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A: Environmental, Combustion, and Atmospheric Chemistry; Aerosol Processes, Geochemistry, and Astrochemistry

Effects of Gas-Particle Partitioning on Refractive Index and Chemical Composition of m-Xylene Secondary Organic Aerosol Kun Li, Junling Li, Weigang Wang, Jiangjun Li, Chao Peng, Dong Wang, and Maofa Ge J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b12792 • Publication Date (Web): 12 Mar 2018 Downloaded from http://pubs.acs.org on March 13, 2018

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The Journal of Physical Chemistry

Effects of Gas-Particle Partitioning on Refractive Index and Chemical Composition of m-Xylene Secondary Organic Aerosol

1 2 3 4 5

Kun Li,†,‡,ǁ Junling Li,†,‡ Weigang Wang,*,†,‡ Jiangjun Li,‡,§ Chao Peng,†,‡ Dong

6

Wang,‡,§ and Maofa Ge*,†,‡

7 8 9 10 11 12 13



State Key Laboratory for Structural Chemistry of Unstable and Stable Species, CAS

Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. ‡

University of Chinese Academy of Sciences, Beijing 100049, P. R. China.

§

Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute of

Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China.

14 15 16

Abstract

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The formation of secondary organic aerosol (SOA) contains partitioning processes of

18

the oxidation products between gas and particle phase, which could change the

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particle-phase composition when particles grow. However, the effects of these

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processes on the optical properties of SOA remain poorly understood. In this study,

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we performed smog chamber experiments to investigate the effects of gas-particle

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partitioning (GPP) on the refractive index (RI) and chemical composition of the

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m-xylene SOA. Here, we show that the GPP processes, as organic mass increases, can

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increase the proportions of semi- and intermediate volatile organic compounds

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(SVOCs and IVOCs) in the particle phase and result in the decrease of SOA RI real

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part for 0.09 ± 0.02 (without seeds) and 0.15 ± 0.02 (with seeds). This indicates that

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the SOA optical properties are closely related to the total organic mass and

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molecular-level composition. In addition, the presence of inorganic seeds promotes

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the GPP to the particle phase and hence further decreases the RI real part for 0.05 ±

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0.02. As pre-existing aerosols are ubiquitous in the ambient atmosphere, it is

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suggested that there should be a certain correction when applying the SOA RI of

32

previous laboratory studies to air quality and climate models.

33

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Introduction

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As an important type of atmospheric fine particulate matter, secondary organic

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aerosol (SOA) plays a vital role in visibility, climate change, and human health.1-2 The

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formation and evolution of SOA were the foci of atmospheric chemistry in the past

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few decades.3-8 During the formation of SOA, gas-particle partitioning (GPP, i.e.

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condensation and evaporation between gas and particle phase) was found to be a

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traditional and dominant approach.9-12 The partitioning between gas and particle phase

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of a component i is given by: ∗



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 = 1 +   ;  = ∑  

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where  is the partitioning coefficient, which means the fraction in particle phase,

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∗ is the effective saturation concentration of the compound,  is the

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concentration of organic aerosol, and  is concentration of the compound (in both

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gas and particle phase).5,

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increase as well, which will lead to the higher partitioning coefficient  according to

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the equation, so Equation (1) should be applied iteratively. One consequence of this

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partitioning process is that the organic aerosol composition and oxidation state will be

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changed as the total organic concentration increases, which has been proved by

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laboratory chamber studies.13-14 It may have important impacts, because the changing

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composition could influence the physical properties such as hygroscopic and optical

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coefficients, which are important parameters related to visibility and aerosol climate

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effects.



11-12

(1)

If the organic concentration  increases,  will

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Due to the complex compositions and various mixing states, the optical properties

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of SOA and their effects on global radiative balance remain poorly quantified.1, 15-16

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Refractive index (RI) is one of the most important factors of aerosol optical

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properties.16 As other optical factors like scattering and extinction efficiencies can be

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calculated using the RI, quantitatively understanding the RI of SOA is essential.15-17

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In recent years, some laboratory studies have been carried out to determine the RIs of

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SOA.17-28 The studied SOA precursors contain both biogenic (monoterpenes, isoprene

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etc.) and anthropogenic (toluene, xylene, alkanes etc.) volatile organic compounds

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(VOCs). Some of these studies find that the SOA RI changes as organic mass loading

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increases (i.e. particles grow).17, 19 Kim et al. found that the RI of limonene SOA

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increased as particles grow, while the RI of α-pinene SOA didn’t change obviously.19

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The Journal of Physical Chemistry

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Li et al. found that SOA generated from m-xylene decreased dramatically among the

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four studied aromatic precursors.17 However, the driving force of this kind of

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changing in optical properties is still unclear.

