Matthieu Riva
email: matthieu.riva@ircelyon.univ-lyon1.fr

Yuzhi Chen

Yue Zhang

Ziying Lei

Nicole E Olson

Hallie C Boyer #

Shweta Narayan

Lindsay D Yee

Hilary S Green

Tianqu Cui

Zhenfa Zhang

Karsten Baumann

Mike Fort

Sri H Budisulistiorini

Caitlin A Rose

Igor O Ribeiro

Rafael L E Oliveira

Erickson O Dos

Cristine M D Machado

Sophie Szopa

Yue Zhao ¬,''

Eliane G Alves $

Suzane S De Sá

Weiwei Hu

Eladio M Knipping

Stephanie L Shaw •••

Sergio Duvoisin

Rodrigo A F De Souza

Brett B Palm

Jose-Luis Jimenez

Marianne Glasius

Allen H Goldstein ∇

Havala O T Pye

Avram Gold

Barbara J Turpin

William Vizuete

Scot T Martin

Joel A Thornton

Cari S Dutcher #

Andrew P Ault
email: aulta@umich.edu

Jason D Surratt
email: surratt@unc.edu

Increasing Isoprene Epoxydiol-to-Inorganic Sulfate Aerosol (IEPOX:Sulf inorg ) Ratio Results in Extensive Conversion of Inorganic Sulfate to Organosulfur Forms: Implications for Aerosol Physicochemical Properties

Keywords: SOA, mass spectrometry, heterogeneous chemistry, reactive uptake

Acid-driven multiphase chemistry of isoprene epoxydiols (IEPOX), key isoprene oxidation products, with inorganic sulfate aerosol yields substantial amounts of secondary organic aerosol (SOA) through the formation of organosulfur compounds. The extent and implications of inorganic-to-organic sulfate conversion, however, are unknown. In this report, we demonstrate that extensive consumption of inorganic sulfate occurs, which increases with the IEPOX-to-inorganic sulfate concentration ratio (IEPOX:Sulf inorg ), as determined by laboratory measurements. Characterization of total sulfur aerosol observed at Look Rock, Tennessee, from 2007-2016 shows that organosulfur mass fractions will likely continue to increase with ongoing declines in anthropogenic Sulf inorg , consistent with our laboratory findings. We further demonstrate that organosulfur compounds greatly modifies critical aerosol properties, such as acidity, morphology, viscosity, and phase state. These new mechanistic insights demonstrate that changes in SO 2 emissions, especially in isoprene-dominated environments, will significantly alter biogenic SOA physicochemical properties. Consequently, IEPOX:Sulf inorg will play an important role in understanding historical climate and determining future impacts of biogenic SOA on global climate and air quality.

ASSOCIATED CONTENT

The materials and methods provide additional information on the collection of PM 2.5 performed during the different field campaigns. In addition, more details on the chemical characterization of the organosulfur compounds are also proposed. Finally, the thermodynamic model of aerosol pH is explained. Figures S1 andS2 show the representative Taylor plots and the interfacial tensions of the main IEPOX-derived SOA components, respectively. Figures S3-S5 show the mass concentrations of aerosol sulfate and organosulfur compounds as well as their contribution to the mass of sulfate. Figure S6 compares the modelled aerosol pH when considering vs not considering IEPOX-OS. Figure S7 shows AFM amplitude and phase images, and SEM images of IEPOX-OA. Figure S8 shows the diel-profiles of the RH in the amazon forest during the wet and dry season as well as the concentration of isoprene. Figure S9 presents the interfacial tension of 2-MT. Figure S10 shows the gas-phase concentration of IEPOX during chamber experiments. The tables provide a summary of experimental conditions as well as a summary of the different organosulfates identified in SOA collected during the different field campaigns. This material is available free of charge via the Internet at http://pubs.acs.org/.

Introduction

Secondary organic aerosol (SOA) formed through the oxidation of volatile organic compounds is a major and globally ubiquitous component of atmospheric fine particulate matter (PM 2.5 ; aerosol particles ≤ 2.5 μm in aerodynamic diameter). [START_REF] Jimenez | Evolution of Organic Aerosols in the Atmosphere[END_REF] Chemical composition of fine particles determines aerosol physicochemical properties, such as viscosity and phase state, and plays a central role in the effects of SOA on air quality and climate. [START_REF]Pope III CA Lung Cancer, Cardiopulmonary Mortality, and Long-Term Exposure to Fine Particulate Air Pollution[END_REF][START_REF] Shiraiwa | Global Distribution of Particle Phase State in Atmospheric Secondary Organic Aerosols[END_REF] Understanding how SOA forms and interacts with other gas-and particle-phase species is crucial to accurately evaluating its importance in the Earth's climate system and adverse effects on public health. Inorganic sulfate species (e.g., SO 4 2-, HSO 4 -) are also a significant PM 2.5 component with the capacity to impact atmospheric composition and climate, in part, because of its predicted impact on aerosol acidity, hygroscopicity, visibility and cloud nucleation. [START_REF] Jimenez | Evolution of Organic Aerosols in the Atmosphere[END_REF][START_REF]Climate Change[END_REF] The oxidation of sulfur dioxide (SO 2 ) to sulfuric acid (H 2 SO 4 ) increases aerosol acidity, which enhances SOA formation. [START_REF]Climate Change[END_REF][START_REF] Surratt | Effect of Acidity on Secondary Organic Aerosol Formation from Isoprene[END_REF][START_REF]Jang M Heterogeneous Atmospheric Aerosol Production by Acid-Catalyzed Particle-Phase Reactions[END_REF][START_REF] Offenberg | Jaoui M Influence of Aerosol Acidity on the Formation of Secondary Organic Aerosol from Biogenic Precursor Hydrocarbons[END_REF][START_REF] Zhang | Surratt JD Organosulfates as Tracers for Secondary Organic Aerosol (SOA) Formation from 2-Methyl-3-Buten-2-Ol (MBO) in the Atmosphere[END_REF] Sulfur in the form S(VI) in aerosols was generally assumed to be primarily present as inorganic sulfate (SO 4 2-and HSO 4 -) ions until more recent studies revealed the presence of organosulfur components in PM 2.5 . [START_REF] He | Mulawa P The Characteristics of PM2.5 in Beijing, China[END_REF][START_REF] Wu | Tang U Chemical Characteristics of Airborne Particulate Matter near Major Roads and at Background Locations in Macao, China[END_REF][START_REF] Lukács | Hartyáni Z Quantitative Assessment of Organosulfates in Size-Segregated Rural Fine Aerosol[END_REF][START_REF] Tolocka | Contribution of Organosulfur Compounds to Organic Aerosol Mass[END_REF][START_REF] Shakya | Peltier RE Non-Sulfate Sulfur in Fine Aerosols across the United States: Insight for Organosulfate Prevalence[END_REF] Despite observations that organosulfur compounds are important contributors to SOA mass in a range of environments globally, [START_REF] Pratt | Surratt JD Organosulfates in Cloud Water above the Ozarks' Isoprene Source Region[END_REF][START_REF] Boone | Pratt KA Aqueous Processing of Atmospheric Organic Particles in Cloud Water Collected via Aircraft Sampling[END_REF][START_REF] Altieri | Organosulfates, and Nitrooxy Organosulfates in Rainwater Identified by Ultra-High Resolution Electrospray Ionization FT-ICR Mass Spectrometry[END_REF][START_REF] Gómez-González | Characterization of Organosulfates from the Photooxidation of Isoprene and Unsaturated Fatty Acids in Ambient Aerosol Using Liquid Chromatography/(-) Electrospray Ionization Mass Spectrometry[END_REF][START_REF] Hawkins | Carboxylic Acids, Sulfates, and Organosulfates in Processed Continental Organic Aerosol over the Southeast Pacific Ocean during VOCALS-REx 2008[END_REF][START_REF] Hansen | Organosulfates and Organic Acids in Arctic Aerosols: Speciation, Annual Variation and Concentration Levels[END_REF][START_REF] Glasius | Organosulfates in aerosols downwind of an urban region in central Amazon[END_REF] estimations of aerosol acidity and liquid water content typically assume that only inorganic sulfate plays a role. [START_REF] Weber | Nenes A High Aerosol Acidity despite Declining Atmospheric Sulfate Concentrations over the Past 15 Years[END_REF] Correctly identifying the chemical form of sulfur (i.e., inorganic vs. organic), and representing it accurately in atmospheric models is essential as the different forms lead to different aerosol physicochemical properties that will have different predicted impacts on air quality and climate. [START_REF] Shiraiwa | Global Distribution of Particle Phase State in Atmospheric Secondary Organic Aerosols[END_REF] Laboratory studies have demonstrated that acid-driven multiphase chemistry (reactive uptake) of isoprene epoxydiols (IEPOX) is key to explaining the chemical form and extent of SOA formation from photochemical oxidation of isoprene [START_REF] Bates | Gas Phase Production and Loss of Isoprene Epoxydiols[END_REF][START_REF] Paulot | Unexpected Epoxide Formation in the Gas-Phase Photooxidation of Isoprene[END_REF][START_REF] Surratt | Seinfeld JH Reactive Intermediates Revealed in Secondary Organic Aerosol Formation from Isoprene[END_REF][START_REF] Gaston | Reactive Uptake of an Isoprene-Derived Epoxydiol to Submicron Aerosol Particles[END_REF] and field measurements have confirmed this is the predominant pathway. [START_REF] Rattanavaraha | Assessing the Impact of Anthropogenic Pollution on Isoprene-Derived Secondary Organic Aerosol Formation in PM2.5 Collected from the Birmingham, Alabama, Ground Site during the 2013 Southern Oxidant and Aerosol Study[END_REF][START_REF] Budisulistiorini | Examining the Effects of Anthropogenic Emissions on Isoprene-Derived Secondary Organic Aerosol Formation during the 2013 Southern Oxidant and Aerosol Study (SOAS) at the Look Rock, Tennessee Ground Site[END_REF][START_REF] Hu | Characterization of a Real-Time Tracer for Isoprene Epoxydiols-Derived Secondary Organic Aerosol (IEPOX-SOA) from Aerosol Mass Spectrometer Measurements[END_REF][START_REF] Robinson | Evidence for a Significant Proportion of Secondary Organic Aerosol from Isoprene above a Maritime Tropical Forest[END_REF][START_REF] Xu | Ng NL Effects of Anthropogenic Emissions on Aerosol Formation from Isoprene and Monoterpenes in the Southeastern United States[END_REF][START_REF] De Sá | Martin ST Influence of Urban Pollution on the Production of Organic Particulate Matter from Isoprene Epoxydiols in Central Amazonia[END_REF] While chamber studies have shown that organosulfur compounds, specifically organosulfates (OS) [START_REF] Surratt | Seinfeld JH Organosulfate Formation in Biogenic Secondary Organic Aerosol[END_REF] formed by the reactive uptake of IEPOX with particulate inorganic sulfate (Sulf inorg ), contribute significantly to IEPOX-SOA, [START_REF] Lin | Surratt JD Isoprene Epoxydiols as Precursors to Secondary Organic Aerosol Formation: Acid-Catalyzed Reactive Uptake Studies with Authentic Compounds[END_REF][START_REF] Riva | Zelenyuk A Effect of Organic Coatings, Humidity and Aerosol Acidity on Multiphase Chemistry of Isoprene Epoxydiols[END_REF] the extent and implications of sulfate conversion to organic forms have remained unknown.

