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<front>
<journal-meta>
<journal-id journal-id-type="publisher-id">Front. Environ. Chem.</journal-id>
<journal-title>Frontiers in Environmental Chemistry</journal-title>
<abbrev-journal-title abbrev-type="pubmed">Front. Environ. Chem.</abbrev-journal-title>
<issn pub-type="epub">2673-4486</issn>
<publisher>
<publisher-name>Frontiers Media S.A.</publisher-name>
</publisher>
</journal-meta>
<article-meta>
<article-id pub-id-type="publisher-id">732219</article-id>
<article-id pub-id-type="doi">10.3389/fenvc.2021.732219</article-id>
<article-categories>
<subj-group subj-group-type="heading">
<subject>Environmental Chemistry</subject>
<subj-group>
<subject>Original Research</subject>
</subj-group>
</subj-group>
</article-categories>
<title-group>
<article-title>
<italic>In-Situ</italic> FTIR Study of Heterogeneous Oxidation of SOA Tracers by Ozone</article-title>
<alt-title alt-title-type="left-running-head">Wang et&#x20;al.</alt-title>
<alt-title alt-title-type="right-running-head">Heterogeneous Oxidation of Tracer</alt-title>
</title-group>
<contrib-group>
<contrib contrib-type="author">
<name>
<surname>Wang</surname>
<given-names>Runhua</given-names>
</name>
<uri xlink:href="https://loop.frontiersin.org/people/1254479/overview"/>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Huang</surname>
<given-names>Yajuan</given-names>
</name>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Hu</surname>
<given-names>Qian</given-names>
</name>
</contrib>
<contrib contrib-type="author" corresp="yes">
<name>
<surname>Cao</surname>
<given-names>Gang</given-names>
</name>
<xref ref-type="corresp" rid="c001">&#x2a;</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Zhu</surname>
<given-names>Rongshu</given-names>
</name>
</contrib>
</contrib-group>
<aff>Shenzhen Key Laboratory of Organic Pollution Prevention and Control, Harbin Institute of Technology (Shenzhen), <addr-line>Shenzhen</addr-line>, <country>China</country>
</aff>
<author-notes>
<fn fn-type="edited-by">
<p>
<bold>Edited by:</bold> <ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/817932/overview">Fudong Liu</ext-link>, University of Central Florida, United&#x20;States</p>
</fn>
<fn fn-type="edited-by">
<p>
<bold>Reviewed by:</bold> <ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/951041/overview">Giovanni Cagnetta</ext-link>, Tsinghua University, China</p>
<p>
<ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/1437050/overview">Biwu Chu</ext-link>, Research Center for Eco-Environmental Sciences (CAS), China</p>
<p>
<ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/961233/overview">Tu Binh Minh</ext-link>, VNU University of Science, Vietnam</p>
</fn>
<corresp id="c001">&#x2a;Correspondence: Gang Cao, <email>caogang@hit.edu.cn</email>
</corresp>
<fn fn-type="other">
<p>This article was submitted to Organic Pollutants, a section of the journal Frontiers in Environmental Chemistry</p>
</fn>
</author-notes>
<pub-date pub-type="epub">
<day>29</day>
<month>10</month>
<year>2021</year>
</pub-date>
<pub-date pub-type="collection">
<year>2021</year>
</pub-date>
<volume>2</volume>
<elocation-id>732219</elocation-id>
<history>
<date date-type="received">
<day>28</day>
<month>06</month>
<year>2021</year>
</date>
<date date-type="accepted">
<day>14</day>
<month>10</month>
<year>2021</year>
</date>
</history>
<permissions>
<copyright-statement>Copyright &#xa9; 2021 Wang, Huang, Hu, Cao and Zhu.</copyright-statement>
<copyright-year>2021</copyright-year>
<copyright-holder>Wang, Huang, Hu, Cao and Zhu</copyright-holder>
<license xlink:href="http://creativecommons.org/licenses/by/4.0/">
<p>This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these&#x20;terms.</p>
</license>
</permissions>
<abstract>
<p>Secondary organic aerosols (SOA) play an important role in global climate change and air quality, and SOA tracers can directly characterize the source and reaction mechanism of SOA. However, it is not well known that whether the tracers can be oxidized or how the instability of the tracers in the atmosphere. In this paper, <italic>in-situ</italic> FTIR was used to analyze the chemical structure changes of erythritol, analogue of 2-methyl erythritol (AME) that is, a tracer of isoprene SOA, and 2, 3-dihydroxy-4-oxopentanoic acid (DHOPA), a tracer of toluene SOA, when exposed to high concentration of ozone for short periods. Under the condition of 20&#xa0;ppm ozone exposure for 30&#xa0;min, the change rate of absorption area of AME at 3,480 and 1700&#xa0;cm<sup>&#x2212;1</sup> was &#x2212;0.0134 and 0.00117 int.abs/s, respectively, and the change rate of the absorption area of DHOPA at 1,640 and 3340cm<sup>&#x2212;1</sup> was &#x2212;0.00191 and 0.00218 int.abs/s, respectively. The pseudo-first-order reaction rate constant <italic>k</italic>
<sub>
<italic>app</italic>
