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Evolution of the complex refractive index of secondary organic aerosols during atmospheric aging Quanfu He, Nir Bluvshtein, Lior Segev, Daphne Meidan, J. Michel Flores, Steven S. Brown, William H. Brune, and Yinon Rudich Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05742 • Publication Date (Web): 20 Feb 2018 Downloaded from http://pubs.acs.org on February 20, 2018
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Evolution of the complex refractive index of secondary organic aerosols during
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atmospheric aging
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Quanfu He†, Nir Bluvshtein†, Lior Segev†, Daphne Meidan†, J. Michel Flores†, Steven S. Brown‡,§, William Brune∥, and Yinon Rudich†* †
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Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot 76100, Israel Cooperative Institute for Research in Environmental Sciences, University of Colorado, 216 UCB, Boulder, CO 80309, USA § Chemical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, 325 Broadway, Boulder, CO 80305, USA ∥ Department of Meteorology and Atmospheric Science, The Pennsylvania State University, University Park, PA 16802-5013
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Abstract
‡
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The wavelength-dependence of the complex refractive indices (RI) in the visible spectral range of secondary
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organic aerosols (SOA) are rarely studied, and the evolution of the RI with atmospheric aging is largely unknown. In
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this study, we applied a novel white light-broadband cavity enhanced spectroscopy to measure the changes in the RI
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(400–650 nm) of β−pinene and p−xylene SOA produced and aged in an oxidation flow reactor, simulating daytime
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aging under NOx-free conditions. It was found that these SOA are not absorbing in the visible range, and that the
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real part of the RI, n, shows a slight spectral dependence in the visible range. With increased OH exposure, n first
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increased and then decreased, possibly due to an increase in aerosol density and chemical mean polarizability for
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SOA produced at low OH exposures, and a decrease in chemical mean polarizability for SOA produced at high OH
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exposures, respectively. A simple radiative forcing calculation suggests that atmospheric aging can introduce more
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than 40% uncertainty due to the changes in the RI for aged SOA.
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1 Introduction
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Atmospheric aerosols strongly influence Earth’s energy budget by aerosol-radiation interactions and aerosol-
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cloud interactions.1 A large fraction of the submicron aerosols in the troposphere consists of organic aerosol
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components, of which a large portion is secondary organic aerosols (SOA) originated from biogenic and
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anthropogenic precursors.2-5 However, the optical properties of SOA are still poorly constrained.6,7 The optical
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properties of SOA have been studied for various precursor volatile organic compounds (VOCs).8-13 Besides
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precursor type and oxidation environment, the chemical composition of SOA changes with oxidation time. At the
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early stage of atmospheric oxidation, the addition of oxygen-containing functional groups (functionalization) and
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autoxidation dominate SOA formation. As oxidation proceeds, cleavage of C–C bonds (fragmentation) becomes
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more important.14-16 During this aging process, the chemical composition (e.g. the ratios of oxygen, carbon, and
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hydrogen atoms) of SOA changes significantly, potentially leading to changes in the aerosol optical properties.
