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Effect of oxidation process on complex refractive index of secondary organic aerosol generated from isoprene Tomoki Nakayama, Kei Sato, Takashi Imamura, and Yutaka Matsumi Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05852 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 6, 2018
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Environmental Science & Technology
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Effect of oxidation process on complex refractive
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index of secondary organic aerosol generated from
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isoprene
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Tomoki Nakayama*,†, Kei Sato‡, Takashi Imamura‡, Yutaka Matsumi†
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†
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Nagoya 464-8601, Japan
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‡
Institute for Space-Earth Environmental Research, Nagoya University, Furo-cho, Chikusa-ku,
National Institute for Environmental Studies, 16-2, Onogawa, Tsukuba 305-8506, Japan
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Abstract. Oxidation of isoprene by hydroxyl radical (OH), ozone (O3), or nitrate radical
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(NO3) leads to formation of secondary organic aerosol (SOA) in the atmosphere. This SOA
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contributes to radiation balance by scattering and absorbing solar radiation. In this study, the
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effect of oxidation process on the wavelength-dependent complex refractive index (RI) of
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SOA generated from isoprene was examined. Oxidation conditions did not have a large effect
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on magnitude and wavelength dependence of the real part of the RI. In the case of SOA
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generated in the presence of sulfur dioxide (SO2), significant light absorption at short visible
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and ultraviolet wavelengths with the imaginary part of the RI, up to 0.011 at 375 nm, was
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observed during oxidation with OH. However, smaller and negligible values were observed
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during oxidation with O3 and NO3, respectively. Moreover, in the absence of SO2, light
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absorption was not observed regardless of the oxidation process. There was an empirical
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correlation between the imaginary part of the RI and the average degree of unsaturation of
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organic molecules. The results obtained herein demonstrate that oxidation processes should be 1 ACS Paragon Plus Environment
Environmental Science & Technology
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considered for estimating the radiative effect of isoprene-derived SOA.
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1. Introduction
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Formation of secondary organic aerosol (SOA) during the oxidation of isoprene is
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considered to be an important source of fine particles in the atmosphere. Current
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observational studies have suggested that SOA generated from isoprene (hereafter referred to
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as isoprene-SOA) accounts for a large fraction (to up ∼40%) of total organic aerosols,
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especially in or around temperate and tropical forests during summer.1−3 If mixing ratios of
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hydroxyl radical (OH), ozone (O3), and nitrate radical (NO3) are assumed to be 0.06 pptv, 30
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ppbv, and 20 pptv, respectively, lifetimes of isoprene for reactions with OH, O3, and NO3 are
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estimated to be 1.7 h, 1.3 day, and 0.8 h, respectively.4 Thus, in the troposphere, isoprene is
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expected to be predominantly oxidized by OH during day and NO3 during night; however, the
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reaction with O3 possibly contributes to oxidation under specific conditions such as under dry
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conditions or with low concentrations of nitrogen oxides (NOx, i.e., NO and NO2).
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Meanwhile, a number of oxidation products are generated from these oxidation
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processes, some of which contribute to SOA formation.5,6 The yield of the isoprene-SOA
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depends on the oxidation process. In addition, recent studies have suggested that acidic sulfate
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seeds lead to a high yield for isoprene-SOA.7−9
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During the photooxidation (OH oxidation) of isoprene under low NOx conditions, the
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SOA mass yield increases from 1.3% to 28.6% in the presence of acidic seed particles, which
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is partially related to the enhanced reactive uptake of oxidation products such as isoprene
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epoxydiols (IEPOX) generated from isoprene hydroxy hydroperoxides (ISOPOOH).8
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Meanwhile, during the photooxidation of isoprene under high NOx conditions,
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hydroxymethyl-methyl-α-lactone
(HMML),
which
is
generated
from 2
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Environmental Science & Technology
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methacryloylperoxynitrate (MPAN), is considered to be an important intermediate during the
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formation of SOA.10 On the other hand, the formation of IEPOX under high NOx conditions
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has also been reported recently.11 Hydroxyhydoperoxides, C5-alkene triols, 2-methyl tetrols,
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acids, oligomers, organosulfates, and hydroxynitrate have been reported as the major products
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of isoprene. The yield of each compound greatly depends on experimental conditions,
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including seed particle type, NOx level, and relative humidity (RH).5,8, 12−16 For example, a
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number of organosulfate species and their total fraction are considered to increase with the
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generation of SOA in the presence acidic sulfate particles (or SO2).8,12,15
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During the ozonolysis of isoprene in the presence of OH scavenger, stabilized
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Criegee intermediates (sCIs) are generated, which can contribute to SOA formation through
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oligomerization reactions with other oxidation products such as carboxylic acids.17−19 A low
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mass yield for SOA (