</sub> were 1.89 &#xd7; 10<sup>&#x2212;8</sup> and 2.12 &#xd7; 10<sup>&#x2212;7</sup>&#xa0;s<sup>&#x2212;1</sup>, and the uptake coefficients of ozone on the surface of AME and DHOPA were (1.3&#x20;&#xb1; 0.8) &#xd7; 10<sup>&#x2212;8</sup> and (4.5&#x20;&#xb1; 2.7) &#xd7; 10<sup>&#x2212;8</sup>, respectively. These results showed the oxidation processes of AME and DHOPA were slow in the presence of high concentrations of ozone, which implied that AME and DHOPA could be considered to be stable in the atmospheric environment with ozone as the main oxidant.</p>
</abstract>
<kwd-group>
<kwd>SOA tracers</kwd>
<kwd>
<italic>in-situ</italic> FTIR</kwd>
<kwd>ozone</kwd>
<kwd>uptake coefficient</kwd>
<kwd>pseudo-first-order reaction 2</kwd>
</kwd-group>
<contract-sponsor id="cn001">Foundation for Innovative Research Groups of the National Natural Science Foundation of China<named-content content-type="fundref-id">10.13039/501100012659</named-content>
</contract-sponsor>
</article-meta>
</front>
<body>
<sec id="s1">
<title>Introduction</title>
<p>Organic tracers have been used widely for source apportionment of organic aerosols under the assumption that they are not reactive in the atmosphere (<xref ref-type="bibr" rid="B20">Katrib et&#x20;al., 2005</xref>; <xref ref-type="bibr" rid="B22">Kleindienst et&#x20;al., 2007</xref>; <xref ref-type="bibr" rid="B3">Ding et&#x20;al., 2012</xref>; <xref ref-type="bibr" rid="B23">Lai et&#x20;al., 2014</xref>). However, previous studies have indicated that some of those tracers may not remain stable, which leads to the inaccurate estimation of source contributions (<xref ref-type="bibr" rid="B1">And and Smith, 2004</xref>; <xref ref-type="bibr" rid="B38">Weitkamp et&#x20;al., 2008a</xref>; <xref ref-type="bibr" rid="B14">Hoffmann et&#x20;al., 2010</xref>; <xref ref-type="bibr" rid="B25">Lambe et&#x20;al., 2012</xref>; <xref ref-type="bibr" rid="B37">Wang et&#x20;al., 2020</xref>). Weitkamp et&#x20;al. studied the ozone oxidation of primary organic tracers of cooking oil emission (oleic acid, palmitoleic acid, and cholesterol etc.) through a series of chamber experiments. The results showed that the rate constant of ozone heterogeneous oxidation of oleic acid was 1.5 &#xd7; 10<sup>&#x2212;11</sup>&#xa0;cm<sup>3</sup> molec<sup>&#x2212;1</sup> sec<sup>&#x2212;1</sup>, which was ten times of cholesterol and four times of palmitoleic acid (<xref ref-type="bibr" rid="B39">Weitkamp et&#x20;al., 2008b</xref>). Lambe et&#x20;al. studied the effective reaction rate of oxidation reaction of norhopane, an organic tracer of motor oil by hydroxyl radical, and the rate constant of norhopane was 8.4 &#xd7; 10<sup>&#x2212;12</sup>&#xa0;cm<sup>3</sup> molec<sup>&#x2212;1</sup> s<sup>&#x2212;1</sup> (<xref ref-type="bibr" rid="B24">Lambe et&#x20;al., 2009</xref>). The study of Hennigan et&#x20;al. indicated that rate constant of levoglucosan was 1.1 &#xd7; 10<sup>&#x2212;11</sup>&#xa0;cm<sup>3</sup> molecule<sup>&#x2212;1</sup>s<sup>&#x2212;1</sup> when biomass burning particles were exposed to 1&#x20;&#xd7; 10<sup>6</sup> molecules&#xa0;cm<sup>&#x2212;3</sup> of OH (<xref ref-type="bibr" rid="B13">Hennigan et&#x20;al., 2010</xref>).</p>
<p>The above studies were focused on the heterogeneous oxidation of primary organic tracers, while such investigations on the secondary organic tracers have seldom been reported. And there is no clear conclusion about the complexity of heterogeneous oxidation of secondary tracers. Kessler et&#x20;al. used the erythritol as a surrogate for 2-methyltetrols and studied heterogeneous oxidation of pure erythritol particles by gas-phase OH radicals with an effective OH uptake coefficient, <italic>&#x3b3;</italic>
<sub>eff</sub>, of 0.77&#x20;&#xb1; 0.1 and a corresponding chemical lifetime of &#x223c;13.8&#x20;&#xb1; 1.4&#xa0;days at a relative humidity (RH) of 30&#x2009;% (<xref ref-type="bibr" rid="B21">Kessler et&#x20;al., 2010</xref>). However, Xu et&#x20;al. investigated the heterogeneous OH oxidation of pure erythritol aerosols that contained erythritol and ammonium sulfate (AS) at different dry inorganic-to-organic mass ratios (IOR) in an aerosol flow tube reactor at a high relative humidity of 85&#x2009;%. Their kinetic data would suggest that 2-methyltetrols in atmospheric particles were likely chemically stable against heterogeneous OH oxidation under humid conditions (<xref ref-type="bibr" rid="B42">Xu et&#x20;al., 2020</xref>).</p>
<p>The laboratory studies on atmospheric heterogeneous oxidation reactions, using Knudsen cell (<xref ref-type="bibr" rid="B34">Seisel et&#x20;al., 2006</xref>; <xref ref-type="bibr" rid="B44">Zhou and Wang, 2014</xref>), flow tube reactor (<xref ref-type="bibr" rid="B28">Leli&#xe8;vre et&#x20;al., 2004</xref>; <xref ref-type="bibr" rid="B21">Kessler et&#x20;al., 2010</xref>), FTIR (<xref ref-type="bibr" rid="B43">Zeng et&#x20;al., 2013</xref>; <xref ref-type="bibr" rid="B10">He et&#x20;al., 2016</xref>), smog chamber (<xref ref-type="bibr" rid="B27">Lee et&#x20;al., 2004</xref>; <xref ref-type="bibr" rid="B9">Hartz et&#x20;al., 2007</xref>; <xref ref-type="bibr" rid="B40">Weitkamp et&#x20;al., 2007</xref>; <xref ref-type="bibr" rid="B7">Ge et&#x20;al., 2016</xref>), have developed rapidly in the determination of kinetic constants of trace