OH-Initiated Oxidation of m-Xylene on Black Carbon Aging

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OH-Initiated Oxidation of m-Xylene on Black Carbon Aging Song Guo, Min Hu, Yun Lin, Mario Gomez-Hernandez, Misti Levy Zamora, Jianfei Peng, Don Collins, and Renyi Zhang Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 06 Jul 2016 Downloaded from http://pubs.acs.org on July 6, 2016

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Environmental Science & Technology

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OH-Initiated Oxidation of m-Xylene on Black Carbon Aging

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Song Guo*1, Min Hu1, Yun Lin2, Mario Gomez-Hernandez3, Misti L. Zamora2, Jianfei Peng1,

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Donald R. Collins2, Renyi Zhang*1,2,3 1

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State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China

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Department of Atmospheric Sciences, Texas A&M University, College Station, TX 77843, USA 3

Department of Chemistry, Texas A&M University, College Station, TX 77843, USA

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Abstract. Laboratory experiments are conducted to investigate aging of size-classified black

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carbon (BC) particles from OH-initiated oxidation of m-xylene. The variations in the particle

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size, mass, effective density, morphology, optical properties, hygroscopicity, and activation

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as cloud condensation nuclei (CCN) are simultaneously measured by a suite of aerosol

12

instruments, when BC particles are exposed to the oxidation products of the OH-m-xylene

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reactions. The BC aging is governed by the coating thickness (∆rve), which is correlated to

14

the reaction time and initial concentrations of m-xylene and NOx. For an initial diameter of

15

100 nm and ∆rve = 44 nm, the particle size and mass increase by a factor of 1.5 and 10.4,

16

respectively, and the effective density increases from 0.43 to 1.45 g cm-3 due to organic

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coating and collaping of the BC core. The BC particles are fully converted from a highly

18

fractal to nearly spherical morphology for ∆rve = 30 nm. The scattering, absorption, and

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single scattering albedo of BC particles are enhanced accordingly with organic coating. The

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critical supersaturation for CCN activation is reduced to 0.1% with ∆rve = 44 nm. The results

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imply that the oxidation of m-xylene exhibits larger impacts in modifying the BC particle

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properties than those for the OH-initiated oxidation of isoprene and toluene.

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Key Words: Black Carbon, aging, m-xylene-OH oxidation, coating thickness, light

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absorption

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1. Introduction

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Black carbon (BC) particles produced from incomplete combustion of fossil fuel and

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biomass1,2 account for a significant fraction of the tropospheric particulate matter, especially

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in urban areas3-7. Due to the strong ability to absorb light over a broad range of the solar

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spectra, BC particles profoundly impact solar radiation transfer in the atmosphere8; it has

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been suggested that BC represents the second most important climate-warming agent after

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carbon dioxide9. Also, the presence of BC stabilizes the atmosphere by cooling the surface

34

and heating aloft, resulting in weak convection, less cloud fraction, and less precipitation10,11.

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Furthermore, BC modifies the near-surface photochemistry and ozone production12,13. In the

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atmosphere, BC undergoes various aging processes6,14,15, which significantly alter its

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chemical and physical properties16. In particular, formation of coatings on BC particles from

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gas-phase pollutants and/or their oxidation products, e.g., sulfate, nitrate, and secondary

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organic aerosol (SOA) constituents17, dramatically alters the particle size, mass, and

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morphology16,18-23. Modeling simulations9,24,25, laboratory19,26-28, and field measurements29-32

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have indicated that coatings of sulfate and organics significantly enhance the light scattering

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and absorption coefficient of BC particles. Fresh BC is largely hydrophobic, but coating of

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water-soluble material significantly enhances hygroscopicity and the potential to serve as

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cloud condensation nuclei (CCN)16,23,33-38. Hence, aging of BC considerably impacts its direct

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and indirect forcing on climate10,39.

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The changes in the BC particle properties have been shown to depend on coating

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materials. For example, uptake of sulfuric acid16,18, α-pinene ozonolysis products21,40, and



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toluene-OH oxidation products27 increase the size and mass of BC particles and significantly

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modify their morphology and hygroscopicity. On the other hand, coating of the isoprene-OH

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oxidation products changes the morphology and hygroscopicity of the BC particles, but

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decrease the particle size28. When BC is coated by succinic acid19,41, oleic acid20, or

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anthracene20, the morphology and hygroscopicity remain unchanged. Furthermore, a recent

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field measurement has suggested negligible absorption enhancement of atmospheric BC

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particles under variable mixing states22.

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Aromatic hydrocarbons, such as benzene, toluene, xylenes, and tri-methylbenzene,

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are important anthropogenic volatile organic compounds (VOCs) in the urban atmosphere,

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accounting for 20-30% of the total VOCs in the urban environments42. Photochemical

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oxidation of aromatic hydrocarbons represents an important contributor to ozone and SOA

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formation in urban cities4,43-48. Experimental studies have shown that the aromatic

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hydrocarbons exhibit a large SOA formation potential and likely dominate urban SOA

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formation49-56. A recent experimental study has investigated the changes of physical and

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optical properties of BC particles exposed to OH-initiated toluene oxidation products27,

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showing that coating of toluene oxidation products efficiently modifies the properties of BC

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particles. Previous oxidation studies of anthropogenic VOCs have shown that the SOA yield

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(35.7 ± 1.0%) from m-xylene oxidation is higher than that from toluene (29.8 ± 1.6%)55,56.