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As the only intrinsic optical property of a particle, the complex refractive index (RI; m = n + ik) is fundamental to
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aerosol optical parameters such as the single scattering albedo and asymmetry parameter which are employed in
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radiative transfer estimation. Knowing the value of RI is critical in modeling the optical properties of aerosols and
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predicting radiative forcing on regional and global scales. The real (n) and imaginary (k) parts of the complex RI are
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indicative of scattering and absorption, respectively, which are determined by the particles’ chemical composition
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and mixing state. Previous studies investigated the relationship between the complex RI and particle growth (as a
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proxy for oxidation time) over several hours.17-19 The authors found that the real part of RI of SOA from photo-
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oxidation of limonene and α-pinene increased as particles grew up to ∼300 nm; and with additional particle growth,
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the RI either remained constant or decreased. Cappa et al. observed an increase in the RI of SOA produced by
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squalane and azelaic acid following heterogeneous OH oxidation.20 A more recent study by Flores et al. showed that
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for SOA from a mixture of α-pinene, limonene, and isotopically-labeled p-xylene, the RI increased as the SOA aged
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for 29 h (including 14.5 h dark condition) in an outdoor atmospheric simulation chamber.21 Lambe et al. measured
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the RI for SOA generated from naphthalene, guaiacol, and α-pinene aged under a wide range of OH radical
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oxidation in an oxidation flow reactor (OFR).22 They found that the real part of RI decreased as the OH exposure
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increased for naphthalene and α-pinene SOA, while the real part of RI for guaiacol SOA increased and then
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decreased with increasing OH exposure. Excluding experiments by Lambe et al., all measurements in the other three
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studies were performed either at the initial aerosol growth stage or for slightly aged aerosols. From these, we
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conclude that studies that explore a wide range of oxidation times for different types of SOA are needed for better
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understanding the aging effects on the complex refractive index. However, it is difficult to conduct long time
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oxidation experiments in atmospheric simulation chambers. Recently, the Potential Aerosol Mass (PAM) OFR has
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been used for simulating ambient atmospheric aging. Detailed studies have corroborated its ability to simulate
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atmospheric aging.23-26 The OFR enables the production of aerosols that have experienced a wide range of OH
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radical exposures from days to weeks of equivalent atmospheric OH and to directly explore the effects of aging on
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the SOA complex refractive index.
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Conventional techniques to measure the wavelength-dependent optical properties are typically based on offline
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methods, including median-based collection or solvent extraction.27-30 Several online methods have been developed
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by using multiple laser wavelengths and channels, in which the aerosol flow is diverted to different cells31-34 or by
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switching the light source to perform measurements at different single wavelengths in the same aerosol chamber.35-37
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These online methods measure at only several single, discrete wavelengths. An example for a broadband instrument
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is the Aerosol Extinction Differential Optical Absorption Spectrometer (AE-DOAS) that combines a white-type
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multi-pass gas cell with a grating and a diode array detector that measures aerosol extinction from 235 to 700 nm.38
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The average detection limit for the AE-DOAS is 32.5 Mm−1. Therefore, an instrument with a higher sensitivity and
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accuracy is needed to characterize spectrally resolved aerosol extinction in the visible over a range more
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representative of atmospheric values. Recent advances in cavity enhanced spectroscopy, which combines a high-
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intensity, broadband light source and a multichannel detector coupled with a spectrometer allows sensitive
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measurements of optical properties as a function of wavelength.39 Initial cavity measurements have reported aerosol
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extinction in the wavelength range of 315–345 nm,40,41 360–390 and 385–420 nm,42 415–485 nm,43 and 655–690
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nm.44 As of now, there is no instrument that covers the entire visible spectrum.
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In this study, we employ an instrument that extends the ability of current broadband cavity enhanced
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spectroscopy (BBCES) to measure aerosol extinction as a function of wavelength. Through size selection, we
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measure the extinction cross section for several particle sizes which enables retrieval of RI as a function of
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wavelength. This technique is verified by measuring the Rayleigh scattering cross section of CO2 and by retrieving
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complex RI for particles that are purely scattering (polystyrene latex spheres), slightly absorbing (Suwannee River
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fulvic acid), and highly absorbing (nigrosin dye). The new technique is employed to determine the RI for β−pinene
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and p−xylene SOA produced under various OH exposures, in order to investigate the effect of aging on the complex
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refractive index of model SOA from biogenic and anthropogenic origins.
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2 Experimental
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2.1 The new 400−650 nm cavity enhanced spectrometer channel (BBCESVis)
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The BBCES system consists of two channels encompassing different wavelength regions. One of the channels
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measures aerosol extinction at 385−425 nm (BBCES405) and has been described by Washenfelder et al..42 The other
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channel measures aerosol extinction at 400–650 nm (BBCESVis) and is described below. The optical system, aerosol
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generations, and data analysis are described in detail below and presented schematically in Fig. S1.