gases and adsorption reactions on particle surfaces (<xref ref-type="bibr" rid="B26">Lee and Harris, 2006</xref>; <xref ref-type="bibr" rid="B8">Goldstein et&#x20;al., 2008</xref>; <xref ref-type="bibr" rid="B32">Nieto et&#x20;al., 2008</xref>; <xref ref-type="bibr" rid="B11">He and Zhang, 2019</xref>). In recent years, <italic>in-situ</italic> FTIR technology has been widely used in heterogeneous oxidation studies of various compounds by monitoring functional group transformation as reactions progressed (<xref ref-type="bibr" rid="B43">Zeng et&#x20;al., 2013</xref>; <xref ref-type="bibr" rid="B10">He et&#x20;al., 2016</xref>; <xref ref-type="bibr" rid="B6">Gao et&#x20;al., 2019</xref>). The evolution of the FTIR absorption peak with time is generally used to estimate the uptake coefficient, which refers to the fraction of gaseous oxidants irreversibly reacted in the surface of compounds after collision (<xref ref-type="bibr" rid="B15">Hudson et&#x20;al., 2001</xref>; <xref ref-type="bibr" rid="B35">Smith et&#x20;al., 2002</xref>; <xref ref-type="bibr" rid="B16">Hung et&#x20;al., 2005</xref>). It is an important physicochemical parameter to characterize the heterogeneous reaction in the atmosphere, and an important quantitative index for the surface uptake ability of atmospheric particles (<xref ref-type="bibr" rid="B30">Moise and Rudich, 2002</xref>; <xref ref-type="bibr" rid="B36">Thornberry and Abbatt, 2004</xref>; <xref ref-type="bibr" rid="B45">Ziemann, 2005</xref>; <xref ref-type="bibr" rid="B17">Hung and Tang, 2010</xref>). The heterogeneous reaction rate between gaseous oxides and the condensed compounds can be calculated based on a pseudo-first-order rate equation with the corresponding uptake coefficient (<xref ref-type="bibr" rid="B41">Worsnop et&#x20;al., 2002</xref>). Gao et&#x20;al. studied the heterogeneous reactions of ozone with oleic acid via a flow system combined with ATR-FTIR, and reported the uptake coefficient <italic>&#x3b3;</italic> of ozone on oleic acid was (4.6&#x20;&#xb1; 1.0) &#xd7; 10<sup>&#x2212;4</sup> (<xref ref-type="bibr" rid="B6">Gao et&#x20;al., 2019</xref>). The uptake coefficients of ozone on oleic acid in previous studies ranged from (7.8&#x20;&#xb1; 2) &#xd7; 10<sup>&#x2212;3</sup> to (1.9&#x20;&#xb1; 0.6) &#xd7; 10<sup>&#x2212;5</sup> (<xref ref-type="bibr" rid="B9">Hartz et&#x20;al., 2007</xref>; <xref ref-type="bibr" rid="B12">Hearn and Smith, 2004</xref>; <xref ref-type="bibr" rid="B31">Nash et&#x20;al., 2006</xref>; <xref ref-type="bibr" rid="B35">Smith et&#x20;al., 2002</xref>; <xref ref-type="bibr" rid="B33">Rosen et&#x20;al., 2008</xref>).</p>
<p>In this paper, in order to study the ozone heterogeneous oxidations of Erythritol, Analogue of 2-Methyl Erythritol (AME) and 2, 3-Dihydroxy-4-oxopentanoic Acid (DHOPA) which are secondary organic tracers of the largest natural source of non-methane hydrocarbons (isoprene) and the representative precursor of anthropogenic SOA (toluene) respectively, an <italic>in-situ</italic> FTIR was used to analyze the chemical structure changes of AME and DHOPA, when exposed to high concentrations of ozone for short periods. The uptake coefficients of ozone on the surface of AME and DHOPA were measured and the pseudo-first-order reaction rate constant of AME and DHOPA were estimated.</p>
</sec>
<sec sec-type="materials|methods" id="s2">
<title>Materials and Methods</title>
<sec id="s2-1">
<title>Materials and Instruments</title>
<p>All chemicals used for this study had purity levels greater than 99%. -AME was supplied by Aladdin, and DHOPA was supplied by TRC. Ozone was generated photolytically using a Xonics ozone generator (Jelight Model 600, Irvine, CA) and the ozone concentration was measured with a photometric ozone detector (2BTechnologies Model 106L). The experiment was carried out at room temperature, in that low constant temperature water baths (DHC-0505-A, qiwei), which controlled the temperature of ambient chamber at 25&#xb0;C. Analyses of heterogeneous oxidation of the AME and DHOPA were carried out in an <italic>in-situ</italic> FTIR (Is-50FT-IR, Thermo). The <italic>in-situ</italic> FTIR was composed of a reaction chamber and a Harrick Praying Mantis. The reaction chamber consisted of an ambient chamber and a dome which equipped with two round KBr observation windows. The external facilities of the Harrick Praying Mantis included observation windows, purge door and purge line fitting, as well as two tilted mirrors and four horizontal mirrors in the Praying Mantis.</p>
<p>
