiratory illness and some are implicated as carcinogens18-22. PAH are produced by the

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inefficient combustion of hydrocarbon fuel sources, primarily as biomass burning15,

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exhaust25-27 and industrial complex smokestack byproducts28-29, where PM concentrations of 500 ppm are

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observed near urban regions23. Due to their extended aromatic structure, PAH have an inherent stability in

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the atmosphere4,

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globally15-16, 30.

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PAH also demonstrate varying levels of photosensitivity as some forms produce oxidation products

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(oxPAH) upon exposure to light16,

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oxidations, resulting in hydroxylated groups such as carbonyls, hydroxides, and ethers in varying

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combinations, all of which have been observed downwind of PAH sources34-37. Naturally occurring OH

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and other oxidants are implicated with these transformations are gas-phase reactive oxygen species

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(ROS), including ozone, hydroxyl radicals, and superoxide radicals38-40.

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23-24,

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which allows them to persist for hours to weeks and, therefore, be distributed

31-33.

During transport, PAH may go through several of these



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PAH such as naphthalene, anthracene, and phenanthrene are commonly observed to oxidize into several

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degradation products indicating the probability of many reaction pathways. Suggested mechanisms in the

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oxidation and oligomerization of phenolic compounds, for example, describe an array of pathways

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involving oxygen-centered radical intermediates41-42. Irradiated aromatic alcohols may oxidize through an

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ozonation43 or excited state carbonyl mechanism44 to obtain characteristics similar to atmospheric humic-

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like substances (HULIS)45. This may serve as analogous to reactive pathways in similarly oxidized

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aromatics, like oxPAH. Anthracene observed in atmospheric samples is commonly associated with

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quinones38,

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branched oligomers are also observed48.

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The semi-volatile nature and mass-range of PAH allows for solid-vapor equilibria of many different vapor

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pressures49. This allows the smaller PAH to exist in vapor form50 and larger PAH to adsorb onto or

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comprise solid particulate matter (PM)51-52, with the solid PAH cores acting as cloud condensation nuclei

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(CCN)53. The volatile smaller PAH are also generally more water soluble54 and can therefore partition

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from the gas phase into the cloud water layer. The combination of PAH vapor-phase partitioning and

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cloud water leaching from particles will increase dissolved PAH concentrations until equilibria is

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attained, representing a saturated suspension.

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Given the structural similarities to phenol, the primary objective of this study is to determine if oxPAH

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undergo similar photochemistry towards oligomerization within bulk model cloud water systems, which

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may be an unexplored route to BrC, and to elucidate potential reaction pathways of these oxidations. To

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achieve this, saturated suspensions of parent chain PAH, specifically naphthalene, anthracene,

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phenanthrene, and pyrene, are exposed to simulated sunlight, and are analyzed for molecular mass,

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functional groups, light absorption, polarity, as a proxy for degree of oxidation, and number of reaction

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products.

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of varying forms, and naphthalene is frequently observed with naphthols33 and larger

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MATERIALS AND METHODS

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Materials

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All experiments and sample preparation were performed under a laminar-flow hood using HEPA-filtered

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air. To reduce cross-contamination in the reactor between reactions, Teflon beaker liners were cleaned

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through a succession of solvent treatments. This process starts with an acetone rinse followed by an

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overnight bath of 100% HPLC-grade acetonitrile, then a final overnight bath in 5% trace-metal grade

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nitric acid including a pre- and post-bath triplicate rinse with 18.2 MΩ purified water. Plastic tubes,

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bottles, and syringes were prepared using a rinse with 100% acetone and dried before sample addition.

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Reagents

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Pure PAH reagents used for these reactions include naphthalene (NAP), anthracene (ANT), phenanthrene

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(PHE), and pyrene (PYR). Oxidized PAH products used include benzoic acid, phthalic acid, 1-naphthol,

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1,4-naphthoquinone, 9,10-anthracenediol, 1,4 and 9,10-anthraquinone (Sigma, Fisher). All reagents were

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used as received without further purification. Ultrapure water was used throughout (ThermoScientific

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Nanopure, Waltham, MA) and collected at a resistance of 18.2 MΩ.