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In the present work, we present laboratory experiments to investigate the impacts of

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OH-initiated m-xylene oxidation on BC aging. Size-resolved BC particles are exposed to

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m-xylene-OH photochemical oxidation products, and the changes in particle size, mass,


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effective density, organic mass fraction, dynamic shape factor, coating thickness, optical

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properties, hygroscopicity, and CCN activation are simultaneously measured. Comparison of

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BC aging from the oxidation of several VOCs (i.e., isoprene, toluene, and m-xylene) is made,

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and the parameter that better quantifies the BC aging processes under atmospheric conditions

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is identified.

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2. Experimental

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The experiments were performed in a collapsible environmental chamber, as

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previously described27. Briefly, BC particles generated from incomplete combustion of

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propane were diluted, dried, and size-classified by a differential mobility analyzer (DMA) 6.

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The monodisperse BC particles passed through two denuders to remove gas contaminations

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before being introduced to the chamber. The initial BC number concentration was 1000 ±

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200 particle cm-3, and the BC particles contained less than 5% organics, which was estimated

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by comparing the masses of heated (450 °C) and non-heated particles. H2O2 and m-xylene

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were flushed into the chamber in a flow of purified air. Eighteen black lights were used to

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initiate H2O2 photolysis to produce OH radical. H2O2 from an aqueous solution (16 wt %, 50

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µL) was flushed into the chamber with purified air. The nominal concentration of H2O2 in the

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chamber was 5 ppm to yield a steady OH concentration, which was estimated from the

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xylene decay curve. The estimated OH concentration ranged in (3 – 4) x 106 molecule cm-3

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without NOx and (4 – 5) x 106 molecule cm-3 and (8 - 9) x 106 molecule cm-3 with 50 ppb and

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300 ppb NOx, respectively. The chamber was operated at the relative humidity (RH) of 20 ±

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3%. The detailed experimental procedures have also been described elsewhere27,28.



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The BC particle size, mass, hygroscopicity, and CCN activation were measured with a

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suite of integrated aerosol instruments. The particle number size distribution and mass were

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measured by a scanning mobility particle sizer (SMPS, DMA-CPC) and an aerosol particle

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mass analyzer (APM, DMA-APM-CPC), respectively. The particle diameter and mass

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growth factors (Gfd/Gfm) are expressed as Dp/D0 and mp/m0, respectively, where D0 (m0) and

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Dp (mp) are the particle diameter (mass) before and after aging. The organic mass fraction

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(forg) is expressed as 1-1/Gfm. The material density of particles is calculated using those for

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BC and m-xylene SOA from the literature (ρBC = 1.77 g cm-3, ρorg = 1.40 g cm-3)54,55. The

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volume equivalent diameter is calculated by using the material density. The volume

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equivalent coating thickness is expressed as the change in Dve (ref 20). The dynamic shape

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factor χ characterizes the particle irregularity by comparing the drag force of an irregular

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particle to that of a spherical particle with an equivalent volume, when particles move at the

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same speed.

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A tandem differential mobility analyzer (TDMA) system was used to measure the

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changes in the diameter of BC cores by removing the coated organics in a thermal denuder at

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300 °C. The hygroscopicity of aged BC particles was also measured using TDMA by

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humidifying particles to 90% RH.

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The optical properties were measured by an integrated instrument consisting of a

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commercial Nephelometer (TSI, 3563) and a cavity ring-down spectrometer (CRDS)26. The

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light scattering coefficient (bsca) and the extinction coefficient (bext) were determined at 532

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nm. The light absorption coefficient babs was calculated from the difference between bext and


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bsca. The particle scattering (Csca) and absorption (Cabs) cross sections were derived by

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dividing the corresponding optical coefficient by particle number concentration (N).

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Homogenous nucleation of SOA particles was negligible in our experiments, because only

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the BC peak was measured during the aging experiments. The mass absorption cross-section

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(MAC) was calculated dividing the absorption coefficient (babs) with the coated BC particle

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mass (M). Critical supersaturation of particles were measured by a CCN counter (Droplet

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Measurement Technologies, CCN 100). The calibration and operation conditions were

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similar to those described previously28.

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3.

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3.1

Results and Discussion Hydrocarbon and NOx Concentrations on BC Aging

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The changes in the particle size (Gfd), mass (Gfm), effective density (ρeff), and

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volume equivalent coating thickness (∆rve) under different initial m-xylene concentrations are

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shown as a function of the reaction time (integrated OH exposure time) in Figure 1 (Figure

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S1). It is evident that Gfd, Gfm, ρeff, and ∆rve increase with the reaction time (the integrated

125

OH exposure time) and the initial m-xylene concentration, which regulate the amount of

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condensable products formed from the m-xylene oxidation. The changes in the BC properties

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are directly related to the oxidation level of the m-xylene reaction system, increasing with the

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amount of the reaction products. In Figure 1c (Figure S1c), when the reaction time is longer

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than 250 minutes (OH exposure time is longer than 8 x1010 molecule cm-3 s), the effective

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density converges at a value of about 1.45 g cm-3, which is to the material density of the



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condensable products from the m-xylene oxidation (1.40 g cm-3)55. This level-off behavior in

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the effective density likely reflects complete re-structuring of the BC core.

