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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
17
The formation of secondary organic aerosol (SOA) contains partitioning processes of
18
the oxidation products between gas and particle phase, which could change the
19
particle-phase composition when particles grow. However, the effects of these
20
processes on the optical properties of SOA remain poorly understood. In this study,
21
we performed smog chamber experiments to investigate the effects of gas-particle
22
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
24
increase the proportions of semi- and intermediate volatile organic compounds
25
(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
29
the GPP to the particle phase and hence further decreases the RI real part for 0.05 ±
30
0.02. As pre-existing aerosols are ubiquitous in the ambient atmosphere, it is
31
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: ∗
42
= 1 + ; = ∑
43
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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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
148
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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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.
159 160
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
189
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
199
for both seed conditions: the first one is when SOA RIs are just detectable and
200
particles are relatively small (~110 min for seeded experiments; ~170 min for
201
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
203
Fig. 3. In general, the mass spectra are similar at different sizes, no matter for
204
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
207
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
214
conditions, during when the HOMs may be generated by multi-step gas phase OH
215
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
217
negligible. The reason may be that the semi- or low volatile compounds mostly
218
condensed into particle phase in the presence of seeds before they could be oxidized
219
to HOMs in the gas phase.
220
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
224
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
229
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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