<xref ref-type="fig" rid="F1">Figure&#x20;1</xref> showed the experimental setup for measuring the uptake coefficients of ozone on the surface of AME or DHOPA solid powder using <italic>in-situ</italic> FTIR. The general procedures of the experiments were briefly outlined below: Firstly, a transmittance spectrum with the Praying Mantis in the sample compartment was collected when the wavenumber at 2,500&#xa0;cm<sup>&#x2212;1</sup> reached the maximum value. Then, dry air was blown for 5&#xa0;min, and the background was collected. Finally, 1&#xa0;mg AME/DHOPA solid powders were spread out evenly at the bottom of the sample cup of the ambient chamber, and 20&#xa0;ppm ozone was injected for 30&#xa0;min continuously. The characteristic infrared absorption peaks of surface of solid powder during the process of the ozone oxidation of AME or DHOPA were monitored online under the FTIR operational conditions of the infrared resolution of 4&#xa0;cm<sup>&#x2212;1</sup>, the background scanning of 64 times, the series sample scanning of 32 times, the sample interval of 41.27&#xa0;s, and the scanning range of 4,000&#x2013;400&#xa0;cm<sup>&#x2212;1</sup>. After each experiment completed, the residual ozone in the ambient chamber was removed by blowing dry air through the outlet.</p>
<fig id="F1" position="float">
<label>FIGURE 1</label>
<caption>
<p>
<italic>in-situ</italic> FTIR experimental&#x20;setup.</p>
</caption>
<graphic xlink:href="fenvc-02-732219-g001.tif"/>
</fig>
</sec>
<sec id="s2-2">
<title>Calculation of Uptake Coefficient</title>
<p>The method used to measure the uptake coefficients in this work was similar to that used in previous studies on ozone oxidations of oleic acid and linoleic acid (<xref ref-type="bibr" rid="B19">Jaoui et&#x20;al., 2004</xref>; <xref ref-type="bibr" rid="B16">Hung et&#x20;al., 2005</xref>; <xref ref-type="bibr" rid="B5">Engelke et&#x20;al., 2010</xref>). Zeng et&#x20;al. studied the heterogeneous reaction of linoleic acid with ozone. It was confirmed that the rate constant of linoleic acid oxidation was in accordance with the pseudo-first-order reaction rate constant. When the sample was exposed to 250&#xa0;ppb ozone concentration at 30% RH, <italic>k</italic>
<sub>
<italic>app</italic>
</sub> was 5.98 &#xd7; 10<sup>&#x2212;4</sup>&#xa0;s<sup>&#x2212;1</sup> and the uptake coefficient was 5.79 &#xd7; 10<sup>&#x2212;4</sup> (<xref ref-type="bibr" rid="B43">Zeng et&#x20;al., 2013</xref>).</p>
<p>In our experiments, the molar ratio of ozone to AME or DHOPA remained at more than three orders of magnitude. Under such pseudo-first order reaction, the second-order reaction of ozone (as A in <xref ref-type="disp-formula" rid="e1">Eq. 1</xref>) and AME or DHOPA (as B in <xref ref-type="disp-formula" rid="e1">Eq. 1</xref>) can be considered as the pseudo-first-order reaction, the rate constant for which can be calculated by <xref ref-type="disp-formula" rid="e2">Eq. 2</xref> (<xref ref-type="bibr" rid="B6">Gao et&#x20;al., 2019</xref>).<disp-formula id="e1">
<mml:math id="m1">
<mml:mrow>
<mml:mi mathvariant="normal">A&#x2b;B</mml:mi>
<mml:mo>&#x2192;</mml:mo>
<mml:mi mathvariant="normal">P</mml:mi>
</mml:mrow>
</mml:math>
<label>(1)</label>
</disp-formula>
<disp-formula id="e2">
<mml:math id="m2">
<mml:mrow>
<mml:mfrac>
<mml:mrow>
<mml:mi>d</mml:mi>
<mml:mrow>
<mml:mo>[</mml:mo>
<mml:mi>B</mml:mi>
<mml:mo>]</mml:mo>
</mml:mrow>
</mml:mrow>
<mml:mrow>
<mml:mi>d</mml:mi>
<mml:mi>t</mml:mi>
</mml:mrow>
</mml:mfrac>
<mml:mo>&#x3d;</mml:mo>
<mml:msub>
<mml:mtext>k</mml:mtext>
<mml:mrow>
<mml:mtext>app</mml:mtext>
</mml:mrow>
</mml:msub>
<mml:mrow>
<mml:mo>[</mml:mo>
<mml:mi>B</mml:mi>
<mml:mo>]</mml:mo>
</mml:mrow>
<mml:mtext>&#xa0;when&#xa0;</mml:mtext>
<mml:msub>
<mml:mi>k</mml:mi>
<mml:mrow>
<mml:mi>a</mml:mi>
<mml:mi>p</mml:mi>
<mml:mi>p</mml:mi>
</mml:mrow>
</mml:msub>
<mml:mrow>
<mml:mo>[</mml:mo>
<mml:mtext>B</mml:mtext>
<mml:mo>]</mml:mo>
</mml:mrow>
<mml:mo>&#x3d;</mml:mo>
<mml:mi>k</mml:mi>
<mml:mrow>
<mml:mo>[</mml:mo>
<mml:mi>A</mml:mi>
<mml:mo>]</mml:mo>
</mml:mrow>
<mml:mrow>
<mml:mo>[</mml:mo>
<mml:mi>B</mml:mi>
<mml:mo>]</mml:mo>
</mml:mrow>
</mml:mrow>
</mml:math>
<label>(2)</label>
</disp-formula>
<list list-type="simple">
<list-item>
<p>
<inline-formula id="inf1">
<mml:math id="m3">
<mml:mrow>
<mml:msub>
<mml:mi>k</mml:mi>
<mml:mrow>
<mml:mi>a</mml:mi>
<mml:mi>p</mml:mi>
<mml:mi>p</mml:mi>
</mml:mrow>
</mml:msub>
</mml:mrow>
</mml:math>
</inline-formula>&#x2014;&#x2014;Pseudo-first-order rate constant (s<sup>&#x2212;1</sup>);</p>
</list-item>
<list-item>
<p>
<italic>k</italic>&#x2014;&#x2014;Second order rate constant (cm<sup>3</sup>&#xb7;molecule<sup>&#x2212;1</sup>&#xb7;s<sup>&#x2212;1</sup>);</p>
</list-item>
</list> The uptake coefficient <italic>&#x3b3;</italic> was calculated using the change rate&#x20;of the integral area of characteristic absorption peaks at specific wavelengths of AME or DHOPA based on <xref ref-type="disp-formula" rid="e3">Eq. 3</xref> (<xref ref-type="bibr" rid="B41">Worsnop et&#x20;al., 2002</xref>; <xref ref-type="bibr" rid="B6">Gao et&#x20;al., 2019</xref>).<disp-formula id="e3">
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</mml:mrow>
<mml:mfrac>
<mml:mrow>
<mml:msub>
<mml:mi mathvariant="normal">S</mml:mi>
<mml:mi mathvariant="normal">A</mml:mi>
</mml:msub>
</mml:mrow>
<mml:mi mathvariant="normal">V</mml:mi>
</mml:mfrac>
</mml:mrow>
</mml:math>
<label>(3)</label>
</disp-formula>
<list list-type="simple">
<list-item>
<p>[<inline-formula id="inf2">
<mml:math id="m5">
<mml:mrow>
<mml:msubsup>
<mml:mi>C</mml:mi>
<mml:mi>i</mml:mi>
<mml:mi>a</mml:mi>
</mml:msubsup>