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Figures 2a,b show that the particle size and mass growth factors also are correlated

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with the initial NOx concentration: the increases in Gfd and Gfm are more pronounced under

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a higher NOx condition. Both Gfd and Gfm also exhibit the similar variations with the

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integrated OH exposure time (Figs. S1a,b). Because of an increasing OH concentration in the

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presence of NOx, i.e., (3.2 ± 0.1) x 106, (4.3 ± 0.1) x 106, and (8.7 ± 0.8) x 106 molecule cm-3

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at 0, 50, and 300 ppb NOx, respectively, NOx facilitates the OH cycling and accelerates the

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xylene reaction. In our experiment, the reacted m-xylene concentrations within a reaction

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time of 300 minutes are 103 ± 1, 136 ± 1, and 183 ± 7 ppb, under the initial NOx

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concentrations of 0, 50 and 300 ppb, respectively. Hence, a larger amount of the oxidation

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products at an increasing NOx level is responsible for the larger size and mass growth factors

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for BC particles. Previous studies have also shown that a higher initial hydrocarbon

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concentration enhances BC aging27,28. In addition, Ng et al. (2007) and Song et al. (2007)

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have suggested that NOx promotes the m-xylene SOA formation, resulting in a higher SOA

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yield; the presence of NOx promotes OH production and reduces the reaction time to achieve

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a certain oxidation level, i.e., the product formation56,57.

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In contrast, when plotted as a function of ∆rve, the changes in the BC properties

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exhibit an identical trend (Figs. 1e-f), independent of the initial m-xylene concentration.

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Similar behaviors are also evident when the initial NOx concentration is varied (Figs. 2c,d).

151

Hence, our results indicate that ∆rve governs the BC aging: the initial m-xylene and NOx


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concentrations determine the time scale to form a certain ∆rve, which subsequently regulates

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the variation in the particle properties.

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3.2

Evolution in BC Properties

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The change in ∆rve with the reaction time is depicted in Figure 3a for the initial BC

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mobility diameters of 80, 100, 150, and 240 nm. Figure 3a exhibits a similar trend of ∆rve

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with the increasing reaction for different initial particle diameters, indicating that the

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formation of the coating thickness is independent on the initial particle size. The changes in

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Gfd, Gfm, ρeff, heated ρeff (300°C), and χ are shown in Figures 3b-f as a function of the

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coating thickness. The particle size, mass, and effective densities increase with ∆rve, because

161

of increasing coating of the m-xylene oxidation products on BC particles. With an organic

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coating of 30 nm on BC particles and for an initial diameter of 100 nm, the particle size and

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mass increase considerably by the factors of 1.3 and 6.1, respectively, and the effective

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density increases from 0.43 to 1.45 g cm-3. The changes in mass and effective density are

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larger than those in particle size, reflecting restructuring of the BC core during the aging

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process. Fresh BC particles are fractal and consist of chain-like aggregates of spherical

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elemental carbon particles. Previous studies have demonstrated that, when coated by

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condensable materials such as sulfuric acid18, glutaric acid41, and products from α-pinene

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ozonolysis21, toluene-OH reaction27, and isoprene-OH reaction28, the BC core varies from a

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fractal to a compact form16,58. However, little restructuring has been shown in the case of

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succinic acid41. The BC core density or the heated effective density, ρeff(heated), also reflects

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the restructuring process. In our experiment, the increase of ρeff(heated) indicates the BC core



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collapses during aging. Clearly, the mass and density changes are more representative of BC

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aging than that of the mobility diameter.

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Figure 3b reveals that the variation of Gfd with ∆rve is strongly size-dependent. Gfd

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increases with ∆rve for 80 and 100 nm particles, becomes nearly invariant with ∆rve for 150

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nm particles, but decreases with ∆rve for 240 nm particles. Smaller particles (i.e., 80 and 100

178

nm) are less fractal, and restructuring is minimal. The decrease in Gfd for particles of the

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initial diameter of 240 nm is explained by shrinking of the BC core. In addition, because of

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the fractal structure of larger BC particles, condensable oxidation products may initially

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occupy the voids of BC and do not increase the particle size3. Hence, both restructuring and

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filling the voids of larger BC particles likely contribute to the decreasing particle size with

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∆rve. In contrast, Gfm for different initial particle sizes increase monotonically, indicating that

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the particles gain mass from coating of the m-xylene oxidation products. Gfm increases more

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pronouncedly for smaller particles, attributable to a relatively larger mass ratio of organic

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coating to the BC core.

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The effective density ρeff of nascent BC is much lower than the BC material density

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because of the fractal structure16. The initial ρeff for fresh BC is in the range of 0.40 to 0.42 g

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cm-3 in our experiments, and ρeff increases by a factor of 2.0-3.5 after aging. The increase in

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ρeff reflects jointly the restructuring of the BC core and formation of organic coatings with a

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higher material density (for the m-xylene-OH oxidation products ρ = 1.40 ± 0.10 g cm-3)55.

