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Environmental Processes

Bouncier Particles at Night: Biogenic Secondary Organic Aerosol Chemistry and Sulfate Drive Diel Variations in the Aerosol Phase in a Mixed Forest Jonathan H. Slade, Andrew P Ault, Alexander Bui, Jenna Ditto, Ziying Lei, Amy L. Bondy, Nicole Olson, Ryan Cook, Sarah Desrochers, Rebecca Harvey, Matthew Erickson, Henry Wallace, Sergio Alvarez, James Flynn, Brandon Boor, Giuseppe A Petrucci, Drew R. Gentner, Robert J. Griffin, and Paul B. Shepson Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b07319 • Publication Date (Web): 19 Apr 2019 Downloaded from http://pubs.acs.org on April 20, 2019

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Environmental Science & Technology

SLADE ET AL

AEROSOL PHASE STATE DIEL CYCLE

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Bouncier Particles at Night: Biogenic Secondary Organic Aerosol Chemistry

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and Sulfate Drive Diel Variations in the Aerosol Phase in a Mixed Forest

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Jonathan H. Sladea†*, Andrew P. Aultb,c* Alexander T. Buid, Jenna C. Dittoe, Ziying Lei,b Amy L.

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Bondyc, Nicole E. Olsonc, Ryan D. Cookc, Sarah J. Desrochersa, Rebecca M. Harveya, Matthew

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H. Ericksonf, Henry W. Wallaced, Sergio L. Alvarezf, James H. Flynnf, Brandon E. Boorg,

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Giuseppe A. Petruccih, Drew R. Gentnere, Robert J. Griffind,i, Paul B. Shepsona,j,k,‡

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aDepartment

of Chemistry, Purdue University, West Lafayette, IN 47907

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bDepartment

of Environmental Health Sciences, University of Michigan, Ann Arbor, MI 48109

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cDepartment

of Chemistry, University of Michigan, Ann Arbor, MI 48109

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dDepartment

of Civil and Environmental Engineering, Rice University, Houston, TX 77005

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eDepartment

of Chemical and Environmental Engineering, Yale University, New Haven, CT

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06520

13

fDepartment

14

gLyles

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hDepartment

of Chemistry, University of Vermont, Burlington, VT 05405

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iDepartment

of Chemical and Biomolecular Engineering, Rice University, Houston, TX 77005

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jDepartment

of Earth, Atmospheric and Planetary Sciences, Purdue University, West Lafayette,

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IN 47907

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kPurdue

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†Now

21

CA 92093

22

‡Now

23

11794

of Earth and Atmospheric Sciences, University of Houston, Houston, TX 77204

School of Civil Engineering, Purdue University, West Lafayette, IN 47907

Climate Change Research Center, Purdue University, West Lafayette, IN 47907

at Department of Chemistry and Biochemistry, University of California San Diego, La Jolla,

at School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY

1 ACS Paragon Plus Environment


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Correspondence and requests for materials should be addressed to J.H.S. (email:

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[email protected]) or to A.P.A. (e-mail: [email protected]).

26 27

TOC Figure

28 29


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Abstract

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Aerosol phase state is critical for quantifying aerosol effects on climate and air quality. However,

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significant challenges remain in our ability to predict and quantify phase state during its evolution

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in the atmosphere. Herein, we demonstrate that aerosol phase (liquid, semi-solid, solid) exhibits a

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diel cycle in a mixed forest environment, oscillating between a viscous, semi-solid phase state at

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night and liquid phase state with phase separation during the day. The viscous nighttime particles

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existed despite higher relative humidity and were independently confirmed by bounce factor

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measurements and atomic force microscopy. High-resolution mass spectrometry shows the more

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viscous phase state at night is impacted by the formation of terpene-derived and higher molecular

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weight secondary organic aerosol (SOA) and smaller inorganic sulfate mass fractions. Larger

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daytime particulate sulfate mass fractions, as well as a predominance of lower molecular weight

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isoprene-derived SOA, lead to the liquid state of the daytime particles and phase separation after

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greater uptake of liquid water, despite the lower daytime relative humidity. The observed diel cycle

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of aerosol phase should provoke rethinking of the SOA atmospheric lifecycle, as it suggests diurnal

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variability in gas–particle partitioning and mixing timescales, which influence aerosol multiphase

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chemistry, lifetime, and climate impacts.