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A laser-driven white light source (LDLS EQ−99CAL, Energetiq Technology, Inc., MA, USA) with output spectra
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from 170 to 2100 nm is employed in this system. Light from the lamp first passes through a filter (10CGA−295,
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Newport Corporation, CA, USA) that removes light with wavelengths shorter than 294 nm. A dichroic beamsplitter
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(400 nm, Dichroic long pass filter, Edmund Optics Inc., NJ, USA) is employed to reflect 290–400 nm light which
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can be used in the second UV cavity. Light with wavelength longer than 400 nm is optically filtered using bandpass
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filters (MLD Technologies, Inc., CA, USA) and reflected into a second channel (BBCESVis). To maintain a stable
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optical power output, the lamp’s temperature is controlled by an aluminum block constantly cooled by 20℃
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circulating water, while the lamp house is purged with 2 L min−1 ultra-pure nitrogen gas to reduce ozone formation.
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The light is coupled into a cavity consisting of two 2.5 cm, 1 m radius of curvature mirrors (MLD Technologies,
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Inc., CA, USA). The manufacturer’s reported mirror total loss varies from 140 ppm to 910 ppm within the
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wavelength range of 400–700 nm. The size of the light beam at the input mirror is adjusted by an iris. Following the
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cavity, the light is directly collected using a 0.1 cm F/2 fiber collimator (74−UV, Ocean Optics, Dunedin, FL, USA)
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into one lead of a two-way 100 mm core HOH−UV−VIS fiber (SR−OPT−8015, Andor Technology, Belfast, UK)
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that is linearly aligned along the input slit of the grating spectrometer. Spectra were acquired using a 163 mm focal
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length Czerny−Turner spectrometer (Shamrock SR−163, Andor Technology, Belfast, UK) equipped with a charge-
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coupled device (CCD) detector (DU920P−BU, Andor Technology, Belfast, UK) maintained at –50℃. Details about
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the spectrometer and CCD were given in Flores et al..45 In this study, the grating was rotated to acquire spectra over
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the region of 292–807 nm with a resolution of 0.5 nm, although only a portion of that spectral region was used in
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these experiments.
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2.2 Instrument validation
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The new BBCESvis channel was thoroughly validated by determining the Rayleigh scattering cross section of CO2,
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and by measuring the optical properties of several aerosol standards such as polystyrene latex spheres, Suwannee
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River fulvic acid, and nigrosin that serve as proxies for pure scattering, slightly absorbing, and highly absorbing
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aerosols, respectively. The details of these validation experiments are provided in the Supporting Information (SI)
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accompanying this paper.
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2.3 SOA generation by the OFR
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SOA particles were generated in the OFR by homogeneous nucleation and condensation following OH oxidation
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of β−pinene and p−xylene. The SOA was directed to the BBCESVis for extinction measurement. The OFR consists
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of a horizontal 13.3 L aluminum cylindrical chamber,23,46 displayed in Fig. S1a. Details of the SOA production
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conditions are listed in Table 1, including initial O3 concentration, the relative humidity (RH), and related water
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vapor mixing ratio, and precursor VOCs mixing ratios.
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Ozone is generated by irradiating high purity O2 with a mercury lamp (78−2046−07, BHK Inc., CA, USA)
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outside the reactor. Inside the OFR, O(1D) radicals are generated by UV photolysis of O3 using two mercury lamps
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(82−934−08, BHK Inc., CA, USA) with peak emission at λ= 254 nm. These lamps are mounted in Teflon-coated
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quartz cylindrical sleeves and continually purged with pure nitrogen. Water vapor is introduced into the reactor
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using a temperature-controlled Nafion membrane humidifier (Perma Pure LIC, NJ, USA). Dry carrier gas of N2 was
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mixed with a wet carrier gas (humidified N2) to provide a controllable RH in the reactor. A total flow of 6.3 Lmin−1
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of N2 and 0.3 L min−1 O2 with RH of 36−38% was used. The temperature inside the reactor was 22.5±0.3 ℃.