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The dynamic shape factor (χ) reflects the regularity of the particle morphology. A

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spherical particle has a χ value of unity, and χ is larger than unity for irregular particles. As


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shown in Figure 3f, χ decreases during BC aging. For smaller particles (80 and 100 nm), χ

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approaches unity when ∆rve reaches about 20 nm, i.e., 1.01 for 80 nm and 1.03 for 100 nm.

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For particles of 150 nm, χ decreases to 1.02, when ∆rve reaches about 30 nm. With ∆rve

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varying from 0 nm to 26.8 nm for 240 nm particles, χ decreases from 1.74 to 1.18. Our

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measured change in χ from the m-xylene oxidation (0.60) is more pronounced than those

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from coating of succinic acid (0.74) (refs 19 and 41), isoprene-OH oxidation products (0.75)

200

(ref 28), but is less than coating of the toluene-OH oxidation products (0.48) (ref 27). The

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decrease in χ likely includes two contributions, i.e., restructuring of BC aggregates and

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filling of the voids between primary spheres by the oxidation products. With an increasing

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amount of coatings by the organics, the particles become more spherical, and χ approaches

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unity.

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3.3

Optical Properties and CCN Activity

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The changes in light extinction and scattering are measured during the BC aging

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process (Figs. 4a,b), showing the particle scattering (Csca) and absorption (Cabs) cross-section

208

as a function ∆rve for the initial diameters of 150 nm (Fig. 3a) and 240 nm (Fig. 3b). Fresh

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BC scatters weakly because it consists of small primary spheres (i.e., 12-20 nm)16. The single

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scattering albedo is low for fresh particles, ranging from 0.13 to 0.20. With an increasing

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coating thickness, there is a significant enhancement in scattering: Csca for an initial diameter

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of 150 nm increases by a factor of 11.8 and 16.5 in the absence and presence of 50 ppb NOx,

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respectively. Similarly, when plotting the scattering coefficient as a function of ∆rve, the



214

variation of Csca exhibits a similar trend with and without NOx, suggesting that scattering

215

only depends on the organic coating.

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Our results show that the change of BC light absorption is non-monotonic during

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aging. For an initial diameter of 150 nm and in the absence of NOx (Fig. 3a), Cabs decreases

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initially due to coating of insulated materials on the primary spheres, which weakens the

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electromagnetic interaction among the primary spheres. This interaction recurs for additional

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aging due to the restructuring of the BC core, leading to an increase of Cabs. However, with

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further restructuring, the BC spheres in the outer layer shield the inner spheres from

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absorbing light, resulting in the decrease of Cabs (ref 3). For a larger coating thickness, Cabs

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increases ultimately due to the lensing effects. Interestingly, in our experiment with the

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presence of 50 ppb NOx, Cabs does not exhibit the initial decrease because of a faster reaction,

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but shows the subsequent (decreasing and increasing) features at a larger coating thickness.

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Cabs is enhanced by 9% and 20% in the absence and presence of NOx (i.e., 50 ppb),

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respectively. The MAC variations of BC particles with ∆rve and the MAC enhancement with

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the ratio of the coating to core mass are shown in Figure S2. Our measured variations in the

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BC optical properties are consistent with those from previous theoretical and experimental

230

studies. For example, theoretical calculation results have indicated that fresh BC is a strong

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light absorber because the interaction between primary spheres increases absorption by 30%

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compared to the sum absorption of individual spheres. Because of electrical insulation of the

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organic coating, this interaction may be weakened25. Previous experimental measurements

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have also confirmed the theoretical calculations, showing that when thin organic coatings are


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removed from aged BC particles, the absorption increases by 4-14% (refs 19 and 26). When

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coated by sufficient transparent coatings, BC particles absorb more light due to the lensing

237

effect19.

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Fresh BC is commonly hydrophobic16,40,60. In our study, the hygroscopic growth

239

factor for fresh BC at 90% RH is 1.00 ± 0.01. Figure S3a shows the variation in the growth

240

factor with ∆rve. Initially, for thin organic coating (∆rve < 20 nm), the growth factor decreases

241

with ∆rve, because condensation of a small amount of water restructures the BC particles.

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With additional aging, the BC core is fully compacted, and sufficient hydrophilic organic

243

coatings are formed, leading to an increase of the hygroscopic growth factor. The growth

244

factors remain unchanged when particles are fully aged.

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Coating of hydrophilic organics significantly enhances the CCN activity of BC

246

particles. Figure S3b shows the changes in fractions of activated particles (CCN/CN) with

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time at different supersaturations (SS). Less than 0.1% of the fresh BC particles serve as

248

CCN at SS of 1%. For the coating thicknesses of 11.5 and 35.7 nm after the reaction times of

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60 min and 170 min, 50% of the particles activate at SS of 0.4% and 0.2%, respectively. The

250

critical supersaturation (SSc), at which 50% of the particles are activated as CCN, is

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calculated and presented in Figure 4c as the function of the volume equivalent diameter (Dve).