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In addition, when investigating the refractive index of SOA, previous studies

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mostly focus on the SOA particles in the absence of aerosol seeds,15-26 though it has

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been found that seeds could influence the GPP process and enhance the SOA

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yields.29-31 Pre-existing aerosols can provide the surface for organic materials to

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condense on, and increase the proportion of more volatile compounds in the particle

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phase. They are ubiquitous in the ambient atmosphere; hence the impacts of

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pre-existing aerosol seeds on SOA optical properties should be further studied. A

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recent study investigated the effects of seeds on optical properties of SOA derived

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from long-chain alkanes, a type of intermediate VOC (IVOC), attempting to relate the

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chemical composition to RIs.27 However, as the more classical and widely studied

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precursors of anthropogenic SOA, the aromatics, the effects of aerosol seeds on

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optical properties of their SOA remain unknown.

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In this study, we performed smog chamber experiments to investigate the effects

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of the GPP process with and without inorganic seeds on the RI of SOA generated by

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photo-oxidation

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High-resolution electrospray ionization mass spectrometry was used to analyze the

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molecular composition of the SOA particles. By combining the high-resolution

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chemical analysis with optical detection, we try to find the relationship between

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chemical composition and RI. We show the impacts of GPP and pre-existing

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inorganic seeds on composition and RI of m-xylene SOA, and discuss their

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atmospheric implications.

of

m-xylene,

a

typical

anthropogenic

aromatic

precursor.

90 91

Experimental

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The experiments discussed here have been described in detail in a previous

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publication.28 Briefly, the experiments were performed with a dual-reactor smog

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chamber. The combined light sources including 340 nm broadband lamps and 365 nm

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narrowband black lamps are used.32 The temperature was maintained at 293 ± 1K.

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The RH in the reactor was controlled at < 5%. The size distribution of aerosol was

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measured by the scanning mobility particle sizer (SMPS; Model 3936, TSI). The NOx

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and O3 concentration were monitored by trace gas analyzers (T200UP and T400,

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99 100

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Teledyne API). The concentration of m-xylene was measured by the quadrupole proton transfer reaction mass spectrometry (PTR-MS; HS500, Ionicon).

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Zero air was injected into the chamber before each experiment. NOx was

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introduced into the chamber from a standard gas cylinder, and the concentration in the

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reactor was 300 ± 50 ppb. A known volume of m-xylene (≥ 99%, Sigma-Aldrich) was

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injected into a glass U-tube, and then it was introduced into the chamber with the zero

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air flow. The concentration of m-xylene in the reactor was around 4 ppm. For the

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seeded condition, the atomizer which contained inorganic salts (namely ammonium

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sulfate, AS and sodium chloride, NaCl; ≥ 99%, Sigma-Aldrich) solution was used to

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generate the inorganic aerosols. The seed particles were dried by a diffusion dryer and

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then introduced into the reactors without size selecting. The seeds are solid under the

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< 5% RH in the chamber, which can exclude the influence of liquid phase reactions

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on compositions and optical properties.28 The number concentration of seeds in the

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reactors is (1.6 ± 0.3) × 104 cm-3. The non-sizing particles have a mode of 63 ± 4 nm,

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with geometric standard deviation (σg) of 1.8 ± 0.05. The UV lights were turned on 30

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min after the NOx, m-xylene, seeds were introduced into the chamber to make sure

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that they are well mixed. The representative reaction profiles are shown in Fig. S1.