</mml:mrow>
</mml:math>
</inline-formula>]&#x2014;&#x2014;&#x20;the initial concentration of i (molecules&#xb7;cm<sup>&#x2212;3</sup>);</p>
</list-item>
<list-item>
<p>
<inline-formula id="inf3">
<mml:math id="m6">
<mml:mrow>
<mml:mfrac>
<mml:mrow>
<mml:mi>d</mml:mi>
<mml:mrow>
<mml:mo>[</mml:mo>
<mml:mrow>
<mml:msubsup>
<mml:mi>C</mml:mi>
<mml:mi>i</mml:mi>
<mml:mi>a</mml:mi>
</mml:msubsup>
</mml:mrow>
<mml:mo>]</mml:mo>
</mml:mrow>
</mml:mrow>
<mml:mrow>
<mml:mtext>dt</mml:mtext>
</mml:mrow>
</mml:mfrac>
</mml:mrow>
</mml:math>
</inline-formula>&#x2014;&#x2014;&#x20;the rate of molecular change of i (molecules&#xb7;cm<sup>&#x2212;3</sup>&#xb7;s<sup>&#x2212;1</sup>);</p>
</list-item>
<list-item>
<p>
<inline-formula id="inf4">
<mml:math id="m7">
<mml:mrow>
<mml:mover accent="true">
<mml:mtext>c</mml:mtext>
<mml:mo stretchy="true">&#xaf;</mml:mo>
</mml:mover>
</mml:mrow>
</mml:math>
</inline-formula>&#x2014;&#x2014;&#x20;average rate of ozone in the gas phase (cm&#xb7;s<sup>&#x2212;1</sup>);</p>
</list-item>
<list-item>
<p>N<sub>A</sub>&#x2014;&#x2014;&#x20;Avogadro&#x2019;s number (6.02 &#xd7; 10<sup>23</sup> molecules&#xb7;mol<sup>&#x2212;1</sup>);</p>
</list-item>
<list-item>
<p>
<inline-formula id="inf5">
<mml:math id="m8">
<mml:mrow>
<mml:msub>
<mml:mi>P</mml:mi>
<mml:mrow>
<mml:msub>
<mml:mtext>O</mml:mtext>
<mml:mn>3</mml:mn>
</mml:msub>
</mml:mrow>
</mml:msub>
</mml:mrow>
</mml:math>
</inline-formula>&#x2014;&#x2014;&#x20;atmospheric pressure of ozone (Pa &#x3d; kg&#xb7;m<sup>&#x2212;1</sup>&#xb7;s<sup>&#x2212;2</sup>);</p>
</list-item>
<list-item>
<p>R&#x2014;&#x2014;&#x20;gas constant or proportionality constant (8.314&#xa0;J&#xa0;mol<sup>&#x2212;1</sup>&#xa0;K<sup>&#x2212;1</sup>);</p>
</list-item>
<list-item>
<p>T&#x2014;&#x2014;&#x20;temperature (K);</p>
</list-item>
<list-item>
<p>
<inline-formula id="inf6">
<mml:math id="m9">
<mml:mrow>
<mml:mfrac>
<mml:mrow>
<mml:msub>
<mml:mi>S</mml:mi>
<mml:mi>A</mml:mi>
</mml:msub>
</mml:mrow>
<mml:mi>V</mml:mi>
</mml:mfrac>
</mml:mrow>
</mml:math>
</inline-formula>&#x2014;&#x2014;&#x20;the specific surface area of i (cm<sup>&#x2212;1</sup>).</p>
</list-item>
</list>
</p>
<p>In this study, the surface area of the reaction chamber (20&#xa0;cm<sup>&#x2212;1</sup>) was assumed as the specific surface area for collision. However, the collision area corresponds to the sample preparation methods and affects the uptake coefficient. In order to distinguish the changes of FTIR absorption peaks during the process of the oxidation reactions, enough amounts of samples was used, resulting in the formation of pores within the solid powder for ozone to permeate. As a result, our experiments measured the upper bounds of the uptake coefficients of AME and DHOPA.</p>
</sec>
</sec>
<sec sec-type="results|discussion" id="s3">
<title>Results and Discussion</title>
<sec id="s3-1">
<title>Ozone Oxidation of AME</title>
<p>
<xref ref-type="fig" rid="F2">Figure&#x20;2</xref> showed the infrared spectrum of AME at a room temperature of 25&#xb0;C and 30%&#x20;RH.</p>
<fig id="F2" position="float">
<label>FIGURE 2</label>
<caption>
<p>Infrared spectrum of AME at room temperature.</p>
</caption>
<graphic xlink:href="fenvc-02-732219-g002.tif"/>
</fig>
<p>To resolve the broad O&#x2013;H stretching band of carboxyl groups that was superimposed on the peaks of interest between 3,700 and 2,500&#xa0;cm<sup>&#x2212;1</sup>, the six superimposed peaks were separated by 8.5 origin Gaussian fitting, including the stretching bands of -OH at 3,180&#xa0;cm<sup>&#x2212;1</sup> and 3,480&#xa0;cm<sup>&#x2212;1</sup>, -CH<sub>2</sub> antisymmetric stretching at 2,910&#xa0;cm<sup>&#x2212;1</sup>, -CH stretching at 2,823&#xa0;cm<sup>&#x2212;1</sup>, -CH<sub>2</sub> symmetric stretching at 2,718&#xa0;cm<sup>&#x2212;1</sup>, and -CH stretching at 2,663&#xa0;cm<sup>&#x2212;1</sup>, as shown in <xref ref-type="fig" rid="F3">Figure&#x20;3</xref> (<xref ref-type="bibr" rid="B18">Injae et&#x20;al., 1998</xref>; <xref ref-type="bibr" rid="B29">Luo et&#x20;al., 2003</xref>; <xref ref-type="bibr" rid="B2">Branca et&#x20;al., 2016</xref>). The detailed assignment of spectral bands for AME was summarized in <xref ref-type="table" rid="T1">Table&#x20;1</xref>.</p>
<fig id="F3" position="float">
<label>FIGURE 3</label>
<caption>
<p>Gaussian fitting results of AME in the 3,700&#x2013;2500cm<sup>&#x2212;1</sup> region.</p>
</caption>
<graphic xlink:href="fenvc-02-732219-g003.tif"/>
</fig>
<table-wrap id="T1" position="float">
<label>TABLE 1</label>
<caption>
<p>IR frequencies and assignments for functional groups in AME.</p>
</caption>
<table>
<thead valign="top">
<tr>
<th align="left">Wavenumber (cm<sup>&#x2212;1</sup>)</th>
<th align="center">Band assignment</th>
</tr>
</thead>
<tbody valign="top">
<tr>
<td align="left">3259</td>
<td align="left">-OH stretching</td>
</tr>
<tr>
<td align="left">2910</td>
<td align="left">-CH<sub>2</sub> antisymmetric stretching</td>
</tr>
<tr>
<td align="left">2823</td>
<td align="left">-CH stretching</td>
</tr>
<tr>
<td align="left">2718</td>