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Keywords

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aerosol phase│ viscosity │ glass transition│ secondary organic aerosol │ diel cycle



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Introduction

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Secondary organic aerosol (SOA), formed from both gas-phase oxidation and condensed-phase

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reactions involving anthropogenic and biogenic volatile organic compounds (BVOCs), constitutes

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a significant fraction of the global aerosol budget and the most abundant type of organic aerosol

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in the atmosphere.1 SOA impacts climate directly by scattering or absorbing radiation and

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indirectly by altering the radiative properties of clouds and serving as ice nuclei2. However, there

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are order-of-magnitude differences in the global SOA burden according to atmospheric models,

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leading to significant uncertainties regarding SOA impacts on the radiative budget3.

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The uncertainties associated with SOA impacts on climate result in part from limited knowledge

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of the chemical transformations and physical state of aerosol during its lifetime in the atmosphere.4,

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5 While SOA has traditionally been assumed to be in a homogeneous liquid state, recent laboratory,

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field, and modeling studies have demonstrated that SOA formed from a variety of BVOC–oxidant

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systems and in different environments can exist as amorphous solids, semi-solids, and phase-

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separated particles.6-16 Shiraiwa, et al.

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varies spatially around the globe and with altitude depending on temperature, relative humidity,

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and particle composition (molar mass and characteristics such as the oxygen-to-carbon ratio of the

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organic matter, O/C) on an annual average. Such variations in particle phase state can significantly

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impact aerosol reactivity,17-20 the long-range transport of toxic substances,21, 22 and the uptake and

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partitioning of semi-volatile organic compounds and water vapor.8, 23-28 These processes affect the

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lifetime of SOA and its optical29 and cloud condensation/ice nuclei activation properties.30-33

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Field studies have aimed to characterize the phase state of atmospheric SOA particles based on the

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analysis of particle bounce factors (BF) in different forested environments, including the boreal

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forest region of northern Finland,6 the Amazon rain forest of Brazil,34 and a mixed

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used a model to predict that the phase state of particles


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deciduous/coniferous forest in the southeastern US.12 While these studies together suggest phase

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state varies by location, with solid-like particles in the colder, monoterpene-dominated boreal

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forests, and liquid-like particles in the humid, isoprene-dominated forests of the Amazon, it has

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remained unclear if phase state changes over the diel cycle, despite the obvious contrasts in

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temperature, relative humidity, boundary layer height and atmospheric transport, light availability,

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and BVOC emissions.35, 36 For example, monoterpenes and the nitrate radical can become the

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dominant BVOC–oxidant system in forests at night, generating SOA with very different molecular

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properties and viscosity than that formed from daytime isoprene photooxidation.13, 16, 23, 28, 37-39

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Furthermore, the viscosity of SOA can be very sensitive to modulations in aerosol liquid water

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(ALW) content, which depend on the ambient relative humidity, water-soluble organic carbon

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content, and inorganic mass fraction.9, 10

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Herein, we analyze the ambient particle phase state over multiple daily cycles in a mixed

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deciduous/coniferous forest during the Atmospheric Measurements of Oxidants in Summer

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(AMOS) study in 2016 at the University of Michigan Biological Station (UMBS) PROPHET

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research site.40 This work expands on prior BF field measurements by integrating high-frequency

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(1 Hz) day and night BF measurements with an electrical low-pressure impactor (ELPI) and single

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particle and bulk chemical composition, phase, and morphology analyses employing a suite of

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microscopy

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measurements provided detailed quantitative information on the daily cycles of water-soluble ions,

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total organic mass, and other composition measures important when assessing compositional

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drivers of particle phase state, including average molar mass (M) and O/C ratios, and estimates of

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aerosol liquid water (ALW).

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Materials and Methods

and

high-resolution

mass

spectrometry

instrumentation.