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OH radicals are produced via the mechanism as follows:
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O + hν → O + O D λ = 254 nm
(1)
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O D + H O → 2OH
(2)
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The OH concentration was varied by changing the UV light intensity. Equations for estimating exposures
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(products of the oxidants’ concentration and the average residence time) from OH, non-OH radicals, and photolysis
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were obtained by fitting box-modeled exposure data against the input and output O3 concentration, water mixing
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ratios, and external OH reactivity,24,47,48 and are presented in Table 1. Suppression by reaction with β-pinene or p-
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xylene was included in OH exposure (OHexp) estimation. During the experiments, OHexp ranged from 6.48×1010 to
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1.73×1012 molecules cm−3 s, which is approximately 0.5−13.3 days of equivalent atmospheric exposure at a typical
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ambient OH radical concentration of 1.5×106 molecules cm−3.49 The ratios of O3 exposure/OH exposure, 254 nm
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photon flux exposure /OH exposure, O(1D) exposure /OH exposure, and O(3P) exposure /OH exposure are smaller
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than 7.9×105, 1.4×106, 5.8×10−6, 4.9×10-2, respectively, indicating that the oxidation in the OFR is dominated by OH
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radical as shown by Peng et al..24
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2.4 Measurement of SOA chemical composition and density
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A high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS, Aerodyne Research Inc., Billerica,
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MA, USA) operated alternatingly between the high-sensitivity V mode and the high-resolution W mode, was used to
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measure the chemical compositions of SOA.50 The toolkit Pika 1.16I was used for determining the chemical
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compositions of the SOA.51,52 Correction for the elemental ratios was achieved by the Improved-Ambient method.52
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Assuming that the SOA particles are spherical and non-porous, the effective density (ρeff) was calculated from Eq. (3)
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by comparing the vacuum aerodynamic diameter (dva, obtained from the HR-ToF-AMS) and the mode mobility
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diameter (dm, size measured by the SMPS):53
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=
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where ρ0 is the standard density (1 g cm-3).
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2.5 Refractive index retrieval for aerosols
! " #
(3)
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The refractive index retrieval method has been described previously.42,54-57 The complex RI of the aerosols can be
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retrieved by measuring several particle diameters, assuming similar composition for each selected diameter, and by
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fitting a theoretical Mie curve to the measured extinction cross sections at each specific wavelength.42,56 The general
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expression that relates the aerosol optical cross section, σ%&' (, )* , +, is determined by Eq. (4):
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σ%&' (, )* , + =
,-./ 0,12 ,3
(4)
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Dp is the particle mode diameter, m is the complex refractive index, and N(Dp) is the particle number
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concentration (cm−3). To measure the optical cross section of the size-selected particles, aerosol from the OFR was
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sampled, passed through an ozone scrubber and a diffusion dryer (RH< 20%), size selected with a DMA (between
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175 and 400 nm in 25 nm steps), and directed into the BBCES, while counted by the CPC. The inlet of the BBCES
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is located at the center of the cavity with two outlets at both ends.40 This configuration significantly reduces the
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uncertainty associated with the ratio of cavity length to aerosol-filled length. Optical measurement for each size of
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particle is an average of 105 spectra integrated for 95 s. The measured extinction cross sections were corrected for
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multiple-charged particles. To retrieve the real and imaginary components of the RI, the retrieval algorithm was
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limited to searching for n ≥ 1 and k ≥ 0, their physical boundaries.