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Methods

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Aqueous suspensions of organic samples were created by adding 100 mg of organic crystals to 200 mL of

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ultrapure water. These mixtures are then capped, inverted ten times, and stored at room temperature in the

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dark for 24 hours in order to allow the PAH suspension to achieve equilibrium33, 55-56. The reaction vessels

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are comprised of 100 mL Teflon liners inside jacketed glass beakers which are temperature controlled by

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water flow from a chiller pump.

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During the reaction, samples were stirred at 25°C for 16 hours under a xenon lamp passing through an Air

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Mass, AM 1.5 Global filter (Oriel Sol 1A, Newport Solar Simulator). Output light consisted of 5.4% UV,

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54.7% visible, and 39.9% IR resulting in a spectrum equivalent to sunlight, and calibrated for a flux of 5 ACS Paragon Plus Environment


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1550 Wm-2 to simulate one sun at sea level at its zenith. Control samples of identical composition were

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conducted in the dark by covering an additional reaction vessel with commercially available aluminum

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foil. Sample aliquots of 4 mL were removed at the beginning and end of the 16 hour reaction period.

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Sample suspensions following the photoreaction are evaluated for oxidized products and their properties

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by HPLC, UV-vis, ATR-FTIR, ESI/APCI-MS, and SPE analyses, as described below.

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Analysis

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UV-vis (Shimadzu UV-1800) absorption spectra are collected by adding bulk extract to 1 cm quartz

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cuvettes after baseline calibration using ultrapure water. All samples are analyzed from 220 to 700 nm

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with maximum absorbance set to 4.0 absorbance units.

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HPLC (Agilent 1100) analysis was performed using a reversed-phase retention gradient mobile phase

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method. Separation was obtained using 1 mL/min flow rate, 0.1% TFA:ACN gradient of 90:10 to 0:100

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over 22.5 min plus hold to 28 min, 100 μL sample injection through a Hydro-RP 250 x 4.6 mm column

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(Phenomenex), and detection at 254 nm. The chosen column uses a C18 stationary phase on beads with

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80Å pore size. Therefore each peak represents a compound or set of compounds that match a particular

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degree of oxidation (i.e, polarity).

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Functional groups were characterized using attenuated total reflectance Fourier transform infrared

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spectroscopy (ATR-FTIR, ThermoScientific iS5), analyzed from 700 to 4000 cm-1. The ATR-FTIR

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analyses of functional groups on light and dark samples are performed on solid residues following

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aqueous evaporation. The diamond crystal was cleaned and prepared for analysis using a swab of

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isopropanol and background set after drying. Obtained residues were applied directly to the crystal and

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firm contact was attained by a torque-controlled plunger.

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RESULTS AND DISCUSSION

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Photochemistry of Parent PAH

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Figure 1 presents the UV-vis spectra of the pure PAH parent chains before and after exposure to

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simulated sunlight. Here, we observe that the illuminated NAP (Fig.1a) UV-vis spectrum demonstrates a

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dramatic increase in absorption relative to the dark reaction, with the illuminated sample displaying a

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featureless decay from the UV and throughout the visible range. Illumination of ANT shows almost

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identical behavior (Fig.1a) compared to NAP, with the dark also maintaining a single absorption peak in

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the UV region with no absorption in wavelengths higher than 300 nm. Visually, illumination of NAP and

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ANT result in a transition from clear and colorless to a yellow to orange colored solution.

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Figure 1: UV-vis analysis of saturated suspensions of PAH, (a) naphthalene, in blue, and anthracene, in red, (b) phenanthrene, in green, and pyrene, in purple, in ultrapure water following 16 hour reaction period. Lighted samples are represented in the solid spectra, the dashed spectra represents the samples kept in the dark.