Combined laboratory, field, and modeling studies described in this study reveal the hitherto unrecognized effect that acid-driven multiphase chemistry of IEPOX may result in substantial conversion of Sulf inorg to organosulfur compounds. Laboratory experiments reveal that the extent of Sulf inorg conversion is greater for high IEPOX:Sulf inorg ratios (such as observed in the Amazon) than for lower ratios (such as observed in the Southeastern United States (SE U.S.)). Assuming IEPOX concentration remains constant, continuing Sulf inorg reductions in the Northern Hemisphere as a consequence of SO 2 emission controls are thus expected to greatly increase the fraction of Sulf inorg converted in the future. High IEPOX:Sulf inorg scenario also likely characterized pre-industrial atmospheric conditions. Changes in the ratio over time and the consequent changes in SOA chemical composition and physicochemical properties will increase understanding of the evolution of climate change and inform projections for the future.

Experimental Methods

Smog Chamber Experiments.

Experiments were performed in the indoor environmental chamber facility at the University of North Carolina. The experimental setup and analytical techniques used in this work have been described in detail elsewhere [START_REF] Lin | Surratt JD Isoprene Epoxydiols as Precursors to Secondary Organic Aerosol Formation: Acid-Catalyzed Reactive Uptake Studies with Authentic Compounds[END_REF][START_REF] Riva | Chemical Characterization of Secondary Organic Aerosol Constituents from Isoprene Ozonolysis in the Presence of Acidic Aerosol[END_REF] . Briefly, experiments were carried out under dark and wet conditions (50 ± 4 %, relative humidity (RH)) at 296 ± 1 K in a 10-m 3 Teflon chamber. A summary of the experimental conditions is provided in Table S1. Prior to each experiment, the chamber was flushed continuously with clean air for ~ 24 hours corresponding to a minimum of seven chamber volumes until the particle mass concentration was < 0.01 μg m -3 . Chamber flushing also reduced VOC concentrations below the detection limit (i.e., ~ 75 ppt for IEPOX). Temperature and RH in the chamber were continuously monitored using a dew point meter (Omega Engineering Inc.). Acidified ammonium sulfate seed aerosols were injected into the pre-humidified chamber using a custom-built atomizer with an aqueous solution of 0.06 M (NH 4 ) 2 SO 4 (aq) and 0.06 M H 2 SO 4 (aq) until the desired total aerosol mass concentration was achieved. After seed injection, the chamber was left static for at least 30 min to ensure that the seed aerosol stability and uniform mixing. The IEPOX vapor was generated by using a high-purity N 2 (g) flow of 2 L min -1 for 10 min and then 4 L min -1 for 50 min through a heated manifold (60°C) containing a known amount of pure trans-β-IEPOX (synthesized in house [START_REF] Zhang | Gold A Technical Note: Synthesis of Isoprene Atmospheric Oxidation Products: Isomeric Epoxydiols and the Rearrangement Products Cisand Trans-3-Methyl-3,4-Dihydroxytetrahydrofuran[END_REF] ) dissolved in ethyl acetate. This approach to introduce IEPOX has been used in previous studies. [START_REF] Lin | Surratt JD Isoprene Epoxydiols as Precursors to Secondary Organic Aerosol Formation: Acid-Catalyzed Reactive Uptake Studies with Authentic Compounds[END_REF][START_REF] Riedel | Surratt JD Heterogeneous Reactions of Isoprene-Derived Epoxides: Reaction Probabilities and Molar Secondary Organic Aerosol Yield Estimates[END_REF][START_REF] Riedel | Constraining Condensed-Phase Formation Kinetics of Secondary Organic Aerosol Components from Isoprene Epoxydiols[END_REF] Aerosol size distributions were continuously measured using a differential mobility analyzer (DMA, BMI model 2002) coupled to a mixing condensation particle counter (MCPC, BMI model 1710) in order to monitor aerosol number, surface area, and volume concentration within the chamber. SOA generated from the reactive uptake of IEPOX was collected using a particle-into-liquid sampler (PILS, BMI model 4001) throughout each experiment. PILS samples were collected at 5 min intervals for offline chemical analysis by ion chromatography (IC) and ultra-performance liquid chromatography interfaced to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ ESI-HR-Q-TOFMS). The PILS vials were stored in the dark at 2°C immediately after collection and analyzed within 24 h without further pretreatment. PILS operating conditions and dilution correction are described in the SI.

Flow Tube Experiments.