<td align="left">-CH<sub>2</sub> symmetric stretching</td>
</tr>
<tr>
<td align="left">2663</td>
<td align="left">-CH Stretching</td>
</tr>
<tr>
<td align="left">2348</td>
<td align="left">Carbonyl (C&#x3d;O)</td>
</tr>
<tr>
<td align="left">2122</td>
<td align="left">Carbonyl (C&#x3d;O)</td>
</tr>
<tr>
<td align="left">1970</td>
<td align="left">Carbonyl (C&#x3d;O)</td>
</tr>
<tr>
<td align="left">1414</td>
<td align="left">Deformation of C-O-H</td>
</tr>
<tr>
<td align="left">1366</td>
<td align="left">Deformation of C-O-H</td>
</tr>
<tr>
<td align="left">1305</td>
<td align="left">-OH in-plane</td>
</tr>
<tr>
<td align="left">1255</td>
<td align="left">-OH in-plane</td>
</tr>
<tr>
<td align="left">1217</td>
<td align="left">C-C Stretching</td>
</tr>
<tr>
<td align="left">1055</td>
<td align="left">C-O Stretching</td>
</tr>
<tr>
<td align="left">968</td>
<td align="left">Deformation of C-H</td>
</tr>
<tr>
<td align="left">884</td>
<td align="left">Deformation of C-H</td>
</tr>
</tbody>
</table>
</table-wrap>
<p>The heterogeneous oxidation of ozone on AME was carried out in a reaction chamber of FTIR under the same ambient conditions. The FTIR spectra of the AME were recorded as a function of time. <xref ref-type="fig" rid="F4">Figure&#x20;4A</xref> showed that the area of the absorption peak at wavenumber 3480&#xa0;cm<sup>&#x2212;1</sup> (-OH stretching) decreased with time when exposed to ozone. In addition, a small absorption peak at 1700&#xa0;cm<sup>&#x2212;1</sup> appeared and the area of this peak increased with time, as shown in <xref ref-type="fig" rid="F4">Figure&#x20;4B</xref>. It was likely the infrared absorption caused by the C&#x3d;O stretching of the reaction product. Changes in AME absorption peaks at 3,480&#xa0;cm<sup>&#x2212;1</sup> and at 1700&#xa0;cm<sup>&#x2212;1</sup> during the experiments indicated that&#x2013;OH at the end chain of AME was oxidized to C&#x3d;O bond by ozone. Based on the changes in the absorption peak area at 3,480 and 1700&#xa0;cm<sup>&#x2212;1</sup>, the change rates of peak areas were calculated as &#x2212;0.00134 int. abs/s and 0.00117 int. abs/s, respectively.</p>
<fig id="F4" position="float">
<label>FIGURE 4</label>
<caption>
<p>Changes of the AME absorption peak at 3,480 and 1700&#xa0;cm<sup>&#x2212;1</sup> in the course of reactions. <bold>(A)</bold> 3,480&#xa0;cm<sup>&#x2212;1</sup> <bold>(B)</bold> 1700&#xa0;cm<sup>&#x2212;1</sup>.</p>
</caption>
<graphic xlink:href="fenvc-02-732219-g004.tif"/>
</fig>
</sec>
<sec id="s3-2">
<title>Ozone Oxidation of DHOPA</title>
<p>The infrared spectrum of DHOPA at a room temperature of 25&#xb0;C and 30% relative humidity was presented in <xref ref-type="fig" rid="F5">Figure&#x20;5</xref>. To resolve the broad O&#x2013;H stretching band of alcohol OH and carboxylic acid -OH that were superimposed on the peaks of interest between 4,000 and 3,000&#xa0;cm<sup>&#x2212;1</sup>, as well as the peaks between 1800 and 1,600&#xa0;cm<sup>&#x2212;1</sup>, 8.5 origin Gaussian fitting was used for peak separation. The resolved peaks, included the stretching bands of -OH at 3,540 and 3,340&#xa0;cm<sup>&#x2212;1</sup>, C&#x3d;O of carboxylic acid (COOH) and of ketone at 1730 and 1,640&#xa0;cm<sup>&#x2212;1</sup> respectively as shown in <xref ref-type="fig" rid="F6">Figure&#x20;6</xref>. A comprehensive interpretation of FTIR spectra was tabulated in <xref ref-type="table" rid="T2">Table&#x20;2</xref>, which summarized the detailed assignment of spectral bands for DHOPA.</p>
<fig id="F5" position="float">
<label>FIGURE 5</label>
<caption>
<p>Infrared absorption spectrum of DHOPA at room temperature.</p>
</caption>
<graphic xlink:href="fenvc-02-732219-g005.tif"/>
</fig>
<fig id="F6" position="float">
<label>FIGURE 6</label>
<caption>
<p>Gaussian fitting results of DHOPA in the 4,000&#x2013;3,000 and 1800&#x2013;1,600&#xa0;cm<sup>&#x2212;1</sup> region.</p>
</caption>
<graphic xlink:href="fenvc-02-732219-g006.tif"/>
</fig>
<table-wrap id="T2" position="float">
<label>TABLE 2</label>
<caption>
<p>IR frequencies and assignments for functional groups in DHOPA.</p>
</caption>
<table>
<thead valign="top">
<tr>
<th align="left">Wavenumber (cm<sup>&#x2212;1</sup>)</th>
<th align="center">Band assignment</th>
</tr>
</thead>
<tbody valign="top">
<tr>
<td align="left">3430</td>
<td align="left">-OH Stretching</td>
</tr>
<tr>
<td align="left">2930</td>
<td align="left">-CH<sub>3</sub> antisymmetric stretching</td>
</tr>
<tr>
<td align="left">2342</td>
<td align="left">Carbonyl (C&#x3d;O)</td>
</tr>
<tr>
<td align="left">1730</td>
<td align="left">Carboxylic acid (COOH)</td>
</tr>
<tr>
<td align="left">1640</td>
<td align="left">Carbonyl (C&#x3d;O)</td>
</tr>
<tr>
<td align="left">1371</td>
<td align="left">-CH<sub>3</sub> symmetric stretching</td>
</tr>
<tr>
<td align="left">1255</td>
<td align="left">-OH in-plane</td>
</tr>
<tr>
<td align="left">1133</td>
<td align="left">C-C Stretching</td>
</tr>
<tr>
<td align="left">1090</td>
<td align="left">C-O Stretching</td>
</tr>
<tr>
<td align="left">973</td>
<td align="left">Deformation of C-H</td>
</tr>
<tr>