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The illuminated PHE and PYR (Fig.1b) suspensions create similar, but less intense, discolored solutions,

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relative to the illuminated NAP and ANT. The increase in absorbance following illumination produces an

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elevated baseline into the near-UV and visible region of the spectrum. The single peak in the PHE light

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sample is shown to increase in intensity and shift slightly from 237 to 243 nm, while the PYR sample has

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similarly shifted its peak at 238 nm in the dark to a shoulder at 253 nm in the light. 7 ACS Paragon Plus Environment


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To further explore the chemical composition of the illuminated PAH, the HPLC chromatograms in Figure

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2 display the separations of individual products in the dark and illuminated samples. The NAP (Fig 2a)

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and ANT (Fig 2b) reactions display significantly more products from irradiation than do PHE and PYR

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(S1). Illuminated NAP samples demonstrate a wide range of products across the elution gradient starting

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at retentions of three minutes and displaying an even spread of products until 15 minutes. The most

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significant peaks have retention times of 8, 11, 13, and 15 minutes which, via their retention times, have

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been identified as phthalic anhydride, benzoic acid, phthalic acid, and 1-naphthol, respectively.

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Figure 2: HPLC analysis of post-reaction samples of saturated PAH, (a) NAP and (b) ANT. Lighted samples are represented in the solid spectra, the dashed spectra represents the samples kept in the dark. Numbered peaks are identified products based on retention times of standards dissolved in acetonitrile.1. phthalic anhydride, 2. benzoic acid, 3. phthalic acid, 4. naphthol, 5. naphthalene, 6. phthalic anhydride, 7. benzoic acid, 8. phthalic acid, 9. 1,4-naphthoquinone, 10. 9,10-anthraquinone, 11. anthracene.

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Illuminated ANT samples also show the creation of robust products across this 3 to 15-minute region and

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extend up to 23 minutes. Several peaks are shared between the NAP and ANT reactions, including

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phthalic anhydride, benzoic acid, phthalic acid, and both also contain peaks at 3, 4, and 6 minutes that 8 ACS Paragon Plus Environment

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have not yet been identified. Some key differences in these chromatograms include the introduction of

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new peaks in the ANT reaction within the 4.5 to 6-minute region, and peaks at 14, 15.5, 18.5, and 20

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minutes. Among these new peaks, the 14 and 18.5-minute peak products were identified as 1,4-

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naphthoquinone and 9,10-anthraquinone, respectively. These are compared to the very few peaks

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obtained from the same reactions kept in the dark.

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PHE and PYR produced a very small number of peaks in both reactions, which relate to the formation of

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very few products (Figure S1). Both PHE and PYR share significant peaks at 15 minutes in their

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illuminated and dark samples, but only illuminated PHE shows a strong peak at 16 minutes. The NAP and

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ANT in the light yield several products which correlates well with their elevated UV-vis absorptions.

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Conversely, the PHE and PYR samples demonstrate their lack of responses in UV-vis along with few

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HPLC products.

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Figure 2a shows that illuminated NAP samples display robust creation of various new products. These

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products have a fairly wide mass range of 200 to 800 m/z (LC-MS, Figure S2), suggesting significant

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growth due likely to a combination of oxidation and oligomerization. The indication of this growth is

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consistent with similar reaction mechanisms of benzene, ROS, and carbon-centered radicals41,

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Connecting this information with the UV-vis data in Figure 1 gives the strong suggestion that the curve of

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diminishing light absorption is likely due to shared light absorption between many different products.

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This suggestion seems to be further supported considering the PHE and PYR chromatographic data in S1,

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each displaying very few HPLC peaks in conjunction with a minimal rise in their UV-vis absorption.

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The combination of chemical characteristics discovered, including featureless UV-vis absorption curve,

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multitude of products, and increases in molecular mass, is highly indicative of atmospheric oxidized

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compounds collectively referred to as humic-like substances (HULIS)18. The composition of HULIS is

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significant in that it contains substantial aromatic and oxygenated functional regions18, and is similar in

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nature to terrestrial humic acids, giving them their namesake. This creates an intriguing connection

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considering that there is a strong correlation between BrC and HULIS observed in field samples4.

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Method of Formation of the Higher Molecular Mass Compounds

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To better understand the chemical mechanism towards the more massive oxidized products represented in

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the LCMS, we applied illumination to some of the discovered products in the ANT and NAP reactions.