The reactive uptake coefficient of IEPOX (γ IEPOX ) is an important model-relevant parameter that characterizes the heterogenous chemistry of IEPOX. [START_REF] Surratt | Seinfeld JH Reactive Intermediates Revealed in Secondary Organic Aerosol Formation from Isoprene[END_REF][START_REF] Zhang | Gold A Technical Note: Synthesis of Isoprene Atmospheric Oxidation Products: Isomeric Epoxydiols and the Rearrangement Products Cisand Trans-3-Methyl-3,4-Dihydroxytetrahydrofuran[END_REF] Direct measurement of γ IEPOX onto aqueous ammonium bisulfate (ABS) particles was conducted at the University of Washington in an aerosol flow tube coupled to an iodide-adduct chemical ionization mass spectrometry (CIMS); operating conditions have been described elsewhere and in the SI.

Aerosol Chemical Characterization.

Chemical characterization of PM 2.5 and PILS samples was performed by UPLC/ESI-HR-Q-TOFMS (6520 Series, Agilent) operated in the negative ion mode. [START_REF] Lin | Surratt JD Light-Absorbing Oligomer Formation in Secondary Organic Aerosol from Reactive Uptake of Isoprene Epoxydiols[END_REF][START_REF] Riva | Chemical Characterization of Secondary Organic Aerosol from Oxidation of Isoprene Hydroxyhydroperoxides[END_REF] Total water-soluble organosulfur compound mass was determined from the difference between total watersoluble sulfur measured by isotope ratio inductively coupled plasma mass spectrometry (IR-ICP-MS) and sulfate-sulfur (sulfur in the form of inorganic sulfate/Sulf inorg ) measured by IC on the same sample aliquot. Sample extractions, operating conditions, standard preparation, and uncertainty estimates are described in detail in SI. The National Park Service Improve database [START_REF] Malm | Cahill TA Spatial and Seasonal Trends in Particle Concentration and Optical Extinction in the United States[END_REF] was used to evaluate the inorganic sulfate and organosulfur mass concentrations in the SE-U.S from 2007 to 2016.

Microscopy Imaging.

Morphology of aerosol particles collected before, during, and after IEPOX reactive uptake from smog chamber experiments was characterized by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Additional information regarding operating conditions is provided in SI and prior publications. [START_REF] Ault | Atmospheric Aerosol Chemistry: Spectroscopic and Microscopic Advances[END_REF][START_REF] Bondy | Ault AP Atomic Force Microscopy-Infrared Spectroscopy of Individual Atmospheric Aerosol Particles: Subdiffraction Limit Vibrational Spectroscopy and Morphological Analysis[END_REF][START_REF] Bondy | Ault AP Inland Sea Spray Aerosol Transport and Incomplete Chloride Depletion: Varying Degrees of Reactive Processing Observed during SOAS[END_REF] Biphasic Microfluidics.

Surface and interfacial tensions inform solute surface-bulk partitioning and, therefore, the availability of compounds for interactions with the ambient gaseous phase, such as heterogenous chemistry and water uptake. Lower interfacial tensions due to organic partitioning affect both particle growth and structure. We measured liquid-liquid interfacial tensions of IEPOX SOA tracer 2-methyltetrols (2-MT) and its OS derivative (3-methyltetrol sulfate, IEPOX-OS) using a biphasic microfluidic platform. Droplet microfluidics as a method to measure interfacial tension for aerosol science applications was first introduced for studying the aging of SOA aerosols, [START_REF] Metcalf | Dutcher CS Interfacial Tensions of Aged Organic Aerosol Particle Mimics Using a Biphasic Microfluidic Platform[END_REF] using a microtensiometry technique described in detail in several studies and in the SI. [START_REF] Hudson | Amis EJ Microfluidic Interfacial Tensiometry[END_REF][START_REF] Cabral | Hudson SD Microfluidic Approach for Rapid Multicomponent Interfacial Tensiometry[END_REF][START_REF] Lee | Temperature Controlled Tensiometry Using Droplet Microfluidics[END_REF][START_REF] Narayan | Dutcher CS Removing Water from Diesel Fuel: Understanding the Impact of Droplet Size on Dynamic Interfacial Tension of Water-in-Fuel Emulsions[END_REF][START_REF] Metcalf | Review of Microfluidic Concepts and Applications for Atmospheric Aerosol Science[END_REF] The microfluidic chip is fabricated using standard soft-lithography techniques [START_REF] Mazutis | Heyman JA Single-Cell Analysis and Sorting Using Droplet-Based Microfluidics[END_REF][START_REF] Qin | Whitesides GM Soft Lithography for Micro-and Nanoscale Patterning[END_REF][START_REF] Xia | Soft Lithography[END_REF] and mounted on an inverted microscope. Two immiscible liquid phases separately enter the device by pressure-driven flow, [START_REF] Metcalf | Review of Microfluidic Concepts and Applications for Atmospheric Aerosol Science[END_REF] and droplet breakup is induced by a flow-focusing geometry [START_REF] Anna | Stone HA Formation of Dispersions Using "Flow Focusing[END_REF][START_REF] Garstecki | Stone HA Formation of Monodisperse Bubbles in a Microfluidic Flow-Focusing Device[END_REF][START_REF] Moyle | Anna SL Predicting Conditions for Microscale Surfactant Mediated Tipstreaming[END_REF] Aqueous solutions containing the SOA are the dispersed (droplet) phase, and silicone oil is the carrier (continuous) phase. The droplets of aqueous SOA then enter a contraction geometry in the microfluidic channel, which induces deformation of the liquid-liquid interface. This deformation, is imaged at high speed, and related to material and flow-field properties [START_REF] Cabral | Hudson SD Microfluidic Approach for Rapid Multicomponent Interfacial Tensiometry[END_REF][START_REF] Lee | Temperature Controlled Tensiometry Using Droplet Microfluidics[END_REF][START_REF] Rallison | The Deformation of Small Viscous Drops and Bubbles in Shear Flows[END_REF][START_REF] Taylor | The Formation of Emulsions in Definable Fields of Flow[END_REF][START_REF]Taylor GI The Viscosity of a Fluid Containing Small Drops of Another Fluid[END_REF][START_REF] Martin | Hudson SD Mass Transfer and Interfacial Properties in Two-Phase Microchannel Flows[END_REF] to calculate the interfacial tension between silicone oil and aqueous SOA solutions. The equation governing droplet deformation is:

αη c 5 2η + 3 ε ˙x -u x ∂D x ∂x = σ D x a 0 . (1)
In the above equation, α is a function of the viscosity ratio η, which is a ratio of the dispersed phase viscosity (here, the aqueous phase viscosity is close to that of pure water -1 mPa s -near 100% RH for aerosol droplets), to the continuous phase viscosity η c (here, the viscosity of silicone oil -50 mPa s). For solutions containing 1.55 M ammonium sulfate (AS), the viscosity is 1.215 mPa s. [START_REF] Laliberté | Model for Calculating the Heat Capacity of Aqueous Solutions, with Updated Density and Viscosity Data[END_REF] ε ˙x is the extensional strain rate in the contraction, u(x) is the velocity of the droplet centroid, D(x) is the deformation, a 0 is the un-deformed diameter of the spherical droplet, and σ is the interfacial tension between the aqueous and oil phase. D(x) is defined by Taylor [START_REF] Rallison | The Deformation of Small Viscous Drops and Bubbles in Shear Flows[END_REF][START_REF] Taylor | The Formation of Emulsions in Definable Fields of Flow[END_REF][START_REF]Taylor GI The Viscosity of a Fluid Containing Small Drops of Another Fluid[END_REF] as D x = is linear in the smalldeformation regime [START_REF] Rallison | The Deformation of Small Viscous Drops and Bubbles in Shear Flows[END_REF][START_REF]Taylor GI The Viscosity of a Fluid Containing Small Drops of Another Fluid[END_REF] . A straight line is fit to the linear portion of the curve, known as a "Taylor plot", and the slope of the line is equal to the interfacial tension.