252

The SSc for fully aged particles is as low as 0.10% and 0.11% in the presence of 50 ppb NOx

253

and in the absence of NOx, respectively. Both coating of hydrophilic organics and increase in

254

particle size are responsible for the decreased SSc.

255

The hygroscopic parameter κ is calculated from the CCN measurement61,



κ=

256

4 A3 27 D p3 ln 2 S c

A=

257

4M wσ s / a RT ρ w

(1)

(2)

258

where Dp is the diameter of the particles, Mw is the molar mass of water, σs/a = 0.072 J m-2 is

259

the surface tension of water-air interface, and ρw is the water density. For a multicomponent

260

system, the overall hygroscopic parameter is the sum of the individual values for all

261

components:

κ = ∑ ε iκ i

262

(3)

263

where εi and κi are the volume fraction and hygroscopic parameter for component i.

264

Because BC is hydrophobic, κBC = 0, equation (3) is simplified as:

κ = ε coatingκ coating

265

(4)

266

Figure 4d presents the κ change as the function of ε with and without NOx. The

267

dependence of κ on ε in the absence of NOx is linear (R2 = 0.992). The κ value of the coating

268

material for the m-xylene SOA is 0.12. The small κ value is also attributable to the high

269

molecular weight, in addition to the low hygroscopicity of the OH-m-xylene oxidation

270

products56,57.

271

3.4

Comparison of Coating on BC Properties with Other Species

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Table 1 compares the effects of coatings of various organic and inorganic species on

273

the BC properties. The largest coating thickness measured in our experiments for m-xylene is

274

44.5 nm, which is slightly larger to that for the toluene-OH products (39.1 nm) ref (27) and

275

much larger than those for sulfuric acid (12.0 nm) (refs 16 and18) and isoprene (6.31 nm) (ref 14 ACS Paragon Plus Environment

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28). The particle mass increases by a factor of 10.35 for m-xylene, which is also much larger

277

than those of toluene-OH products (4.57) (ref 27) and isoprene-OH products (1.42) (ref 28).

278

Coating of the m-xylene-OH oxidation products enhances the scattering more pronouncedly

279

than those by the other compounds due to a larger coating thickness. Coatings of sulfuric

280

acid18, α-pinene ozonolysis products21,59 and toluene-OH oxidation products28 increase the

281

absorption by 5-30%. However, little enhancement in absorption is found when BC is

282

exposed to isoprene-OH oxidation products because of an insignificant coating thickness28.

283

Our largest light absorption enhancement is by 9% and 20% by m-xylene (for 150 nm

284

particles without and with NOx, respectively), comparable to that by sulfuric acid. The largest

285

increase in the optical properties corresponds to that of BC aging by the α-pinene-O3

286

oxidation products. Our result also show that SSc is reduced to 0.11% for the m-xylene-OH

287

oxidation, which is much smaller than that for the isoprene-OH oxidation.

288

Figure S4 also shows that coating of OH-initiated oxidation of m-xylene alters the BC

289

particle size and mass more efficiently than those by the other oxidation products. At 200 min,

290

the particle diameter increases by a factor of 1.3 and 1.22 for m-xylene and toluene,

291

respectively. The particle diameter decreases when coated by the isoprene-OH oxidation

292

products due to the shrinking of the BC core21. The mass growth factor is 5.5 for m-xylene at

293

180 min, which is larger than 4.5 for toluene and 1.4 for isoprene. When plotted as a function

294

of the coating thickness, the changes in particle size and mass are similar, except that the

295

particle size decreases when particles are coated by the isoprene-OH products.



296

To further compare the effectiveness of BC aging caused by the oxidation products

297

from the OH-initiated reactions of toluene and m-xylene relevant to ambient conditions4,62,

298

we performed additional BC aging experiments using 200 ppb toluene and compared the

299

results with that using 100 ppb m-xylene. The initial particle sizes for both experiments were

300

100 nm. The largest ∆rve value is 39.1 nm for toluene, and 40.5 nm for m-xylene. The

301

measured largest Gfd, Gfm, Gfh and growth of SSA are 1.32, 7.43, 1.24 and 3.77 for m-xylene

302

and 1.22, 4.57, 0.99 and 2.40 for toluene, respectively. Hence, our results reveal that

303

m-xylene is more efficient in causing BC aging than toluene.

304

3.4

Atmospheric Implications

305

Understanding the variations of BC properties during atmospheric aging is essential

306

in assessing its atmospheric impacts4,30. As summarized in Table 1, the effects of coatings of

307

various organic and inorganic species on the BC properties are highly variable. The

308

m-xylene-OH oxidation products are more efficient in modifying the BC properties than

309

those from the OH-initiated oxidation of isoprene and toluene. Our results show that coating

310

by the oxidation products of the m-xylene-OH reaction considerably increases the BC size,

311

mass, and effective density, dependent of the reaction time and concentrations of m-xylene

312

NOx, and OH. The concentrations of OH radicals and NOx in our experiments are comparable

313

to those in the atmosphere, but the concentration of m-xylene is higher than that in the

314

atmosphere4. There exist unique correlations of the BC size, mass, and effective density with

315

∆rve and ∆rve with the particle size, indicating that ∆rve is applicable to quantify atmospheric

316

BC aging.