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3 Results and Discussion
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3.1 Chemical composition of SOA from β−pinene and p−xylene
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In Fig. S8 we present AMS mass spectra of β−pinene (A1-A3) and p−xylene (B1-B3) derived SOA which
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experienced low, midrange, and high OH exposures. The contributions of CxHy+, CxHyO+, CxHyOz+ groups for SOA
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generated under a variety of OH exposures are shown in Fig. 1. A detailed description of the spectra can be found in
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the SI. Like α−pinene SOA, ions indicative of cycloalkyl fragments (e.g., m/z 27, C2H3+; m/z 41, C3H5+; m/z 55,
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C3H3O+) are present in β−pinene SOA.22 AMS spectra of p−xylene SOA showed significant intensity at m/z 50−51
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(C4H2+, C4H3+), m/z 65 (C5H5+), and m/z 77 (C6H5+), which are characteristics of aromatic compounds.22 At low-OH
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exposures, the mass spectra reflect more of the VOC precursors, and the detected prominent CxHy+ ion signal (an
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indication of hydrocarbon-like organic aerosol, HOA) and CxHyO+ ion signals (an indication of carbonyl compounds)
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provides a proof of the chemical nature of early-generation oxidation products. At higher OH exposures, the SOA
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mass spectra exhibit stronger CxHyOz+ ion signals (an indication of organic acids) and decreased CxHy+ signal,
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indicating the chemical nature of higher-generations oxidation products.58 Additionally, the correlations between the
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obtained SOA mass spectra in this study and unit mass resolution (UMR) spectra of reference ambient HOA,
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semivolatile oxygenated organic aerosol (SV-OOA), and low-volatility oxygenated organic aerosol (LV-OOA)59
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were explored (Fig. S9). As OH exposure increased, weaker correlations between the SOA mass spectra and those
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of HOA and SV-OOA were observed. Meanwhile, the correlations between the spectra of SOA and reference LV-
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OOA became stronger. These indicated increased contribution of LV-OOA components and decreased contributions
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of HOA and SV-OOA components to the generated SOA as OH exposure increased.
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From the AMS spectra, the H/C and O/C ratios are extracted and plotted in a Van Krevelen diagram for SOA
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produced from β−pinene and p−xylene (Fig. 2). The O/C ratio increases with OH exposure (0.5−13.3 days),
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indicating the addition of oxygen-containing functional groups to the carbon skeleton. H/C ratio of β−pinene SOA is
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almost constant (1.60 vs 1.61) when the OH exposure increases from 6.1 ×1010 to 2.0 ×1011 molecules cm−3 s. These
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ratios are similar to the H/C ratio of β−pinene (C10H16, H/C= 1.60), indicating an initial OH addition to the carbon
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skeleton, and/or growth by condensation of semi-volatile oxidized products (e.g., hydroperoxides, acetals,
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hemiacetals) which maintains the H/C ratio.60 As the OH exposure continuously increases (1.5−12.4 days), the
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changes in H/C and O/C ratios of the SOA are well described by linear fits (R2= 0.99) with △(H/C)/△(O/C) slope of
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−0.51. The H/C ratio of p−xylene SOA increased from 1.55 to 1.57, followed by a decrease to 1.46. The O/C ratio
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increases from 0.72 to 0.91 as OH exposure increased. With decreasing H/C ratio, the △(H/C)/△(O/C) exhibits a
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slope of −0.57. The H/C ratio of p−xylene SOA is higher than that of p−xylene (C8H10, H/C= 1.25), indicating
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hydrogen content addition to the precursor.61 The Van Krevelen slopes of aged p−xylene SOA and β−pinene SOA
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are consistent with hydrogen abstraction followed by addition of oxygen-containing functional groups and
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fragmentation reactions that result in carbon and hydrogen loss from the SOA.15,62-64
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3.2 Complex refractive index for β−pinene and p−xylene SOA
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For all of the SOA RI retrievals in this study, the imaginary part was essentially 07#.# #.## at all wavelengths.
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Previous studies also determined near-zero imaginary part of the RI of SOA produced by photo-oxidation from
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α−pinene, α-pinene+limonene mixtures, and m−xylene under NOx free condition for the atmospherically relevant
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wavelength region (λ> 300 nm).10,21,22,30,65 Our result is hence consistent with previous measurements and suggests
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that the fresh and aged SOA generated under NOx free condition is non-absorbing at wavelength range of 400-650
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nm. Thus, only the real part of the refractive index as a function of wavelength between 400−650 nm for the SOA is
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shown in Fig. 3. The plotted real part of RI is averaged every 2 nm and their individual errors are not shown.