Each microfluidic measurement consumes 250-300 μL of aqueous solution, yielding 20-30 droplets. The IEPOX-OS droplet interfacial tensions reported in this work were statistically averaged. All interfacial tension values reported in this work are then normalized by the interfacial tension of pure water with silicone oil measured with microfluidics (30 mN/m). Figures S1 andS2 show Taylor plots and interfacial tensions for 2-MT and IEPOX-OS samples. The surface partitioning of SOA is analogous to that expected to occur in aerosol droplets in the atmosphere, as discussed in Metcalf et al. [START_REF] Metcalf | Review of Microfluidic Concepts and Applications for Atmospheric Aerosol Science[END_REF] Techniques for measurement of interfacial tension using biphasic microfluidics are outlined in prior work. [START_REF] Metcalf | Review of Microfluidic Concepts and Applications for Atmospheric Aerosol Science[END_REF][START_REF] Boyer | Atmospheric Aqueous Aerosol Surface Tensions: Isotherm-Based Modeling and Biphasic Microfluidic Measurements[END_REF] Viscosity Model.

The viscosity of IEPOX-derived OS is calculated based on a modified version of the Vogel-Tammann-Fulcher (VTF) equation (Eqs. 2 & 3) by Angell et al. [START_REF] Angell | Formation of Glasses from Liquids and Biopolymers[END_REF][START_REF]Angell CA Liquid Fragility and the Glass Transition in Water and Aqueous Solutions[END_REF]

η RH = η ∞ e T 0 D T -T 0 (2)
where η ∞ is viscosity at infinite temperature and assumed to be 10 -5 Pa s; T 0 is the Vogel temperature; T is the ambient temperature; and D is the fagility parameter, which controls how closely a material follows the Arrhenius law. [START_REF] Angell | Formation of Glasses from Liquids and Biopolymers[END_REF] When T reaches glass transition temperature T g , η reaches 10 12 Pa s. Then Eq. 2 becomes

T g T 0 = 1 + 0.0255D (3)
Both D and T g are required to calculate η for SOA-water mixtures at a given temperature. The T g of each SOA compound comprising the SOA under dry conditions was estimated based on the improved paramterizaiton described in DeRieux et al. [START_REF] Derieux | Shiraiwa M Predicting the Glass Transition Temperature and Viscosity of Secondary Organic Material Using Molecular Composition[END_REF] Then Gordan-Taylor mix rule (Eq. 8 in the SI) was applied to calculate the T g of SOA mixtures. A detailed description of the workflow and choices of parameters used in calculating the viscosity of IEPOX-derived OS is given in the SI.

Thermodynamic Model of Aerosol pH.

The pH of aerosol particles during the heterogenous reaction processes of IEPOX is calculated with smog chamber data using a thermodynamic model employing two methods. Briefly, the first method uses measured Sulf inorg concentration as well as the NH 4 + concentration at the beginning of the experiment as the input variables. The concentrations of IEPOX-OS and oligomeric OS are not considered. While retaining the inorganic cations and anions considered in the first scenario, the second method takes the anions of IEPOX-OS monomer as well as its dimers and trimers into consideration. The proton balance equation together with the mass balance equations for NH 4 + , SO 4 2-, HSO 4 -, and IEPOX-OS monomers, dimers, and trimers were built to solve the H + ion concentration. The gas-particle equilibrium of NH 3 and NH 4 + was considered using a Henry's law constant of 0.0161 atm M -1 . A growth factor of 1.3 was used to calculate the liquid water content of the particles. Similar to the ISORROPIA-II model, [START_REF] Fountoukis | A Computationally Efficient Thermodynamic Equilibrium Model for K + -Ca 2+ -Mg 2+ -NH 4 + -Na + -SO 4 2--NO 3 -;-Cl --H 2 O Aerosols[END_REF] the activity coefficients for all ions were assumed to be 1 in this case. The uncertainties in the growth factor of the particle, the activity coefficient of each ion, and the pKa values of organosulfur species contribute to the overall uncertainty of this model. These factors may affect the exact values of the pH calculated; however, the drastic difference of the pH values between considering and not considering the organosulfur species remains roughly the same. A detailed description of the thermodynamic model is given in the SI.

Global Modeling.

The IEPOX:Sulf inorg mass ratio has been computed based on numerical simulations performed with the LMDz-OR-INCA global climate-chemistry model. The description of the model representation for sulfate as well as the general features of the LMDz-OR-INCA model can be found in Szopa et al. [START_REF] Szopa | L Aerosol and Ozone Changes as Forcing for Climate Evolution between 1850 and 2100[END_REF] The reaction mechanism of IEPOX is based on St Clair et al. [START_REF] Clair | Wennberg PO Kinetics and Products of the Reaction of the First-Generation Isoprene Hydroxy Hydroperoxide (ISOPOOH) with OH[END_REF] and considers three different isomers of IEPOX produced by three different geometrical configurations of the isoprene hydroxyperoxide (ISOPOOH) precursor produced through the isoprene oxidation.

The LMDz-OR-INCA model is used with a 3.8° lat × 1.87° lon horizontal resolution and 39 vertical levels. The wind fields are nudged on the ECMWF (European Centre for Medium-Range Weather Forecasts) reanalysis for the year of 2010 (considering first a one-year spinup). The anthropogenic emissions are those from the Representative Concentration Pathways (RCP) considering the year 2010 in the 8.5 trajectory (compatible with the evolution of radiative forcing equivalent in 2100 to 8.5 W.m -2 ) [START_REF] Riahi | Nakicenovic N Scenarios of Long-Term Socio-Economic and Environmental Development under Climate Stabilization[END_REF] . The biomass burning emissions correspond to the GFED-v4 inventory for the year of 2010. The anthropogenic, ship and fire emissions of SO 2 are respectively 40.5, 5.9 and 1.1 TgS yr -1 . The biogenic emissions are computed by the ORCHIDEE vegetation model as described elsewhere [START_REF] Messina | Hauglustaine DA Global Biogenic Volatile Organic Compound Emissions in the ORCHIDEE and MEGAN Models and Sensitivity to Key Parameters[END_REF] . They lead to a global annual isoprene emission of 466 TgC yr -1 inducing a production of IEPOX of 115 TgC yr -1 , which is consistent with the one found by St Clair et al. [START_REF] Clair | Wennberg PO Kinetics and Products of the Reaction of the First-Generation Isoprene Hydroxy Hydroperoxide (ISOPOOH) with OH[END_REF]

Results and Discussion

IEPOX conversion of Sulf inorg to organosulfur.