Together,

these


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Site description and field study timeline. The PROPHET-AMOS study was a large-scale field

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campaign conducted at the University of Michigan Biological Station in northern Michigan from

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July 1 to July 31, 2016. The measurements presented herein were made at the PROPHET tower

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research site (45°33’N, 84°42’W) underneath the mixed deciduous/coniferous forest canopy

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(canopy height ~22 m) from July 9 to July 24. The PROPHET research tower is ~5.5 km east of

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the town of Pellston, MI (population < 800) and the nearest major metropolitan areas are Chicago,

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IL (472 km SW), Milwaukee, WI (375 km SW) and Detroit, MI (350 km SE). Other details of the

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site, including forest composition, BVOC emissions, and the impacts of different aerosol sources

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can be found in several previous studies.40-42

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Meteorological and SO2 measurements. Meteorological measurements were conducted ~6 m

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above the ground and were collocated with the trace gas sample inlets for the University of

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Houston Mobile Air Quality Laboratory (UHMAQL). Temperature and relative humidity were

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measured with a R.M. Young 41382 sensor and pressure was measured with a R.M. Young

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61302V pressure sensor. SO2 was measured using a pulsed fluorescence analyzer (Thermo

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Environmental, Inc.; Model 43i-TL). Baseline determinations were made once per hour by flowing

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ambient air over a carbonate-treated filter to selectively remove SO2 from the sample. Instrument

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sensitivity was established by daily challenges with zero-air dilutions of a NIST-traceable gas

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standard (Scott-Marrin, Inc., Riverside, CA).

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Electrical low-pressure impactor sampling. Two electrical low pressure impactors (ELPIs;

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Dekati® ELPI+ and High Resolution-ELPI+43 housed in the University of Michigan Mobile

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Laboratory were employed to determine particle bounce factors (BFs).44 One ELPI was operated

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with sintered substrates,45 and the other with smooth aluminum foil substrates. The two ELPIs

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sampled particles simultaneously at a rate of 10 L min-1 through ~2 m long ½″ ID insulated copper 6 ACS Paragon Plus Environment

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tubing connected to a 12″ L x 4″ ID cylindrical sampling manifold, which was adapted to a ~3 m

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long 1″ ID stainless steel tube oriented vertically on top of the trailer roof. There were no

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differences in the relative humidity or temperature measured outside and at the base of the ELPI

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in the lab during sampling. The ELPIs sampled particles with an aerodynamic diameter between

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0.006 µm to 10 µm across 14 electrically-detected stages at a rate of 1 Hz. The BF data presented

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in this work are 10 min averages of the 1 Hz BF data. It is important to note that BF is used in this

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study to infer changes to particle phase state on relatively shorter timescales than is allowable by

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offline analysis, and should be regarded as a mechanical property, dependent on both the chemical

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and physical properties of the particles as well as the surface characteristics and flow dynamics in

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the impactor.46 Further details regarding the BF calculation can be found in the SI.

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High-resolution time-of-flight aerosol mass spectrometry. A high-resolution time-of-flight

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aerosol mass spectrometer (HR-ToF-AMS, Aerodyne Research Inc.) was used to measure non-

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refractory submicron aerosols.47 In brief, particles were sampled through a 100-micron critical

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orifice and were focused into a particle beam using an aerodynamic lens. After traversing a vacuum

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chamber, the particles in the beam were impacted onto a tungsten vaporizer heated to 600 degrees

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Celsius. Vapors were subsequently ionized by electron impact ionization (70 eV). The HR-ToF-

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AMS was operated in a high mass sensitivity mode, referred to as V-mode. The sampling inlet for

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the HR-ToF-AMS measurement was collocated with the UHMAQL meteorological

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measurements. The inlet was fitted with a PM2.5 cyclone. Further details of ionization efficiency

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calibrations, data analysis, and ALW determination by ISORROPIA can be accessed in the SI.

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Particle collection by MOUDI and analysis by atomic force microscopy (AFM). Particle

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samples for atomic force microscopy (AFM) were collected using a micro-orifice uniform

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deposition impactor (MOUDI, model 110, MSP Corp.) and collocated with the ELPI. Particles 7 ACS Paragon Plus Environment


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were sampled from 5.5 m above ground level through an insulated 1.1 cm inner diameter copper

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tubing inlet using a cylindrical sampling manifold. Particles were impacted on silicon wafers (Ted

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Pella Inc.) on stages 5, 7, 9, and the backup filter of the MOUDI, and samples from 7/10/16 (day)

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and 7/16/16 (night) on stage 9 of the MOUDI were analyzed at the University of Michigan. AFM

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analysis at ambient temperature and pressure was initially performed on a NanoIR2 system

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(Anasys Instruments) followed by analysis with a PicoPlus 5500 AFM (Agilent) after re-