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The real part of RI for β−pinene and p−xylene SOA exhibits a slight spectral dependence with n values that
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decrease with increasing wavelength. For example, n value of β−pinene SOA produced under 0.5 days aging varies
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from n = 1.569 (±0.003) at 400.5 nm to n = 1.538 (±0.004) at 650 nm, with △n= 0.031 (±0.007). While OH
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exposure increased, the △n is 0.044 (±0.004), 0.053 (±0.003), 0.051 (±0.004) for SOA produced at OH exposures
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equivalent to 1.5, 2.6, and 6.8 days, respectively. For highly aged β−pinene SOA, △n is 0.023 (±0.004) and 0.029
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(±0.005) for OH exposure of 8.0 and 12.4 days, indicating a smaller spectral dependence. The same trend is also
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observed for p−xylene SOA, larger △n (0.029−0.065) at lower OH exposures (< 8 days) and relative smaller △n
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(0.014-0.020) for highly aged SOA (>8 days).
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3.3 Comparison of the complex refractive index for β−pinene and p−xylene SOA
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Only a few laboratory studies have determined the refractive indices of SOA generated from β−pinene and
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xylene.8,10,44,66 Liu et al. retrieved the refractive index of SOA produced via photo-oxidation of m-xylene and NOx
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mixtures at different hydrocarbon to NOx (HC/NOx) ratios using spectroscopic ellipsometry.10 At 405 nm, the
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measured values of the real part increased from n = 1.554 (±0.011) at HC/NOx = 0 to n = 1.589 (±0.011) at HC/NOx
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= 4.0. The imaginary components they found in this HC/NOx range were below 1.5 ×10−3. Li et al. determined the
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real part of the complex RI for photo-oxidation generated m-xylene SOA under various NOx levels at 532 nm to be
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1.39−1.56.66 In this study, the retrieved real part of the RI for p−xylene SOA is within the range of 1.465
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(±0.003)−1.594 (±0.006) at 405 nm and 1.461 (±0.004)−1.565 (±0.007) at 532 nm under various OH exposures and
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at NOx free conditions. Kim et al. studied the optical properties of β−pinene SOA generated by ozonolysis without
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OH radical scavenger and photo-oxidation of β−pinene + NOx mixtures by polar nephelometry.8 They found real
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part of the RI ranges between 1.43 and 1.48 for SOA formed by ozonolysis and 1.38−1.52 for SOA produced from
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photo-oxidation at 670 nm. In our study, the working wavelength of BBCESVis for SOA experiment is limited to 650
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nm. The real part of RI retrieved at 650 nm is between 1.452 (±0.003) and 1.546 (±0.003). Our measurements agree
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well with these literature data. Varma et al. studied NO3-initiated oxidation of β−pinene under dry conditions using
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three broadband cavity spectrometers at the SAPHIR atmospheric simulation chamber.44 They measured a real part
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of refractive index n = 1.61 (±0.03) at 655-687 nm, assuming no absorption (k=0). The RI of β−pinene SOA
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generated by OH oxidation in this study is lower than that of β−pinene SOA from NO3 radical oxidation observed by
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Varma et al..44 The higher n value has been attributed to the high proportion (up to 45%) of organic nitrates in the
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particle phase.