Despite the wealth of studies on the reactive uptake of IEPOX, its reactivity remains poorly constrained. [START_REF] Surratt | Seinfeld JH Reactive Intermediates Revealed in Secondary Organic Aerosol Formation from Isoprene[END_REF][START_REF] Gaston | Reactive Uptake of an Isoprene-Derived Epoxydiol to Submicron Aerosol Particles[END_REF][START_REF] Hu | Characterization of a Real-Time Tracer for Isoprene Epoxydiols-Derived Secondary Organic Aerosol (IEPOX-SOA) from Aerosol Mass Spectrometer Measurements[END_REF][START_REF] Lin | Surratt JD Isoprene Epoxydiols as Precursors to Secondary Organic Aerosol Formation: Acid-Catalyzed Reactive Uptake Studies with Authentic Compounds[END_REF][START_REF] Lin | Surratt JD Light-Absorbing Oligomer Formation in Secondary Organic Aerosol from Reactive Uptake of Isoprene Epoxydiols[END_REF][START_REF] Nguyen | Seinfeld JH Organic Aerosol Formation from the Reactive Uptake of Isoprene Epoxydiols (IEPOX) onto Non-Acidified Inorganic Seeds[END_REF] We performed controlled chamber experiments in the presence of ABS seed particles (pH = 1.5) at ~50 % RH using atmospherically-relevant ratios of IEPOX:Sulf inorg (SI Table S1). Figure 1 shows that immediately following IEPOX addition, rapid conversion of Sulf inorg is observed under all conditions measured by the PILS coupled to an IC system operated with 5-minute resolution. The rate of Sulf inorg depletion correlates with the formation of IEPOX-OS and oligomeric-OS (quantified by UPLC/ESI-HR-Q-TOFMS for the same PILS samples), which is further supported by computational chemistry modeling by Piletic et al. that suggests IEPOX would readily produce OSs in acidic aerosols. [START_REF] Piletic | Bartolotti LJ A Computational Study of Acid Catalyzed Aerosol Reactions of Atmospherically Relevant Epoxides[END_REF] IEPOX-OS accounts for most (90-100%) of the Sulf inorg converted within the first 40-60 min under conditions that mimic IEPOX:Sulf inorg ratios relevant to both the SE-U.S. (Figure 1A) and the Amazon (Figure 1B). As shown in Figures 1A andB, ~40% of Sulf inorg injected into the chamber is converted to organosulfur under laboratory conditions that mimic the SE-U.S. conditions, while up to 90% is converted to organosulfur under laboratory conditions that mimic the Amazon. During the next hour following the termination of IEPOX injection, Sulf inorg as well as total organic sulfate has stabilized, indicating inhibition of IEPOX uptake (discussed below), then a decrease of concentrations of IEPOX-OS commences. A net reduction (up to 30% in one hour) of the three quantified OS species indicates that IEPOX-OS are not stable and react to yield as yet uncharacterized organosulfur compounds. One potential class of species, sulfur-containing oligomers, were observed below quantifiable levels in the positive ion mode.

Our results demonstrate the conversion of inorganic-to-organic sulfate is driven primarily by the IEPOX:Sulf inorg ratio as illustrated in Figure 1C. From our laboratory experiments, the IEPOX:Sulf inorg ratio appears to be a critical, previously unrecognized factor in the conversion of inorganic-to-organic sulfate. By extrapolating our laboratory findings to ambient conditions, high IEPOX:Sulf inorg ratios (>2) are common globally, as shown in Figure 1D, especially in equatorial regions and the Southern Hemisphere, and might be important in the formation of organic sulfate 9in such areas. Based on our experimental results, we infer that a higher S(VI) fraction is likely in organic forms in those regions, which are under-sampled relative to the Northern Hemisphere.

Concentration of ambient organosulfur compounds.

Over the past 30 years, air pollution regulations in the Northern Hemisphere have led to decreases in SO 2 concentration of 5.7 % yr -1 in the U.S. and 5.1 % yr -1 in Europe. [START_REF] Xing | Observations and Modeling of Air Quality Trends over 1990-2010 across the Northern Hemisphere: China, the United States and Europe[END_REF] Recent studies project a reduction of SO 4 2-of ~4.5% yr -1 based on current efforts. [START_REF] Attwood | Brown SS Trends in Sulfate and Organic Aerosol Mass in the Southeast U.S.: Impact on Aerosol Optical Depth and Radiative Forcing: Aerosol Mass and Optical Depth in SE US[END_REF][START_REF] Hand | Pitchford ML Particulate Sulfate Ion Concentration and SO 2 Emission Trends in the United States from the Early 1990s through 2010[END_REF] Figure 2A illustrates this trend and presents the SO 4 2-concentration over the last 10 years measured at the Great Smoky Mountain Site (Look Rock, TN) through the IMPROVE monitoring program, [START_REF] Malm | Cahill TA Spatial and Seasonal Trends in Particle Concentration and Optical Extinction in the United States[END_REF] which is strongly influenced by isoprene chemistry. [START_REF] Budisulistiorini | Examining the Effects of Anthropogenic Emissions on Isoprene-Derived Secondary Organic Aerosol Formation during the 2013 Southern Oxidant and Aerosol Study (SOAS) at the Look Rock, Tennessee Ground Site[END_REF] A decrease of Sulf inorg by a factor of 6 is correlated with a subsequent rise in organosulfur as a fraction of total sulfur by a factor of 5. In addition, events with high organosulfur fractions tend to correlate with low Sulf inorg concentrations (Figure 2B), consistent with a previous analysis [START_REF] Tolocka | Contribution of Organosulfur Compounds to Organic Aerosol Mass[END_REF] and corroborating our laboratory findings. Assuming a constant IEPOX concentration and a reduction of SO 4 2-of ~4.5% yr -1 , the increase in the IEPOX:Sulf inorg ratios within the next decades is expected to significantly shift the distribution of S(VI) towards organosulfur (Figure 1C). Since industrialization, Sulf inorg has significantly increased [START_REF] Déandreis | Cozic A Radiative Forcing Estimates of Sulfate Aerosol in Coupled Climate-Chemistry Models with Emphasis on the Role of the Temporal Variability[END_REF][START_REF] Shindell | Lo F Radiative Forcing in the ACCMIP Historical and Future Climate Simulations[END_REF] while the concentration of isoprene has remained constant. [START_REF] Young | Pre-Industrial to End 21st Century Projections of Tropospheric Ozone from the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP)[END_REF] This trend indicates that IEPOX:Sulf inorg was likely much higher during the pre-industrial period than currently, and possibly had a major role in SOA physicochemical properties in isoprene-dominated areas. While the estimated conversion extrapolated from our chamber experiments is likely an upper limit as the reactive uptake of IEPOX is sensitive to aerosol acidity and other factors in the ambient environments (e.g. NO x , organic coating, RH, etc.), the trend identified in this study is central to understanding future air quality and radiative forcing.

Molecular characterization of SOA samples collected in isoprene-rich areas show that OS alone contribute a substantial mass fraction of S(VI) in PM: from 8 to 25% in the SE-U.S. and from 20 to 45% in downwind Manaus (Figure 2C and SI Figure S1). From direct comparison between ICP-MS and AMS sulfate data (Figure S3), we applied a correction factor (i.e., 1.28) in order to account for the subset of organosulfur compounds present within the size range of 1 to 2.5 microns. OS mass ranges from 400-1500 ng m -3 , at the high end of previous studies (SI Tables S2-S4). [START_REF] Rattanavaraha | Assessing the Impact of Anthropogenic Pollution on Isoprene-Derived Secondary Organic Aerosol Formation in PM2.5 Collected from the Birmingham, Alabama, Ground Site during the 2013 Southern Oxidant and Aerosol Study[END_REF][START_REF] Budisulistiorini | Examining the Effects of Anthropogenic Emissions on Isoprene-Derived Secondary Organic Aerosol Formation during the 2013 Southern Oxidant and Aerosol Study (SOAS) at the Look Rock, Tennessee Ground Site[END_REF][START_REF] Surratt | Seinfeld JH Organosulfate Formation in Biogenic Secondary Organic Aerosol[END_REF][START_REF] Nguyen | Glasius M Understanding the Anthropogenic Influence on Formation of Biogenic Secondary Organic Aerosols in Denmark via Analysis of Organosulfates and Related Oxidation Products[END_REF][START_REF] Hettiyadura | Stone EA Qualitative and Quantitative Analysis of Atmospheric Organosulfates in Centreville, Alabama[END_REF] While the sum of OS in the SE-U.S. is significantly different from that of downwind Manaus, isoprene-OS nevertheless represents the predominant OS in both areas. During the 2013 SOAS campaign, total mass concentrations of organosulfur compounds were also determined by IR-ICP-MS (SI Figure S4). The average total sulfur mass multiplied by three (3 × S) and the total Sulf inorg measured by IR-ICP-MS and IC, respectively, differs by 300 ± 200 ngSO 4 m -3 . The difference represents an estimate of total organosulfur compounds and provides an organosulfur/total sulfur ratio similar to that previously observed. [START_REF] Tolocka | Contribution of Organosulfur Compounds to Organic Aerosol Mass[END_REF][START_REF] Shakya | Peltier RE Non-Sulfate Sulfur in Fine Aerosols across the United States: Insight for Organosulfate Prevalence[END_REF][START_REF] Huang | Analysis of Organic Sulfur Compounds in Atmospheric Aerosols at the HKUST Supersite in Hong Kong Using HR-ToF-AMS[END_REF] Although total OS + MSA exhibits a strong correlation (r 2 = 0.78) with the total mass concentration of organosulfur compounds (SI Figure S5), the identified products explain only 50-60% of the total mass of organosulfur quantified in PM 2.5 during the 2013 SOAS field campaign. Assuming similar C:S ratios for identified and unidentified compounds, 16% of the Sulf inorg is converted into organosulfur compounds (SI Figure S5). In fact, using the fit obtained from all chamber experiments (Figure 1C) and an IEPOX:Sulf inorg ratio of 0.24 estimated based on collocated measurements during the 2013 SOAS campaign, [START_REF] Kaiser | Keutsch FN Speciation of OH Reactivity above the Canopy of an Isoprene-Dominated Forest[END_REF] Sulf inorg conversion to organosulfur is estimated to be ~15%. This is in excellent agreement with the characterizations of field SOA samples presented above.