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The light extinction and RIs of aerosols are detected through a custom-built

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cavity ring-down spectrometer (CRDS), see previous work for details.33 The CRDS

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provides the decay time τ directly, and the RI is gained by programs based on Bohren

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and Huffman.34 The detailed process and method are introduced elsewhere.17 Briefly,

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The RI of each diameter is calculated by minimizing the following merit function (χ):  = (, − ,!"!" (#, $))&

(2)

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where n is the real part of RI, k is the imaginary part of RI, and Qext is the extinction

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efficiency. Qext,measured is determined using the following equation:

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, = (

'

+ 



( −, )

)*) !" ,

-

(3)

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where Stot is the total surface area that measured by SMPS, L is the length of the

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cavity, c is the velocity of light, l is the length filled with aerosol and τ0 is the decay

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time when the cavity is filled with zero air. This paper and a previous publication28

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share the same dataset of SOA optical properties. While that paper focuses on effects

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of multiphase reactions, this study mainly investigates the effects of GPP process.

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The Journal of Physical Chemistry

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The morphology of m-xylene SOA particles was determined via atomic force

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microscope (AFM). The aerosol particles were collected by impacting on silicon

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wafers and then were analyzed by a tapping mode AFM (NT-MDT Ntegra) at room

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temperature (~298 K) and ambient RH (~30%). The SOA particles were also collected

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by PTFE membrane filters with a pore size of 0.22 µm (FGLP047, Millipore), and

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then were dissolved in 5 mL methanol. The solution was then analyzed by

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ultra-high-resolution electrospray ionization time-of-flight mass spectrometry

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(UHR-ESI-TOF-MS; Impact HD, Bruker) in positive ion mode to get the mass

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spectra.

138 139

Results and discussions

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The morphology and structure of aerosols in the presence of inorganic seeds are

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important to retrieve the RI, as particle shape and mixing state can influence the

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application and calculation of Mie theory. The studied inorganic salts are not spherical,

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which may introduce some deviation when applying Mie theory. Hence, we at first

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investigate the morphology and mixing state of SOA in the presence of inorganic

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seeds.

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The AFM image of SOA particles in the presence of AS seeds is illustrated in Fig.

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1. It is indicated that the particles are in the core-shell structure, of which the organic

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coating is very soft and was impacted to be a very thin layer on the silicon wafer

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(height < 50 nm). As a result, we speculate that the organic aerosols are likely liquid,

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and they have an approximately spherical shape when suspended in the air.17, 35 This

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finding suggests that there is little deviation when using the Mie theory to calculate RI

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of seeded SOA, which is similar to our previous study about the non-seeded aromatic

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SOA.17

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The Journal of Physical Chemistry

c Height (nm)

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200

section 1

section 2

100

0 0

155

300

600

900

1200

1500

0

300

600

900

1200

1500

Width (nm)

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Figure 1 AFM images of the m-xylene SOA sample in the presence of AS seeds. a. Height image

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with two cross sections; b. phase image, in which core-shell can be observed; c. two cross sections

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in the height image.

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Same as our previous studies, negligible light absorption was found for m-xylene

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SOA at 532 nm, so only the real part of RI are considered here.17, 28 For brevity, “RI”

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represents the real part in the following text. The RIs of seeded experiments are

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retrieved by Mie theory, assuming that the particles are in the core-shell structure.

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However, as the seed is much smaller than the whole particle, there seems to be little

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differences between core-shell assumption and homogeneous assumption (see Fig.

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S2). The RIs of m-xylene SOAs under non-seeded and seeded conditions are shown in

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Fig. 2. It is found that the SOA is generated earlier in the presence of seeds, which

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agrees with previous studies.29 The RIs decrease as time goes on, which is the same

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under both non-seeded and seeded conditions. In addition, at the same reaction time,

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the RIs are smaller for seeded experiments compared with non-seeded experiments. It

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should be noted that the RI data in the companion paper28 are from the period when

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RIs are nearly stable, in this case after 200 min.