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3.4 Relationship between oxidation level and complex refractive index
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Oxidative aging changes the SOA chemical composition, thus the RI changes due to the alteration of the SOA
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oxidation state, density, mean molecular weight, and mean polarizability.15,20,67-69 O/C and H/C ratios are frequently
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used as indications of the oxidation state of the aerosol. Lambe et al. measured the real part of RI of SOA from OH
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oxidation of a biogenic precursor (α−pinene) and anthropogenic precursors (naphthalene, tricyclo[5.2.1.0 ]decane)
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at different oxidation levels.22 They observed that with increasing OH exposure, the O/C ratio increased from 0.42 to
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0.93 and from 0.52 to 1.29 for the α-pinene and naphthalene, respectively, with a corresponding decrease in the real
2,6
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part from 1.51 (±0.02) to 1.45 (±0.04) for α-pinene SOA and from 1.66 (±0.04) to 1.58 (±0.06) for naphthalene SOA
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at 405 nm. In the study conducted by Flores et al., the real part of the complex RI of SOA mixtures from biogenic
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VOCs (BVOCs) and anthropogenic VOCs increased from 1.45 (±0.01) to 1.49 (±0.02) at λ = 420 nm and from 1.49
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(±0.01) to 1.54 (±0.01) at λ = 360 nm under 1.5 to 25.2 h of ageing, as O/C ratio increased from 0.35 to 0.42.21 They
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also observed RI increase at λ>390 nm during 29 h aging for BVOCs particle and p-xylene mixtures when the O/C
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ratio increased from 0.39 to 0.44. Other studies also observed an increasing RI with oxidation for SOA produced via
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heterogeneous OH oxidation of squalane at O/C between 0 and 0.35 20 and SOA generated from the photooxidation
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of toluene with O/C ratios in the range of 0.64−0.75. 70
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Fig. 4 shows the change in the retrieved real part of the complex RI at wavelengths between 400−650 nm as a
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function of OH exposure (Fig. 4 A1, B1), the H/C ratio (Fig. 4 A2, B2) and O/C ratio (Fig. 4 A3, B3). For β−pinene
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SOA, the value of n first slightly increased and then decreased with OH exposure. For less aged SOA, the value of n
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increased as O/C increased from 0.38 to 0.41. For O/C ratio from 0.41 to 0.63, the n value of β−pinene SOA
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decreased. The real part of RI for p−xylene SOA as a function of O/C ratio shows a similar trend to β−pinene SOA.
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The value of n first increased as for O/C between 0.72 and 0.75 and then decreased as O/C ratio increased to 0.91.
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However, in the H/C space, the real part of the complex RI apparently decreased as H/C decreased for both
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β−pinene and p−xylene SOA. Our results are consistent with previous findings19 that the real part of the complex RI
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positively correlated with the early-generation oxidation products markers (p0.5), including the
262
hydrocarbon fragments (e.g. C4H7+, C3H7+, C4H9+, C3H5+) and oxygen-containing fragments (e.g. C2H3O+, C3H3O+,
263
C3H5O+).
264
The presence of oligomers has been observed during SOA formation.71-74 Increasing H/C ratio is consistent with
265
oligomerization process as many oligomers are formed through C-C coupling reactions that release oxygen.75 On the
266
other hand, the predicted refractive indices (n = 1.55–1.60) for these oligomeric compounds are in the upper end of
267
the retrieved values, owing to their higher polarizability.11 Thus, the observed initial increases in H/C ratio and n
268
observed in this study are consistent with oligomerization at low OH exposures. However, the short average
269
residence time of gas-phase species and particles in the OFR (~ 100 s) may limit the slow oligomerization process in
270
SOA formation. During the initial growth stage, the positive slope between H/C and O/C (Fig. 2) indicated SOA
271
formation by functionalization via the addition of OH/OOH functional groups. These less oxygenated semi-volatile
272
species formed by functionalization process consist of a large fraction of hydrocarbon-like substances (confirmed by
273
the abundant of CxHy+ and CxHyO+ fragments in the AMS spectra), that can condense onto the particles and increase
274
the RI.17,19 With increasing OH exposure, the slope between H/C and O/C is ~−0.5, suggesting increased
275
contributions from alcohols, peroxides, and/or carboxylic acids formed by fragmentation, with a corresponding
276
decreased contribution of the less-oxygenated components. Thus, we observe a decrease in RI at elevated OH
277
exposure.
278 279 280
The Lorentz−Lorenz relationship relates the refractive index to the mean polarizability (α), particle density (ρ), and molecular weight (MW)): 89 : 89 7
=
,;
(5)