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317

BC particles (100 nm) are converted from a highly fractal to nearly spherical

318

morphology with ∆rve = 30 nm. The SSc is reduced reduced to 0.1% with ∆rve = 44 nm

319

because of coating of hydrophilic organics and increase in the particle size, indicating that

320

aged BC particles serve efficiently as CCN. The enhanced CCN activity also considerably

321

reduce the atmospheric lifetime of BC particles. Our results illustrate non-monotonic

322

variations in light absorption of BC particles during aging, reflecting the complex

323

morphology variation and related electromagnetic interaction; a large absorption

324

enhancement is only achievable with sufficient coating by the lensing effect. Atmospheric

325

measurements have shown variable MAC enhancement under ambient conditions. For

326

example, a field study has found that the absorption enhancement is insignificant in two

327

California regions22. Liu et al. have observed a significant absorption enhancement of aged

328

BC particles and attributed this enhancement to the formation of brown carbon during winter

329

in United Kindom29. Using a novel environmental chamber method, Peng et al.30 have

330

recently shown that BC aging exhibits two distinct stages, i.e., initial transformation from a

331

fractal to spherical morphology with little absorption variation and subsequent growth of

332

fully compact particles with a large absorption enhancement. The MAC enhancement for BC

333

particles is small in Houston, but reaches a factor of 2.4 under polluted environments in

334

Beijing30. Clearly, under ambient conditions, the variations in BC properties are dependent

335

on the timescale and rate of aging, regulating their impacts on human health, weather, and

336

direct and indirect radiative forcing on climate4,30,62.

337



338

339

Supporting Information

340

Changes in BC properties with the integrated OH exposure time at different initial m-xylene

341

and NOx concentration, variation of hygroscopic growth factor with the reaction time, the

342

variation of CCN fraction with the OH exposure time, and comparison in the evolution of BC

343

particle properties coated by the oxidation products from the xylene-OH with those from the

344

previous studies for toluene-OH and isoprene-OH reactions. This information is available

345

free of charge via the Internet at http://pubs.acs.org/.

346

347

Corresponding Authors:

348

Song Guo: Address: State Key Joint Laboratory of Environmental Simulation and Pollution

349

Control, College of Environmental Sciences and Engineering, Peking University, Beijing,

350

China 100871. Phone: +86-10-62752417, email: [email protected]

351

Renyi Zhang: Address: Department of Atmospheric Science, Texas A&M University,

352

College Station, Texas 77843, USA. Phone: 1-979- 845-7656, email: [email protected]

ASSOCIATED CONTENT

AUTHOR INFORMATION

353 354

Acknowledgements

355

This research was supported by the Robert A. Welch Foundation (A - 1417), National

356

Natural Science Foundation of China (91544214), National Basic Research Program of China

357

(2013CB228503), Special Fund for Strategic Pilot Technology Chinese Academy of Sciences

358

(XDB05010500).

359


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References

361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399

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Jayne, J. T.; Worsnop, D. R.; China, S.; Sharma, N.; Mazzoleni, C.; Xu, L.; Ng, N. L.; Liu, D.; Allan, J. D.; Lee, J. D.; Fleming, Z. L.; Mohr, C.; Zotter, P.; Szidat, S.; Prevot, A. S. H., Enhanced light absorption by mixed source black and brown carbon particles in UK winter. Nat Commun 2015, 6, 8435. 30. Peng, J.; Hu, M.; Guo, S.; Du, Z.; Zheng, J.; Shang, D.; Zamora, L. M.; Zeng, L.; Shao, M.; Wu, Y., Markedly enhanced absorption and direct radiative forcing of black carbon under polluted urban environments. P. Natl. Acad. Sci. USA 2016, 113 (16), 4266–4271. 31. Schwarz, J. P.; Spackman, J. R.; Fahey, D. W.; Gao, R. S.; Lohmann, U.; Stier, P.; Watts, L. A.; Thomson, D. S.; Lack, D. A.; Pfister, L.; Mahoney, M. J.; Baumgardner, D.; Wilson, J. C.; Reeves, J. M., Coatings and their enhancement of black carbon light absorption in the tropical atmosphere. J. Geophys. Res. 2008, 113 (D3), D03203. 32. Shiraiwa, M.; Kondo, Y.; Moteki, N.; Takegawa, N.; Miyazaki, Y.; Blake, D. R., Evolution of mixing state of black carbon in polluted air from Tokyo. Geophys. Res. Lett. 2007, 34 (16), L16803. 33. Khalizov, A. F.; Zhang, R. Y.; Zhang, D.; Xue, H. X.; Pagels, J.; McMurry, P. H., Formation of highly hygroscopic soot aerosols upon internal mixing with sulfuric acid vapor. J. Geophys. Res. 2009, 114 (D5), D05208, doi:10.1029/2008JD010595. 34. Henning, S.; Ziese, M.; Kiselev, A.; Saathoff, H., Hygroscopic growth and droplet activation of soot particles: uncoated, succinic or sulfuric acid coated. Atmos. Chem. Phys. 2011, 11 (10), 28445-28475. 35. Hings, S. S.; Wrobel, W. C.; Cross, E. S.; Worsnop, D. R., CCN activation experiments with adipic acid: effect of particle phase and adipic acid coatings on soluble and insoluble particles. Atmos. Chem. Phys. 2008, 8 (14), 3735-3748. 36. Lambe, A. T.; Ahern, A. T.; Wright, J. P.; Croasdale, D. R.; Davidovits, P.; Onasch, T. B., Oxidative aging and cloud condensation nuclei activation of laboratory combustion soot. J. Aerosol Sci. 2015, 79 (79), 31-39. 37. Liu, D.; Allan, J.; Whitehead, J.; Young, D., Ambient black carbon particle hygroscopic properties controlled by mixing state and composition. Atmos. Chem. Phys. 2012, 13 (4), 2015-2029. 38. Wittbom, C.; Pagels, J. H.; Rissler, J.; Eriksson, A. C.; Carlsson, J. E.; Roldin, P.; Nordin, E. Z.; Nilsson, P. T.; Swietlicki, E.; Svenningsson, B., Cloud droplet activity changes of soot aerosol upon smog chamber ageing. Atmos. Chem. Phys. 2014, 14 (7), 9831-9854. 39. Karcher, B.; Mohler, O.; DeMott, P. J.; Pechtl, S.; Yu, F., Insights into the role of soot aerosols in cirrus cloud formation. Atmos. Chem. Phys. 2007, 7 (16), 4203-4227. 40. Ma, Y.; Brooks, S. D.; Vidaurre, G.; Khalizov, A. F.; Wang, L.; Zhang, R. Y., Rapid modification of cloud-nucleating ability of aerosols by biogenic emissions. Geophys. Res. Lett. 2013, 40 (23), 6293-6297. 41. Xue, H. X.; Khalizov, A. F.; Wang, L.; Zheng, J.; Zhang, R. Y., Effects of Coating of Dicarboxylic Acids on the Mass-Mobility Relationship of Soot Particles. Environ. Sci. Technol. 2009, 43 (8), 2787-2792.