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The UV-vis specta of the illuminated reaction of 1,4 naphthoquinone (Fig. 3a), as well as the 1-naphthol

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reaction (S3), produces robust absorption increases similar to the illuminated NAP and ANT. Specifically,

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they all demonstrate a UV-vis decay curve with higher absorptions at shorter wavelengths and vice versa,

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the only deviation being a broadened shoulder in naphthoquinone. This is in sharp contrast to the

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complete absence of UV-vis absorption seen from 9,10-anthroquinone samples (Fig. 3b). These results

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strongly suggest that NAP derivatives, e.g., naphthol and naphthoquinone, are likely intermediate

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structures in the overall reaction mechanisms. Conversely, the ANT derivatives such as 9,10-

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anthroquinone are shown to be dormant in sunlight and are therefore considered end products.

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Figure 3: UV-vis spectra of post-reaction samples of oxidized reageants. Solid lines represent samples in the light, dashed lines represent samples in the dark, (a) 1,4naphthoquinone, (b) 9,10-anthraquinone. These spectra are displayed in the 500 ppm concentration in which each reaction was produced. 10 ACS Paragon Plus Environment

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The changes in UV-vis spectra across starting materials follow very closely with the reagents observed to

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create peaks in the previous HPLC data. UV-vis data (Figure S3) show the lack of spectra response for

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post-reaction samples for benzoic and phthalic acid. The only change observed between light and dark

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samples of these simple aromatic acids were a more enhanced absorption near 260 nm in the light sample

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versus the dark. Interestingly, anthracene-diol (Figure S3) and 9,10-anthraquinone (Fig. 3b) not only

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show profiles from the light to be essentially unchanged from the dark, but UV-vis spectroscopy shows

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nearly no absorption at all. Neither the single aromatic acids, e.g., phthalic and benzoic acids, nor the

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oxidized ANT structure, 9,10-anthraquinone, seem to demonstrate the further oxidation necessary for BrC

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formation and are therefore considered end points, and do not play a role in HULIS production in the

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context of photoreactions.

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By contrast, photo-reactions of naphthoquinone (Fig.3a) and naphthol (Figure S3) produced a

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significantly greater absorbance profile. These spectra in the dark already contain robust absorption

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peaks; the illuminated naphthol reaction reproduces the UV-vis absorption decay curves seen with the

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absorption in the NAP and ANT reactions. The naphthoquinone has an elevated baseline that fits with the

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reactive PAH curves, however there also includes a pronounced shoulder on this spectra curve.

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The comparison of the UV-vis and HPLC data for the oxPAH offer a similar perspective to the same

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comparison of the parent PAH. Product formation for oxPAH reactions is summarized in Figure 4 and

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Figure S4.



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Figure 4: HPLC chromatograms of post-reaction samples of oxidized reageants. Solid lines represent samples in the light, dashed lines represent samples in the dark, (a) 1,4naphthoquinone, (b) 9,10-anthraquinone.

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The HPLC chromatograms for post-reaction samples for benzoic and phthalic acid (Figure S4) bare a

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similarity to the 9,10-anthroquinone chromatogram (Figure 4b), suggesting that none of these products

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are photo-active in sunlight. These simple aromatic acids are shown to produce very few new products in

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the light. Similarly, anthracene-diol (S4) and 9,10-anthroquinone (Fig.4b) show a product profile from the

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light to be essentially unchanged from the dark side of the reaction in presenting only one main product.

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In strong agreement with their UV-vis spectra, photo-reactions of naphthol (Figure S4) and

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naphthoquinone (Fig.4a) produced a large number of robust products, similar to the parent PAH, NAP

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and ANT. The major production of naphthol reactions look to start eluting at 11 minutes and continuing a

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raised baseline until 28 minutes. Naphthoquinone photoreaction shows steady production of several

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products eluting from about 5 minutes until 20 minutes. In summary, the production of several new peaks

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in 1,4-naphthoquinone (Fig. 4a) and naphthol (Figure S4) are consistent with their respective UV-vis

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absorption decay curves, while the lack of photolytic response in 9,10-anthroquinone can be seen in its

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single significant HPLC peak.

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Aromatic stability and functional groups

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Reactions of oxPAH demonstrate that polycyclic precursors only further oxidize in structures with one

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stabilized single-aromatic center, e.g., naphthol and naphthoquinone. While 9,10-anthraquinone, with two

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stable aromatic centers, shows no oxidation. Among the parent PAH, only the linear species displayed a

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characteristic photolytic oxidation. Among these oxPAH reagents, only multiple-aromatic structures with

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oxygen containing groups on their external rings were observed to undergo further oxidation and produce

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a brown carbon effect. Compounds with oxygen-containing groups on the internal rings did not show a

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response.

