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Jan 31, 2018 - nitrate radical (NO3) leads to the formation of secondary organic aerosol (SOA) in the atmosphere. This SOA contributes to the radiatio...
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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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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 (