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humidification to 80% RH where they equilibrated prior to analysis at 50% RH (similar RH values

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to ambient collection conditions). SEM-EDX analysis was used to confirm that the particles

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analyzed were primarily organic carbon or organic-sulfate mixtures. Particles < 180 nm (stage 9)

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were primarily organic and sulfate, with EDX showing C, O, S, and N, as the predominant

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elements, which prior work at UMBS has shown is organic material and sulfate.42, 48-50 Less than

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3% of daytime particles and 1% of nighttime particles contained markers for salts (other than

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ammonium sulfate) and dust (Na, Mg, K, Ca, P, Al, and Cl), though these are observed in larger

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particles at the site.49, 51 Further details of the AFM and SEM-EDX analysis can be found in the

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SI.

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High-performance liquid chromatography electrospray ionization time-of-flight tandem

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mass spectrometry. A custom-built sampler was positioned on the PROPHET tower 28 m above

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the ground, facing west. Samples were collected on PTFE membrane filters (47 mm diameter, 1.0

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m pores, Tisch Environmental) housed in passivated stainless-steel filter holders (Pall

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Corporation). No upstream inlet tubing was used to minimize potential aerosol loss to tubing

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walls. Samples were collected at 1 m3 hr-1 for maximum collection efficiency. An 84-mesh

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stainless steel screen (McMaster Carr) was installed over the inlet to prevent large particles and

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insects from contaminating filters, and limited particle size to approximately PM10 at a flow rate 8 ACS Paragon Plus Environment

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of 1 m3 hr-1. Samples were collected both during the day (9:00 am-5:00 pm) and at night (9:30 pm-

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5:30 am), avoiding sunrise and sunset to capture products of daytime and nighttime oxidation

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chemistry without the influence of transitional chemistry during dawn and dusk. Field blanks were

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collected alongside all samples in identical holders. Filters were sonicated and extracted in

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methanol and the extracts frozen immediately, then thawed prior to analysis. The extracts were

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analyzed with an Agilent 1260 Infinity HPLC with an Agilent 6550 Q-TOF mass spectrometer,

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using an electrospray ionization (ESI) interface. We note that some fraction of the carboxylic acids

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and carbonyl compounds in the filter samples may have been converted to esters and (hemi)acetals,

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respectively, during extraction in methanol.52 If conversion in the frozen methanol extracts

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occurred, this implies the O/C ratios and viscosity we report in this study could be slightly

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underestimated. Further details can be found in the SI.

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Results and Discussion

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Diel patterns and the impact of mixing state, size, and liquid water on particle bounce. Figure

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1 shows the composite median diel trend in BF from the individual daily BF profiles presented in

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Fig. S1. BF exhibits a maximum in the early morning (between 03:00 and 06:00 EDT), decreasing

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following sunrise to a minimum during the day (between 12:00 and 14:00 EDT), then increasing

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into and through the late evening. For comparison, the median BF values are similar in range to

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those measured in a mixed forest in the southeastern US,12 indicating the particles are characteristic

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of semi-solids, but transition to lower viscosity particles during the day.9, 12, 44



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Figure 1. Diel trend in BF showing median and percentiles for each hour corresponding to all BF

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measurement periods in this study.

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Single particle analysis53 confirmed that BF reflects particle phase state by contrasting BF with the

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morphology and phase of the particles using atomic force microscopy (AFM)54 at ambient pressure

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and RH, with corroboration from scanning electron microscopy (SEM).28 Figure 2 shows

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representative AFM height and phase images of particles impacted onto silicon wafers using a

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micro-orifice uniform deposit impactor (MOUDI) (0.10-0.18 µm) during a low BF period (ranging

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between ~0.2 and ~0.3) during the day and a high BF period (ranging between ~0.6 and ~0.8) at

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night. Because particle size is an important parameter impacting phase transitions,55 phase

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separation,56 and bounce,7, 11 though highly uncertain,57 we examined the impact of particle size

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on the phase state by determining how the spreading ratios (ratio of radius to height, where 1 is a

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hemisphere) of particles with volume equivalent diameters 100 nm differ between

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night and day. This analysis was conducted both as collected and after re-humidifying the samples

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to 80% RH followed by analysis at 50% RH, to reproduce ambient RH. The spreading ratios in 10 ACS Paragon Plus Environment

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Fig. 2e indicate less spreading for the higher BF particles

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