Atmospheric impact of the acid-driven reactive uptake of IEPOX.

In the following section, we show significant alterations of aerosol physicochemical properties due to high levels of inorganic-to-organic sulfate conversion in the chamber experiments. Explored properties include aerosol acidity, morphology, surface tension, viscosity, and reactivity.

Because acidity is one of the governing factors of atmospheric multiphase chemistry, [START_REF]Jang M Heterogeneous Atmospheric Aerosol Production by Acid-Catalyzed Particle-Phase Reactions[END_REF] the response of condensed-phase acidity to inorganic-to-organic sulfate conversion was further investigated. The widely used thermodynamic models treat sulfate as inorganic S(VI), leading to inconsistencies in charge balance in the condensed phase. [START_REF] Weber | Nenes A High Aerosol Acidity despite Declining Atmospheric Sulfate Concentrations over the Past 15 Years[END_REF][START_REF] Fountoukis | Thermodynamic Characterization of Mexico City Aerosol during MILAGRO[END_REF] While organic compounds can reduce the rate of ammonia gas-to-particle partitioning, exclusion of organosulfur compounds such as OSs from thermodynamic models may outweigh this effect and contribute to an even larger discrepancy in acidity. Hence a thermodynamic model was constructed to estimate the acidity of the aerosols from these experiments with and without organosulfur compounds. The model assumes all the components reach thermodynamic equilibrium with the hydrogen ion, while maintaining charge balance and mass balance, similar to previous publications. [START_REF] Fountoukis | A Computationally Efficient Thermodynamic Equilibrium Model for K + -Ca 2+ -Mg 2+ -NH 4 + -Na + -SO 4 2--NO 3 -;-Cl --H 2 O Aerosols[END_REF][START_REF] Clegg | Thermodynamic Model of the System H + -NH 4 + -SO 4 2--NO 3 --H 2 O at Tropospheric Temperatures[END_REF][START_REF] Zuend | Peter T A Thermodynamic Model of Mixed Organic-Inorganic Aerosols to Predict Activity Coefficients[END_REF] The detailed parameterization of the thermodynamic model is described in the SI. As an example, Figure S6 shows the pH of aerosols when not considering the contributions of IEPOX-OS to acidity from the chamber experiments, compared to a thermodynamic box model constructed to take IEPOX-OS into consideration.

Aerosol compositions measured at the end of the chamber experiments were used as input to calculate aerosol pH as shown in Figure S6. If the contributions of IEPOX-OS to acidity were not considered, the aerosol pH would have been overestimated by 3.5 and 9.5 units in the laboratory mimic of SE U.S. and Amazon, respectively. The pK a values of IEPOX-OS remain uncertain. By varying the pK a values of IEPOX-OS in the range of 0-4, the aerosol acidity changes up to 6 times (0.8 pH units), suggesting the importance of further research on determining the pK a values of IEPOX-OS and their roles in governing the aerosol acidity.

Acid-driven multiphase chemistry of IEPOX also leads to a modification of the aerosol morphology from a well-mixed sphere to a core-shell structure, shown by height images obtained by AFM at ambient temperature and pressure (Figures 3A-C), as well as AFM amplitude images, AFM phase images and SEM images (Figure S7) of SOA collected from the chamber experiments. It is noted that the AFM images of the laboratory-derived SOA were collected at ambient pressure and RHs close to chamber conditions (30-40%). SEM data were collected after equilibration under vacuum (10 -3 to 10 -5 Pa). RH was not cycled for these specific particles to check morphology, but has been either cycled or tested at multiple RH values in a recent study. [START_REF] Slade | Shepson PB Bouncier Particles at Night: Biogenic Secondary Organic Aerosol Chemistry and Sulfate Drive Diel Variations in the Aerosol Phase in a Mixed Forest[END_REF] This study demonstrated that the core-shell structure was not substantially altered by the RH cycling beyond efflorescence and deliquescence of the ammonium sulfate core. Additionally, SEM and AFM images were collected under different conditions, but showed similar morphology, supporting the hypothesis that for the more viscous particles in this study, temperature and RH differences during imaging had minimal effect on the particle morphology. These results are consistent with previous theoretical and semi-empirical studies predicting frequent phase separation for particles in the SE-U.S. [START_REF] Pye | Goldstein AH Coupling of Organic and Inorganic Aerosol Systems and the Effect on Gas&Ndash;Particle Partitioning in the Southeastern US[END_REF][START_REF] Pye | On the Implications of Aerosol Liquid Water and Phase Separation for Organic Aerosol Mass[END_REF] Larger IEPOX:Sulf inorg ratios lead to thicker organic shells (e.g., laboratory mimic of Amazon) and can be correlated with the relatively larger amounts of particulate IEPOX-OS and corresponding oligomers (sum of OS corresponds to 27% of Sulf inorg in Figure 1A and 69% in Figure 1B). Hence, OS and oligomeric-OS may lead to a net modification of the morphology of the OA formed in the atmosphere.

As Sulf inorg is consumed and OSs are formed, the aerosol phase becomes more viscous and aerosol particle heights increase (Figure 3D). Height images and profiles from AFM were used as a proxy for liquid, semi-solid, or solid phase of the SOA, which is composed largely of an inorganic sulfate seed and organosulfur coating. The observation of more viscous aerosol is supported by the simulated viscosities of molecular tracers based on recent studies, [START_REF] Shiraiwa | Global Distribution of Particle Phase State in Atmospheric Secondary Organic Aerosols[END_REF][START_REF] Derieux | Shiraiwa M Predicting the Glass Transition Temperature and Viscosity of Secondary Organic Material Using Molecular Composition[END_REF] which suggest that IEPOX-derived OS has viscosity values 1-4 orders of magnitude higher than α-pinene SOA when the RH levels are below 70% (Figure 3D). Hence, IEPOX-derived OS can significantly increase the viscosity of aerosol particles, compared to isoprene-SOA compounds generated through self-nucleation in atmospheric simulation chambers under high concentrations without the addition of acidic sulfate particles. [START_REF] Song | Bertram AK Relative Humidity-Dependent Viscosities of Isoprene-Derived Secondary Organic Material and Atmospheric Implications for Isoprene-Dominant Forests[END_REF] IEPOX-SOA volatility is significantly lower than structure-based vapor pressures of polyols predict, [START_REF] Lopez-Hilfiker | Molecular Composition and Volatility of Organic Aerosol in the Southeastern U.S.: Implications for IEPOX Derived SOA[END_REF] which is consistent with the large inorganic-to-organic sulfate conversion reported here. Given the high modelled viscosity of IEPOX-OS and likely higher viscosity values of oligomeric-OS, the OS coating induced inhibition of multiphase chemistry would be larger than coatings of α-pinene SOA. [START_REF] Zhang | Surratt JD Effect of the Aerosol-Phase State on Secondary Organic Aerosol Formation from the Reactive Uptake of Isoprene-Derived Epoxydiols (IEPOX)[END_REF] In the Amazon rainforest where average RH was higher than 80%, SOA particles were liquid, suggesting limited diffusion will not inhibit multiphase chemistry if RH>80%. [START_REF] Bateman | Martin ST Sub-Micrometre Particulate Matter Is Primarily in Liquid Form over Amazon Rainforest[END_REF] Even though the average RH is close to 80% for both the Amazon and SE-U.S., SI Figure S8 and previous measurements show that the median daytime RH in those regions is consistently below 70% (40-50% during the dry season in the Amazon) during the period of maximum diurnal IEPOX-SOA production. [START_REF] Zhang | Surratt JD Effect of the Aerosol-Phase State on Secondary Organic Aerosol Formation from the Reactive Uptake of Isoprene-Derived Epoxydiols (IEPOX)[END_REF][START_REF] Guo | Weber RJ Fine-Particle Water and PH in the Southeastern United States[END_REF] Hence, diffusion of gaseous molecules limited by the formation of an OS coating will be more likely to affect multiphase chemistry during daytime in isoprene-rich environments, reducing heterogeneous SOA formation from compounds that are generated by photooxidation reactions.