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The series of reactive PAH presented in this study demonstrate a pattern of predictive oxidizing-active

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structures and fits well within the set of Clar's rules that predicts the stability of PAH based on Lewis

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structures58-59. Stated briefly, the overall stability of PAH is based on the number of complete benzene

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rings, or π-sextet, depicted in an aromatic compound’s structure; where the higher number of sextets in

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the structure leads to lower energies of the aromatic system, and therefore greater stability and lower

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reactivity. This is particularly evident in the relatively large brown carbon response from the 3-ring

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anthracene compound, containing only a single sextet structure, and the lack of response from the 3-ring

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phenanthrene, containing two sextets.

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Based on our results and Clar’s rule, we hypothesize that 1,4-anthraquinone, having the oxidized portion

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on the external rings, may contribute a highly oxidizing potential, given the single stabilized sextet.

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Figure S5 shows the results of our photochemical reactor on saturated suspensions of 1,4-anthraquione.

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The UV-vis curve and retention pattern observed in light reactions and several oxidation products

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displays a pattern consistent with Clar’s rules in producing BrC and HULIS character even though 9,10-

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anthroquinone, with two sextets, was stable under these conditions.

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The chemical transformations under sunlight can also be traced by looking at the changes in functional

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groups. Figure 5 shows the addition of functional groups on NAP residues comparing illuminated to dark 13 ACS Paragon Plus Environment


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samples. The FTIR spectrum in Figure 5A displays the growth of peaks similar to the other PAH FTIR

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reactions that display light absorbing characteristics, including naphthoquinone and naphthol (Figure S7),

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and ANT (Figure S6) whose peaks are less pronounced due to decreased surface area and crystal contact.

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Figure 5: ATR-FTIR analysis of NAP products in light and dark conditions. Post-reaction samples are presented for illuminated naphthalene (a) and dark naphthalene (b). Light and dark product analyses are represented by the solid and dashed spectra, respectively. These are performed on a diamond crystal, therefore peak activity in the 2400 cm-1 region is considered background uncertainty.

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Both NAP (Fig 5b) and ANT (Fig S6) dark samples display single peaks at 3048 cm-1 and a group of

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several peaks from 1600 to 900 cm-1, correlating to poly-aromatic hydrogen stretches and bends,

263

respectively. One difference with the NAP dark sample, relative to the ANT dark sample, is the small dip

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centered at 3343 cm-1 which may indicate a hydroxyl hydrogen bend or adsorbed water species.

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Comparing these spectra with the illuminated samples allows for the determination of sunlight-specific

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growth. The NAP light spectrum (Fig 5a) shows significant growth of new peaks at 3219, 1717, and large

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baseline dip from 1360 to 950 cm-1. These would indicate the formation of alcohols, carbonyl groups, and

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ether bridges, respectively. The ANT light spectrum, in S6, displays new groups with modest absorbance

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peaks at 1673 and stretch from 1317 to 1175 cm-1, which also correlate to carbonyl and ether groups,

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respectively. An intriguing difference in functional group character between lighted NAP and ANT

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samples is the lack of hydroxyl stretches in the 3000-3500 cm-1 region of the ANT spectrum. 14 ACS Paragon Plus Environment

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Figure S7 shows bulk functionality on the remaining photo-active systems. Since only oxidized NAP

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derivatives are observed to be active samples under light and produced any character related to brown

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carbon, the FTIR analyses will focus on the structural comparison of those products to known HULIS

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character, demonstrating a potential step in an overall mechanistic pathway of formation. Naphthol and

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naphthoquinone (Figure S7a and b) demonstrate the three main features found in oxidized NAP

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functionality. These peaks of interest are the broad peak at 3300 cm-1, sharp peak at 1700 cm-1, and a

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pronounced broad peak between 1300-1100 cm-1.

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Suggested Photo-Reaction Pathway from PAH to HULIS/Brown Carbon

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The photo-oxidation pathways of NAP and ANT clearly evolved in several directions. Some of these

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directions lead to simple oxidized end points such as benzoic acid, while others are observed to lead to

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further, high molecular mass oxidation products, as evidenced on the LCMS. Figure 6 represents the

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overall collection of information that has come from the NAP and ANT reactions and may act as a

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precursor for further kinetic and mechanistic studies.