<td align="left">884</td>
<td align="left">C-H Deformation</td>
</tr>
</tbody>
</table>
</table-wrap>
<p>The heterogeneous oxidations of DHOPA and ozone were carried out in a reaction chamber of FTIR. The FTIR spectra of the DHOPA were monitored with time as the reactions processed. The results showed that the area of the absorption peak at wavenumber 1640&#xa0;cm<sup>&#x2212;1</sup> (C&#x3d;O stretching) decreased with time when exposed to ozone, as shown in <xref ref-type="fig" rid="F7">Figure&#x20;7A</xref>. Furthermore, the appearance of a small absorption peak at 3,340&#xa0;cm<sup>&#x2212;1</sup> and the increase of this peak area with time were observed, as shown in <xref ref-type="fig" rid="F7">Figure&#x20;7B</xref>. It was likely the infrared absorption caused by the O&#x2013;H stretching of the reaction products. Changes in DHOPA absorption peaks at 1,640&#xa0;cm<sup>&#x2212;1</sup> and at 3,340&#xa0;cm<sup>&#x2212;1</sup> during the experiments indicated that C&#x3d;O of DHOPA was oxidized to CO&#x2013;OH by ozone. Because COOH is more stable than C&#x3d;O, the most reasonable product is 2, 3, 4-trihydroxyvaleric acid. Based on the evolution in the absorption peak area at 1,640 and 3,340&#xa0;cm<sup>&#x2212;1</sup> with time, the change rates of peak areas were calculated as &#x2212;0.00191 int.abs/s and 0.00218 int.abs/s, respectively.</p>
<fig id="F7" position="float">
<label>FIGURE 7</label>
<caption>
<p>Changes of the DHOPA absorption peak at 1,640 and 3,340&#xa0;cm<sup>&#x2212;1</sup> in the course of reactions. <bold>(A)</bold> 1,640&#xa0;cm<sup>&#x2212;1</sup> <bold>(B)</bold> 3,340&#xa0;cm<sup>&#x2212;1</sup>.</p>
</caption>
<graphic xlink:href="fenvc-02-732219-g007.tif"/>
</fig>
</sec>
<sec id="s3-3">
<title>Determination of Uptake Coefficient and Reaction Rate</title>
<p>
<xref ref-type="fig" rid="F8">Figure&#x20;8</xref> showed the standard curve of the number of AME molecules vs. the characteristic absorption peak area of AME at 3,480&#xa0;cm<sup>&#x2212;1</sup>. The equation was <italic>y</italic>&#x20;&#x3d; 6.95 &#xd7; 10<sup>16</sup> <italic>x</italic>&#x20;&#x3d; 2.57 &#xd7; 10<sup>19</sup> with the correlation coefficient of 0.9805. <xref ref-type="fig" rid="F9">Figure&#x20;9</xref> showed the standard curve of the number of DHOPA molecules vs. the characteristic absorption peak area of DHOPA at 1,640&#xa0;cm<sup>&#x2212;1</sup>. The equation was <italic>y</italic>&#x20;&#x3d; 4.51 &#xd7; 10<sup>16</sup> <italic>x</italic>&#x20;&#x3d; 5.33 &#xd7; 10<sup>18</sup>, and the correlation coefficient was 0.9783.</p>
<fig id="F8" position="float">
<label>FIGURE 8</label>
<caption>
<p>Standard curve of number of AME molecules vs. absorption peak area at 3,480&#xa0;cm<sup>&#x2212;1</sup>.</p>
</caption>
<graphic xlink:href="fenvc-02-732219-g008.tif"/>
</fig>
<fig id="F9" position="float">
<label>FIGURE 9</label>
<caption>
<p>Standard curve of number of DHOPA molecules vs. absorption peak area at 1,640&#xa0;cm<sup>&#x2212;1</sup>.</p>
</caption>
<graphic xlink:href="fenvc-02-732219-g009.tif"/>
</fig>
<p>Based on the change rate of the absorption peak area (&#x2212;0.00134 int.abs/s at 3,480&#xa0;cm<sup>&#x2212;1</sup>) obtained in <italic>Ozone Oxidation of AME</italic>, the calculated change rate of number of AME molecules was &#x2212;9.313 &#xd7; 10<sup>13</sup> molecules&#xb7;s<sup>&#x2212;1</sup> according to the standard curve shown in <xref ref-type="fig" rid="F8">Figure&#x20;8</xref>. In the same way, with the change rate of DHOPA peak area at 1,640&#xa0;cm<sup>&#x2212;1</sup> (&#x2212;0.00191 int.abs/s), the calculated change rate of number of DHOPA molecules was &#x2212;8.614 &#xd7; 10<sup>13</sup> molecules&#xb7;s<sup>&#x2212;1</sup>. Under the pseudo-first order reaction of this study, the uptake coefficients <italic>&#x3b3;</italic> and the pseudo-first-order reaction rate constants <italic>k</italic>
<sub>
<italic>app</italic>
</sub> could be calculated using the change rates of the absorption peak areas at 3,480&#xa0;cm<sup>&#x2212;1</sup> of AME and 1,640&#xa0;cm<sup>&#x2212;1</sup> of DHOPA based on <xref ref-type="disp-formula" rid="e2">Eqs 2</xref>, <xref ref-type="disp-formula" rid="e3">3</xref>, using the surface area of the reaction chamber of 20&#xa0;cm<sup>&#x2212;1</sup> as the specific surface area for collision, the concentration of ozone of 20&#xa0;ppm and the average rate of ozone of 115&#xa0;ml/min. The pseudo-first-order reaction rate constant <italic>k</italic>
<sub>
<italic>app</italic>
</sub> were 1.89 &#xd7; 10<sup>&#x2212;8</sup>&#xa0;s<sup>&#x2212;1</sup> and 2.12 &#xd7; 10<sup>&#x2212;7</sup>&#xa0;s<sup>&#x2212;1</sup>, and the uptake coefficients of ozone on the surface of AME and DHOPA were (1.3&#x20;&#xb1; 0.8) &#xd7; 10<sup>&#x2212;8</sup> and (4.5&#x20;&#xb1; 2.7) &#xd7; 10<sup>&#x2212;8</sup>, respectively.</p>