Change in morphology is further supported by measuring the interfacial tension (IFT) depression of the major IEPOX-SOA products in microfluidic platform experiments. Surface tension and IFT are proxy measures of surface concentrations, as tensions will decrease with increased bulk-to-surface partitioning of surface-active components in the aqueous droplets. Each microfluidic measurement consumes 250-300 μL of aqueous solution, yielding 20-30 droplets. The IEPOX-OS droplet interfacial tensions reported in this work were statistically averaged and reported as the circle markers in Figure 4A. Additionally, Figures S2 and S9

show 2-methyltetrols and IEPOX-OS in methanol aqueous solution and 2-methyltetrols in pure water, respectively. IFT depression was observed in all cases (Figure 4A, Figure S2 and Figure S9), and IFT was lower when AS was present. In fact, lowering of the IFT in salty solutions indicates potential salting out [START_REF] Wang | Measuring and Modeling the Salting-out Effect in Ammonium Sulfate Solutions[END_REF][START_REF] Bertram | Zorn SR Predicting the Relative Humidities of Liquid-Liquid Phase Separation, Efflorescence, and Deliquescence of Mixed Particles of Ammonium Sulfate, Organic Material, and Water Using the Organic-to-Sulfate Mass Ratio of the Particle and the Oxygen-to-Carbon Elemental Ratio of the Organic Component[END_REF] of the organics due to an enhancement of organic activity, driving more organic molecules to the surface. The salting out effect subsequently alters the SOA physicochemical properties by changing morphology to a coreshell structure rather than a homogeneously mixed sphere. [START_REF] Zhang | Surratt JD Effect of the Aerosol-Phase State on Secondary Organic Aerosol Formation from the Reactive Uptake of Isoprene-Derived Epoxydiols (IEPOX)[END_REF] Salting out effects are quantified by combining the Setschenow equation and a two parameter surface and IFT model. [START_REF] Wexler | Statistical Mechanics of Multilayer Sorption: Surface Tension[END_REF] The Setschenow constant (K s ) indicates salting out when positive for organicinorganic aqueous systems. The curves in Figure 4A shows model treatment for IEPOX-OS in pure water and in salty water and predicts that IEPOX-OS has a propensity for salting out in the presence of AS, which helps to explain the dependence of shell thickness on the ratio of IEPOX:Sulf inorg . IEPOX-SOA may alter the climate properties of aerosols, for example, enhancing CCN activity by suppressing the surface tension to enhance cloud droplet formation from aerosols in organosulfur-rich particles, causing larger droplets to form before and during cloud activation. [START_REF] Ruehl | Dutcher CS Interfacial Tensions of Aged Organic Aerosol Particle Mimics Using a Biphasic Microfluidic Platform[END_REF] Figure 4B illustrates the limiting effects of IEPOX-SOA products on reactive uptake by aerosol which can most likely be ascribed to a slight decrease in acidity and transformation to a core-shell morphology as a result of the salting out effect induced by organosulfur compounds. The dramatic decrease of γ IEPOX with increasing atmospheric-equivalent exposure time, highlights the profound modification of IEPOX-SOA on aerosol reactivity, explains the presence of residual IEPOX in the gas phase measured by CIMS (SI Figure S10), and the stabilization of Sulf inorg (Figures 1A andB) in our laboratory experiments.

While previous studies have shown that other organic coatings tend to reduce multiphase chemical processes, [START_REF] Gaston | Reactive Uptake of an Isoprene-Derived Epoxydiol to Submicron Aerosol Particles[END_REF][START_REF] Riva | Zelenyuk A Effect of Organic Coatings, Humidity and Aerosol Acidity on Multiphase Chemistry of Isoprene Epoxydiols[END_REF][START_REF] Zhang | Surratt JD Effect of the Aerosol-Phase State on Secondary Organic Aerosol Formation from the Reactive Uptake of Isoprene-Derived Epoxydiols (IEPOX)[END_REF] the results presented in Figure 4B provide direct evidence that uptake of IEPOX by sulfate aerosols has a self-limiting effect and indicate that the formation of a viscous IEPOX-SOA coating likely prevents further SOA formation in the ambient environment, primarily during daytime when RH is lower than 80%.

In sum, this study demonstrates that acid-driven multiphase chemistry of IEPOX converts a significant fraction of Sulf inorg to organosulfur within a range of IEPOX:Sulf inorg ratios relevant to most isoprene-dominated environments. We further demonstrate through laboratory and field measurements, the substantial conversion of sulfate, underlining the major role of IEPOX in controlling the chemical form of S(VI). Retrospective examination of field data in the SE-U.S. consistently shows that the contribution of organosulfur to S(VI) has been increasing with declining Sulf inorg . The measured filter-based organosulfur fraction for Amazon, however, is not as much as that determined by the laboratory experiments conducted at the relevant ratios. There are two possible reasons. First, we conducted experiments mimicking both regions under the same initial condition of aerosol acidity and therefore the observed extent of conversion (up to ~90%) for Amazon relevant ratios likely represents the upper bound in the atmosphere. Second, collocated measurements of gasphase IEPOX have rarely been reported for this area. The actual IEPOX:Sulf inorg could have been lower than the ratios tested in the laboratory mimic of Amazon. This also calls for the need to enrich the inventory of gas-phase IEPOX measurements to better constrain IEPOX:Sulf inorg in regions such as Amazon. From chamber experiments, the organosulfur compounds are shown to impact aerosol acidity (up to 9 pH unit increase compared to the conventional case, where organosulfur compounds are not considered) by our thermodynamic model under assumptions similar to those widely used in models. A similar approach could be adopted in updating current thermodynamic models to assess this impact on predicting pH of ambient SOA. IEPOX reactive uptake results in a core-shell morphology supported by microscopic imaging and biphasic microfluidics. Through non-oxidative chemical processes, IEPOX-OS undergoes transformation to as yet uncharacterized organosulfur compounds, together with highly viscous IEPOX-OS, impeding further reactive uptake of IEPOX. Not examined in this work, though, is how the core-shell morphology may affect the prediction of aerosol acidity. It is obvious that the core and shell need to be treated separately in pH calculations. This warrants a systematic investigation in a future study. Worth noting is that the findings regarding IEPOX-OS and its implications for aerosol physicochemical properties have not been included in the presented global model for calculating the global IEPOX:Sulf inorg . Overall the shift in sulfate distribution within aerosols, changing physicochemical properties, and decreasing multiphase reactivity caused by IEPOX multiphase chemistry have yet to be updated in the regional-and global-scale models to properly predict IEPOX-SOA formation and its impact on air quality and climate.