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42. Calvert, J. G.; Atkinson, R.; H., B. K.; Kamens, R. M.; Seinfeld, J. H.; Wallington, T. J.; Yarwood, G., The Mechanisms of Atmospheric Oxidation of Aromatic Hydrocarbons. Oxford University Press, New York: 2002. 43. Suh, I.; Zhang, D.; Zhang, R. Y.; Molina, L. T.; Molina, M. J., Theoretical study of OH addition reaction to toluene. Chem. Phys. Lett. 2002, 364 (5-6), 454-462. 44. Suh, I.; Zhang, R.; Molina, L. T.; Molina, M. J., Oxidation mechanism of aromatic peroxy and bicyclic radicals from OH-toluene reactions. J. Am. Chem. Soc. 2003, 125 (41), 12655-12665. 45. Zhao, J.; Levitt, N. P.; Zhang, R., Heterogeneous chemistry of octanal and 2,4-hexadienal with sulfuric acid. Geophys. Res. Lett. 2005, 32 (9), L09802, doi:10.1029/2004GL022200. 46. Fan, J. W.; Zhang, R., Atmospheric oxidation mechanism of p-xylene: A density functional theory study. J. Phys. Chem. A 2006, 110 (24), 7728-7737. 47. Zhao, J.; Khalizov A.F.; Zhang, R.; McGraw, R., Hydrogen bonding interaction of molecular complexes and clusters of aerosol nucleation precursors. J. Phys. Chem. 2009, 113 (4), 680–689. 48. Zhao, J.; Zhang, R. Y.; Misawa, K.; Shibuya, K., Experimental product study of the OH-initiated oxidation of m-xylene. J. Photoch. Photobio. A 2005, 176 (1-3), 199-207. 49. Kalberer, M.; Paulsen, D.; Sax, M.; Steinbacher, M.; Dommen, J.; Prevot, A. S. H.; Fisseha, R.; Weingartner, E.; Frankevich, V.; Zenobi, R.; Baltensperger, U., Identification of polymers as major components of atmospheric organic aerosols. Science 2004, 303 (5664), 1659-1662. 50. Cocker, D. R.; Mader, B. T.; Kalberer, M.; Flagan, R. C.; Seinfeld, J. H., The effect of water on gas-particle partitioning of secondary organic aerosol: II. m-xylene and 1,3,5-trimethylbenzene photooxidation systems. Atmos. Environ. 2001, 35 (35), 6073-6085. 51. Martin-Reviejo, M.; Wirtz, K., Is benzene a precursor for secondary organic aerosol? Environ. Sci. Technol. 2005, 39 (4), 1045-1054. 52. Schnitzler, E. G.; Dutt, A.; Charbonneau, A. M.; Olfert, J. S.; Jäger, W., Soot aggregate restructuring due to coatings of secondary organic aerosol derived from aromatic precursors. Environ. Sci. Technol. 2014, 48 (24), 14309-16. 53. Guo, S.; Hu, M.; Guo, Q.; Shang, D., Comparison of Secondary Organic Aerosol Estimation Methods. Acta Chimica 2014, 72 (6), 658-666. 54. Park, K.; Kittelson, D. B.; Zachariah, M. R.; McMurry, P. H., Measurement of inherent material density of nanoparticle agglomerates. J. Nanopart. Res. 2004, 6 (2-3), 267-272. 55. Song, C.; Na, K.; Warren, B.; Malloy, Q.; Cocker, D. R., Secondary organic aerosol formation from m-xylene in the absence of NOx. Environ. Sci. Technol. 2007, 41 (21), 7409-7416. 56. Ng, N. L.; Kroll, J. H.; Chan, A. W. H.; Chhabra, P. S.; Flagan, R. C.; Seinfeld, J. H., Secondary organic aerosol formation from m-xylene, toluene, and benzene. Atmos. Chem. Phys. 2007, 7 (14), 3909-3922.