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These findings may be due to the following reasons. Particle nucleation occurs in

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the absence of seed, and according to the Köhler theory,36 the gaseous components for

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nucleation need relative lower volatility than heterogeneous condensation on the

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surface of seeds. Under non-seeded condition, longer gas-phase oxidation period is

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needed to form lower volatile organic compounds that nucleate to form new particles,

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so the particles are formed later. Previous studies show that the presence of

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pre-existing seeds promotes the condensation of organic vapors.29-30 Therefore, at the

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same reaction time, the particle diameter under the seeded condition is throughout

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larger than the diameters under the non-seeded condition and the aerosol mass is also

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larger (see the color map in Fig. 2). Hence, products with higher volatility could

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condense on the particles under the seeded condition. Likewise, as reaction goes on,

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total organic concentration increases and particles getting larger, so more products

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with higher volatility will condense on the particles. The higher volatility compounds

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possibly have lower RIs, which lead to the lower bulk RIs of SOA under the seeded

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condition, and the decrease of RIs as diameter increases. In addition to the reason

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above, the ongoing oxidation and aging process may change the chemical

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composition of the particles as well, which is another possible explanation for the

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decreased RI with increasing photochemical time.

Non-seeded 1# Non-seeded 2# Seeded 1# Seeded 2# Non-seeded fit Seeded fit

1.60

Real part of SOA RI

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The Journal of Physical Chemistry

1.55

1.50

Dsm (nm) 420

400.0

280

280.0

140

160.0

1.45

1.40 100

150

200

250

300

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t (min)

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Figure 2 Refractive indices (RIs) of SOA under non-seeded and seeded conditions. The X-axis is

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the time after lights were turned on. Particle surface mean diameters (Dsm) are in the range of

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140-420 nm and are shown in the color map.

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To verify the hypothesis above, the SOA particles are collected by filters and are

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analyzed by ultra-high-resolution electrospray ionization time-of-flight mass

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spectrometry (UHR-ESI-TOF-MS). The particles are collected at two different times

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for both seed conditions: the first one is when SOA RIs are just detectable and

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particles are relatively small (~110 min for seeded experiments; ~170 min for

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non-seeded experiments); another one is at the end of an experiment when the

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particles stop growth (~300 min). The mass spectra of these four SOAs are shown in

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Fig. 3. In general, the mass spectra are similar at different sizes, no matter for

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non-seeded or seeded experiments. However, if carefully analyzing the mass spectra

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(Fig. S3), one will find that the bigger particles have more abundant species with

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lower molecular weight (mostly < 200 Da). In addition, the mass spectra are very

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different between non-seeded and seeded experiments, especially for the products

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with higher molecular weight (> 400 Da). The non-seeded SOA contains more species

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with molecular weight of > 400 Da, which are likely the highly oxygenated

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multifunctional compounds (HOMs) formed during new particle formation.37-38

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Though HOMs have been observed mainly from the oxidation of biogenic VOCs,37-40

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recent studies show that they could also be formed from aromatics.41-42 It is noticed

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from Fig. 2 and Fig. S1 that the particles grow about 1 h later under non-seeded

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conditions, during when the HOMs may be generated by multi-step gas phase OH

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oxidation (i.e. autoxidation).41, 43 For seeded experiments, the large molecular weight

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products of > 400 Da are much less than those of non-seeded experiments, even

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negligible. The reason may be that the semi- or low volatile compounds mostly

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condensed into particle phase in the presence of seeds before they could be oxidized

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to HOMs in the gas phase.

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221 222

Figure 3 Mass spectra of SOA under non-seeded and seeded conditions with different diameters. a.

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the non-seeded condition, of which the black and red sticks are at the onset of particles and the

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end of the experiments, namely ~120 nm and ~250 nm. b. the seeded condition, of which the

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The Journal of Physical Chemistry

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black and red sticks are at the onset of particles and the end of the experiments, namely ~150 nm

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and ~390 nm. The enlarged view of this figure is shown in Fig. S3.

227 228

To clearly see the change of chemical compositions when particles grow or when

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with pre-existing seeds, the subtraction plots of mass spectra are shown in Fig. 4.

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These three MS are the differences between large particles and small particles under

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non-seeded condition (a); the differences between large particles and small particles

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under seeded condition (b); and the differences between seeded and non-seeded

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particles at the end of the experiments (c). As discussed above, seeded SOA has more

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low molecular weight products and less high molecular weight products (Fig. 4c). It is

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also found that the most obvious difference when particles grow is the molecules of

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