In isoprene-dominated areas, IEPOX-OS, and potentially other biogenic/anthropogenic OS, likely govern the physicochemical properties of aerosol as well as the distribution of inorganic species such as sulfate or ammonium. Consequently, aerosol growth, multiphase reactions, including aging and reactive uptake of other species, and CCN activity change as surface tension, acidity, hygroscopicity and viscosity are modified. These changes could greatly impact atmospheric composition of biogenic SOA formed over isoprene-dominated areas. Hence, changes in SO 2 emissions at different locations around the world over time may have implications for the physicochemical properties of biogenic-derived SOA. (A) Measured and modeled interfacial tension (IFT) of IEPOX-OS. Microfluidic measurements of IFT (circles); two parameter model treatment of SOA in pure water (solid line), using a binary model [START_REF] Boyer | Atmospheric Aqueous Aerosol Surface Tensions: Isotherm-Based Modeling and Biphasic Microfluidic Measurements[END_REF]100 ; and model treatment of SOA in ammonium sulfate (AS) solution (dashed line), using an adapted form of the binary model adapted for salt-containing organic aqueous solutions that includes known organic model parameters from the solid line and the Setschenow constant as the single adjustable model parameter (dashed line). (B) IEPOX reactive uptake coefficient (γ IEPOX ), obtained from flow tube experiments performed at the University of Washington, on aqueous ammonium bisulfate particles as a function of atmospheric equivalent exposure time defined as the length of the time that an aerosol is exposed to IEPOX gases, assuming gas-phase IEPOX concentration of 1 ppb and an aerosol surface area density of 300 μm 2 cm -3 . The atmospheric equivalent exposure time is obtained from the experimental reaction time by multiplying the ratio of experimental-toambient concentration of IEPOX (=75) represents an upper limit.

,

  where d major and d minor are the major and minor diameters of the deformed droplet respectively, measured using image analysis. A plot of the left side of the equation of motion versus D x a 0

Figure 1 .

 1 Figure 1. Laboratory conversion (RH=50%) of Sulf inorg to organic sulfate during the reactive uptake of IEPOX at IEPOX (μg m -3 ):Sulf inorg (μg m -3 ) ratios atmospherically relevant to (A) a laboratory mimic of the SE-U.S. (ratio=1.5) and (B) a laboratory mimic of the Amazon (ratio=17.1). IEPOX is injected into the chamber at t = 0 and stopped at t = 60. IEPOX-OS dimer and trimer concentrations are shown on and expanded scale for clarity. (C) Filled triangular markers indicate the conversion of Sulf inorg to organic sulfate as a function of IEPOX:Sulf inorg in chamber experiments at RH~50%. Colors correspond to those in Figure 1D. Red open circles correspond to projected IEPOX:Sulf inorg over the next 50 years, based on Sulf inorg reduction rates from Attwood et al. 75 and Hand et al. 76 , and assuming constant IEPOX. (D) Yearly average IEPOX:Sulf inorg across the world for present-day conditions using the LMDz-OR-INCA global climate-chemistry model. White color indicates no sulfate (non-sea salt) or IEPOX concentration.

Figure 2 .

 2 Figure 2. (A) Correlation between total sulfate and organosulfur fraction and (B) Evolution of organosulfur fraction as a function of Sulf inorg at the Great Smoky Mountain Site (Look Rock, TN) during summer (May -September) from 2007-2016, using the National Park Service IMPROVE PM 2.5 database. If particulate organosulfur compounds are not present, the sulfate (SO 4 2-) mass measured by IC should equal three times the sulfur mass concentration measured by X-ray fluorescence (XRF), since the molar mass of sulfur and sulfate are 32 and 96 g/mol, respectively. A value of 3×S/ SO 4 2-lower than 1 is caused by the limitation of the analytical techniques to differentiate 3×sulfur from SO 4 2-. 42 (C) Average mass concentration of the identified organosulfates + methane sulfonic acid (OS + MSA) and total organosulfur compounds (Org-S) in the PM 2.5 samples collected during the 2013 SOAS campaign as well as the average mass concentrations of the sum of OS + MSA

Figure 3 .

 3 Figure 3. Left Column (A), (B), and (C). Atomic force microscopy (AFM) height images of ammonium bisulfate seed (ABS), IEPOX-SOA generated from the reactive uptake of IEPOX in the presence of wet acidic aerosol at RH~50% and IEPOX:Sulf inorg ratios atmospherically relevant to laboratory mimic of SE-U.S. and Amazon, respectively. Middle column (A), (B), and (C). Height maps of impacted particles. (D) Comparison of the measured and predicted viscosity of α-pinene SOA (purple), predicted IEPOX-OS (red) and IEPOX-OS mixtures at 298 K as a function of RH. The data points represent measured viscosity values of α-pinene SOA. The solid lines represent estimated viscosity of α-pinene SOA, 66 IEPOX-OS, and IEPOX-OS mixtures (IEPOX-OS + Oligomeric-OS). The shaded areas represent the upper and lower bounds of the viscosity estimation for each type of SOA. Model parameters are: glass transition temperature of dry SOA (T g,org ); hygroscopicity (κ), fragility (D), and Gordon-Taylor constant (k GT ). Solid line for -pinene SOA at 278.5K: T g,org = 0.1, κ = 10, k GT = 2.5. Solid line for 2-methyltetrol sulfate at 298K; T g,org = 0.12, κ = 13, k GT = 2.5.; Solid line for IEPOX-derived OS mixture at 313 K, T g,org = 0.12, κ = 13, k GT = 2.5. For upper (lower) bounds (shaded regions): α-pinene SOA; T g,org = 300 K (268.5 K), κ = 0.1 (0.1), D = 20 (10), k GT = 2.5 (3.0). 2-Methyltetrol sulfate; T g,org = 320 K (288 K), κ = 0.10 (0.15), D = 20 (10), k GT = 2.5 (3.0). OS mixture; T g,org = 330 K (303 K), κ = 0.10 (0.15), D = 20 (10), k GT = 2.5 (3.0).

Figure 4 .

 4 Figure 4.

37,38 Collection of PM 2.5 samples.

	Ambient SOA (PM 2.5 ) samples from two isoprene-dominated environments (SE-U.S. and
	Amazon forest) were collected onto quartz filters during three field campaigns: (i) during the
	2013 Southern Oxidant and Aerosol Study (SOAS) campaign from 1 June to 15 July 2013 at
	the Centerville, AL (CTR, AL) ground site; (ii) from 18 July through 1 August 2016 from
	downtown Manaus, Brazil and (iii) during the Green Ocean Amazon (GoAmazon2014/5)
	field campaign 39 during intensive operating period 2 (IOP2). Additional information
	regarding filter collection is provided in SI.

Environ Sci Technol. Author manuscript; available in PMC 2020 August 06.

ACKNOWLEDGMENT

The authors wish to thank the Camille and Henry Dreyfus Postdoctoral Fellowship Program in Environmental Chemistry for their financial support. The authors would like to thank the Michigan Center for Materials Characterization for use and help with the SEM and the Banaszak-Holl lab for the use and help with the AFM. The contents of this publication are solely the responsibility of the authors and do not necessarily represent the official views of the U.S. Environmental Protection Agency (U.S. EPA). Further, the U.S. EPA does not endorse the purchase of any commercial products or services mentioned in the publication. The U.S. EPA through its Office of Research and Development collaborated in the research described here. It has been subjected to Agency administrative review and approved for publication but may not necessarily reflect official Agency policy. The authors wish to thank the IMPROVE network. IMPROVE is a collaborative association of state, tribal, and federal agencies, and international partners. U.S. EPA is the primary funding source, with contracting and research support from the National Park Service. The Air Quality Group at the University of California, Davis is the central analytical laboratory, with ion analysis provided by Research Triangle Institute, and carbon analysis provided by Desert Research Institute.

Funding Sources

This work is also funded in part by the U.S. EPA grant R835404, the National Science Foundation (NSF) grants AGS-1703535, AGS-1703019, AGS-1554936, CHE-1404644 and CHE-1404573 and by the National Oceanic and Atmospheric Administration (NOAA) Climate Program Office's AC4 program, award number NA13OAR4310064.

Supplementary Material

Refer to Web version on PubMed Central for supplementary material.