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57. Song, C.; Na, K. S.; Cocker, D. R., Impact of the hydrocarbon to NOx ratio on secondary organic aerosol formation. Environ. Sci. Technol. 2005, 39 (9), 3143-3149. 58. Nakao, S.; Shrivastava, M.; Nguyen, A.; Jung, H. J.; Cocker, D., Interpretation of Secondary Organic Aerosol Formation from Diesel Exhaust Photooxidation in an Environmental Chamber. Aerosol Sci. Technol. 2011, 45 (8), 964-972. 59. Schnaiter, M.; Linke, C.; Mohler, O.; Naumann, K. H.; Saathoff, H.; Wagner, R.; Schurath, U.; Wehner, B., Absorption amplification of black carbon internally mixed with secondary organic aerosol. J. Geophys. Res. 2005, 110 (D19), 19204. 60. Seinfeld, J. H.; Pandis, S. N., Atmospheric chemistry and physics: from air pollution to climate change. Wiley: New York: 2006; Vol. 409, 695. 61. Petters, M. D.; Kreidenweis, S. M., A single parameter representation of hygroscopic growth and cloud condensation nucleus activity. Atmos. Chem. Phys. 2007, 7 (8), 1961-1971. 62. Guo, S.; Hu, M.; Zamora, M. L.; Peng, J.; Shang, D.; Zheng, J.; Du, Z.; Wu, Z.; Shao, M.; Zeng, L.; Molina, M. J.; Zhang, R. Elucidating severe urban haze formation in China. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 17373−17378.

539



540

Table 1. Comparison of the effects of different coatings on the BC size growth factor (Gfd),

541

mass growth factor (Gfm), coating thickness (∆rve), hygroscopic growth factor (Gfh),

542

scattering (Csca), absorption (Cabs), single scattering albedo (SSA), and critical supersaturation

543

(SSC).

Gfd Gfm Gfh Csca/Csca,0 Cabs/Cabs,0 SSA/SSA0 ∆rve Sulfuric acid 12.00 2.80 1.15 2.20 b α-pinene-O3 5.2 1.3 2.5 c,e Toluene-OH 1.22 4.57 39.10 0.99 3.60 1.05 2.40 d Isoprene-OH 0.90 1.42 6.31 0.96 1.44 0.69 1.72 d Isoprene-OH (NOx) 0.94 2.16 11.80 1.02 1.25 0.97 1.21 e Xylene-OH 1.51 10.35 44.50 1.14 11.83 1.09 3.71 2.18 25.50 75.30 1.18 16.53 1.20 4.14 Xylene-OH (NOx)e a b c d 28 e 544 refs 16 and 18, refs 21 and 59, ref 27, ref , and this work for 150 nm particles. The a

545

ratios for Csca, Cabs, and SSA correspond to those between aged and fresh (denoted by 0) BC

546

particles.


Page 24 of 30

SSc 0.60 0.46 0.11 0.10

Page 25 of 30


547 548

Figure captions Figure 1. Changes in BC size growth factor (Gfd), mass growth factor (Gfm), effective

549

density (ρeff), volume equivalent coating thickness (∆rve), and effective density after

550

heating (ρeff(heated)) at different initial m-xylene concentrations as a function of the

551

reaction time (a-d) and coating thickness (e-h). The experimental conditions are: Dp =

552

100 nm and [m-xylene] = 50, 100, or 200 ppb.

553

Figure 2. Changes in BC size growth factor (Gfd) and mass growth factor (Gfm) at different

554

initial NOx concentration as a function of the reaction time (a and b) and coating

555

thickness (c and d). The experimental conditions are: Dp = 100 nm and [NOx] = 0, 50,

556

or 300 ppb.

557

Figure 3. Evolution in BC properties at different initial particle sizes: ∆rve as a function of

558

the reaction time (a) and Gfd (b), Gfm (c), ρeff (d), ρeff(heated) (e), and χ (f) as a function

559

of the coating thickness. The experimental conditions are: Dp = 50, 100, 150, or 240

560

nm and [m-xylene] = 200 ppb.

561

Figure 4. Variation in BC optical and hygroscopic properties during aging. (a and b)

562

Absorption cross section (Cabs, right axis) and scattering cross section (Csca, left axis)

563

as a function of the coating thickness; (c) critical supersaturation (SSc) as a function of

564

the volume equivalent diameter (Dve) ; and (d) hygroscopicity parameter (κ) as a

565

function of the coating volume fraction (ε). The experimental conditions are:

566

[m-xylene] = 200 ppb and [NOx] = 0 or 50 ppb.

567 568



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