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Figure 6: Preliminary suggested reaction pathways in the production of HULIS from (A) naphthalene and (B) anthracene. Solid arrows represent proposed reactions involving HULIS intermediates and dashed arrows represent formation of end products. FAST and SLOW designations are qualitative and based on the prominence of those compounds in the post-reaction sample. Note that all pathways are elucidated from illuminated samples.

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While there are likely several reaction pathways in these reactions, the data presented here offer strong

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evidence for some of the major pathways. Figure 6 displays our proposed preliminary mechanistic

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pathways toward HULIS from NAP and ANT via photochemistry. The shared presence of produced

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brown carbon with naphthoquinone character in both NAP and ANT reactions strongly suggests that the

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conversion of naphthoquinone into brown carbon is likely a faster, favored reaction pathway. Conversely,

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the lack of brown carbon similar to naphthol reactions in NAP reactions indicates that the conversion of 16 ACS Paragon Plus Environment

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naphthol into brown carbon must be a slower process than naphthoquinone. NAP reactions are indicated

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as producing naphthol at a faster rate than they produce naphthoquinone. This is based on residual

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naphthol found in NAP products, which overall demonstrate brown carbon with naphthoquinone

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character (Figure 2).

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The ANT proposed pathways are depicted with a nearly opposite trend. The HPLC product data suggest

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that ANT is likely to produce 1,4-naphthoquinone at a very rapid pace along with its brown carbon

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products, given our previous estimation that these compounds are converted to brown carbon relatively

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quickly. The observation that ANT also produces compounds that are retained similar to naphthol and

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1,4-anthraquinone products further suggests that these are being produced as well, albeit at a slower rate

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relative to naphthoquinone. Since neither of these structures remains in the ANT sample after the

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reaction, we must conclude that they are oxidized at the same rate as they are being produced.

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Since every reaction with a brown carbon response have been observed to produce this only in

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illuminated samples, the specific impact of sunlight cannot be overstated. The excitation of dissolved

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oxygen or water molecules in the presence of PAH by UV radiation, producing varying ROS such as OH

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(as observed by EPR spectroscopy, (Figure S8)), would be a likely component to these reactions. Here,

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we can suggest some reaction scenarios that may arise with produced hydroxyl (OH), superoxide (Ȯ2¯),

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and singlet oxygen (1O2) species. Reactions of OH typically involve the homolytic capture of acidic

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hydrogens to form water and a carbon centered radical. This leaves the hydrocarbon susceptible to attack

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and will result in a hydroxide group at the site of the attack60. PAH reactions likely use OH to produce

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significant naphthol, and potentially use elemental oxygen, such as singlet or superoxide to produce

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quinones via Diels-Alder additions61-62. These are further used as favored intermediates toward the

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formation of brown carbon. The prominence of 1,4 quinones as active intermediates in these reactions

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may relate to known phenolic oligomerization mechanisms via semiquinone redox-cycling 63. A potential

320

keto-enol tautomerization to form the semiquinone would be available to 1,4 diketone positions while

321

unavailable to the 9,10 anthraquinone structure, which is shown to be inactive in this study. 17 ACS Paragon Plus Environment


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322

HULIS Characteristics

323

The characteristic properties observed in brown carbon production in this study show a prominent

324

similarity to behavior in humic-like substances (HULIS), which are seen as a major component of brown

325

carbon64. HULIS is defined as being structurally similar to humic acids, and as such are presented as a

326

collective of compounds containing significant aromatic and oxidized character18. The propensity for

327

atmospheric HULIS material to evolve from combustion processes65 dictates that the bulk structures will

328

contain aliphatic regions of unburned organic fuels66 and significant aromatic areas formed from

329

inefficient combustion23, 67. HULIS samples observed from field campaigns have demonstrated increased

330

average oxidation states of carbon from -2.0 to -0.468 and contain enhanced oxygen:carbon ratios ranging

331

from 0.69 to 1.069-70 presented as oxygen-containing functional groups. Production of these oxidized

332

groups may be due to oxidation during combustion or down field oxidation by atmospheric oxyradicals71,

333

such as hydroxyl and superoxide radicals.

334

The mass distribution of new products formed from NAP and ANT photo oxidation match the expected

335

mass range of HULIS, specifically in regard to major product groups near 150 m/z and extended mass

336

growth up through 800 and 1000 m/z. Supplementary Figure S2 shows the mass range of saturated

337

aqueous suspensions of PAH and oxPAH reactions. Figures S2a and b represent oxidized naphthalene and

338

anthracene LCMS products; while the peaks are more pronounced in the naphthalene masses than the

339

anthracene, their range and overall spread of masses are very similar. The differences between the

340

naphthol (S2c) and naphthoquinone (S2d) are few but pronounced. They both have a similar baseline

341

spread of products across a range of masses from 200 to 800 m/z, however the naphthol has shown

342

interesting bumps in peaks at the 115 and 301 m/z values.

343

To further confirm the HULIS-like nature of these reactions, analyses of eluted products in a HULIS-

344

specific solid phase extraction (SPE) method5, 18, 72 presents further evidence of this activity. Any products

345

retained using this method are detected by UV-vis and HPLC methods as described above, except that


Page 19 of 25


346

these extractions are evaluated in the methanol matrix used for the elution, and are therefore defined as

347

showing HULIS-like retention.

348

Solid phase extraction analyses of NAP and ANT reactions (Figure S9) demonstrate the presence of

349

products with similar retention characteristics to HULIS, confirming the contribution of HULIS in PAH-

350

generated BrC. The UV-vis analyses for each sample demonstrates a similar absorption curve as the

351

original aqueous lighted sample with a mild shoulder, suggesting a similar composition of HULIS

352

products including some prominence of a major product. These features bear no resemblance to the single

353

peak in their aqueous dark samples; therefore this maintains evidence of various HULIS material with

354

some major products of similar composition. The solid phase elution of HULIS products is further

355

confirmed in the HPLC analyses for NAP and ANT extractions, where significant product peaks are

356

observed and thereby demonstrating the presence of individual HULIS-like products.

357

Environmental Implications

358

The discovery of HULIS character in these reactions introduces new potential hazards for PAH. The

359

formation of groups resembling carboxylic acids, i.e. carbonyl and hydroxy groups, suggests a rationale

360

for an increase in solubility and the potential for chelation and mobilization of previously insoluble

361

nutrients73 and metals found in atmospheric particles74. Complexation reactions of environmental

362

nutrients and metals, such as iron, use water soluble organic species with oxygen-containing groups,

363

carboxylic acids for example, to chelate the typically insoluble ferric iron and act as an aqueous-phase

364

system to bring the iron into solution34, 75. This coincides well with relative soluble iron increases near

365

biomass burning regions76-77, and fits with the expected characteristics of oxPAH including phthalic acid's

366

greater aqueous solubility and carboxylic acids groups. The oxygen-containing groups on HULIS and the

367

products of this study may be able to mobilize cytotoxic metals and oxidizers in a manner similar to

368

siderophore compounds that aid organisms in the uptake of iron78-79. The photolytic connection between



Page 20 of 25

369

PAH and increasing ROS emphasizes an intriguing argument for the continued threat these combustion

370

products can have on the environment during their long range transport.

371

Supporting Information. Figures S1-S9 (PDF)

372

Corresponding Author

373

*Phone (303) 871-2986; email: [email protected].

374

ORCID

375

John P. Haynes: 0000-0002-4454-2093

376

Brian J Majestic: 0000-0003-1860-8738

377

Acknowledgements

378

We thank Drs. Benton Cartledge and Gary Bishop for their help and advice on the installation of the

379

photosimulation reactor components. Dr. Bryan Cowen is thanked for his advice on materials preparation

380

for the reactions involving organic aromatic compounds. We thank Drs. Gareth and Sandra Eaton, as well

381

as Dr. Debbie Mitchell for their help in collecting and interpreting the EPR spectra. This study was

382

supported by NSF award 1342599. The authors declare no competing financial interest.

383


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384

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Investigation into Photo-induced Auto-oxidation of Polycyclic Aromatic

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