<p>This experiment is the first time to study the uptake coefficient of AME and DHOPA, so we find the data of ozone heterogeneous oxidation of some other organic substances for comparison. <xref ref-type="table" rid="T3">Table&#x20;3</xref> summarized the uptake coefficients of ozone on the surface of organic compounds reported in the literature and obtained in this study. It clearly showed that the uptake coefficients of ozone on AME and DHOPA were quite different from other organic compounds, which were 2-5 orders of magnitude lower than those of oleic acid, linoleic acid, and cholesterol, as well as no significant correlation with ozone concentration.</p>
<table-wrap id="T3" position="float">
<label>TABLE 3</label>
<caption>
<p>Uptake coefficients of ozone on surface of organic matters.</p>
</caption>
<table>
<thead valign="top">
<tr>
<th align="left">Reactants</th>
<th align="center">concentration (O<sub>3</sub>)</th>
<th align="center">Uptake coefficient (&#x3b3;)</th>
<th align="center">Reference</th>
</tr>
</thead>
<tbody valign="top">
<tr>
<td align="left">AME</td>
<td align="left">20&#xa0;ppm</td>
<td align="center">(1.3&#x20;&#xb1; 0.8) &#xd7; 10<sup>&#x2212;8</sup>
</td>
<td align="left">This work</td>
</tr>
<tr>
<td align="left">DHOPA</td>
<td align="left">20&#xa0;ppm</td>
<td align="center">(4.5&#x20;&#xb1; 2.7) &#xd7; 10<sup>&#x2212;8</sup>
</td>
<td align="left">This work</td>
</tr>
<tr>
<td align="left">Oleic acid</td>
<td align="left">200&#xa0;ppb</td>
<td align="center">(5&#x20;&#xb1; 3) &#xd7; 10<sup>&#x2212;4</sup>
</td>
<td align="left">
<xref ref-type="bibr" rid="B9">Hartz et&#x20;al. (2007)</xref>
</td>
</tr>
<tr>
<td align="left">Oleic acid</td>
<td align="left">90&#xa0;ppm</td>
<td align="center">(7.5&#x20;&#xb1; 1.2) &#xd7; 10<sup>&#x2212;4</sup>
</td>
<td align="left">
<xref ref-type="bibr" rid="B12">Hearn and Smith (2004)</xref>
</td>
</tr>
<tr>
<td align="left">Linoleic acid</td>
<td align="left">90&#xa0;ppm</td>
<td align="center">(1.1&#x20;&#xb1; 0.2) &#xd7; 10<sup>&#x2212;3</sup>
</td>
<td align="left">
<xref ref-type="bibr" rid="B12">Hearn and Smith (2004)</xref>
</td>
</tr>
<tr>
<td align="left">Cholesterol</td>
<td align="left">200&#xa0;ppb</td>
<td align="center">5 &#xd7; 10<sup>&#x2212;4</sup>
</td>
<td align="left">
<xref ref-type="bibr" rid="B9">Hartz et&#x20;al. (2007)</xref>
</td>
</tr>
<tr>
<td align="left">Cholesterol</td>
<td align="left">2&#xa0;ppm</td>
<td align="center">(2.8&#x20;&#xb1; 0.4) &#xd7; 10<sup>&#x2212;6</sup>
</td>
<td align="left">
<xref ref-type="bibr" rid="B4">Dreyfus et&#x20;al. (2005)</xref>
</td>
</tr>
<tr>
<td align="left">Fatty acid</td>
<td align="left">200&#xa0;ppb</td>
<td align="center">(2.08&#x20;&#xb1; 0.04) &#xd7; 10<sup>&#x2212;5</sup>
</td>
<td align="left">
<xref ref-type="bibr" rid="B9">Hartz et&#x20;al. (2007)</xref>
</td>
</tr>
<tr>
<td align="left">Adipic acid</td>
<td align="left">200&#xa0;ppb</td>
<td align="center">(1.67&#x20;&#xb1; 0.08) &#xd7; 10<sup>&#x2212;5</sup>
</td>
<td align="left">
<xref ref-type="bibr" rid="B9">Hartz et&#x20;al. (2007)</xref>
</td>
</tr>
<tr>
<td align="left">Oleyl&#x2009;alcohol</td>
<td align="left">90&#xa0;ppm</td>
<td align="center">7.5 &#xd7; 10<sup>&#x2212;4</sup>
</td>
<td align="left">
<xref ref-type="bibr" rid="B12">Hearn and Smith (2004)</xref>
</td>
</tr>
<tr>
<td align="left">Caprylic acid</td>
<td align="left">200&#xa0;ppb</td>
<td align="center">(1.08&#x20;&#xb1; 0.07) &#xd7; 10<sup>&#x2212;5</sup>
</td>
<td align="left">
<xref ref-type="bibr" rid="B9">Hartz et&#x20;al. (2007)</xref>
</td>
</tr>
</tbody>
</table>
</table-wrap>
</sec>
</sec>
<sec sec-type="conclusion" id="s4">
<title>Conclusion</title>
<p>In this study, <italic>in-situ</italic> FTIR was utilized to study the heterogeneous oxidations of two secondary organic tracers (AME and DHOPA) by ozone. Under the condition of 20&#xa0;ppm ozone exposure for 30&#xa0;min, the heterogeneous oxidations of AME and DHOPA by ozone were carried out in a reaction chamber of FTIR at a room temperature of 25&#xb0;C and 30% relative humidity. The results showed that the AME absorption peak area at 3,480&#xa0;cm<sup>&#x2212;1</sup> (-OH stretching) decreased while a small absorption peak area increased at 1700&#xa0;cm<sup>&#x2212;1</sup> during ozone passing over the AME solid powder. Such changes indicated that&#x2013;OH, at the end chain of AME, was oxidized to C&#x3d;O bond by ozone. For DHOPA, the absorption peak area at 1,640&#xa0;cm<sup>&#x2212;1</sup> (Carbonyl C&#x3d;O stretching) decreased while a small absorption peak area increased at 3,340&#xa0;cm<sup>&#x2212;1</sup> during the ozone oxidation. It was likely the infrared absorption caused by the O&#x2013;H stretching of the reaction products. Based on the changes in the absorption peak area at 3,480 and 1700&#xa0;cm<sup>&#x2212;1</sup> of AME, as well as the changes at 1,640 and 3,340&#xa0;cm<sup>&#x2212;1</sup> of DHOPA, the change rates&#x20;of peak areas were calculated as -0.00134 and 0.00117 int.abs/s for AME, as well as &#x2212;0.00191 and 0.00218 int.abs/s for DHOPA, respectively. The pseudo-first-order reaction rate&#x20;constant <italic>k</italic>
<sub>
<italic>app</italic>
</sub> were 1.89 &#xd7; 10<sup>&#x2212;8</sup>&#xa0;s<sup>&#x2212;1</sup> and 2.12 &#xd7; 10<sup>&#x2212;7</sup>&#xa0;s<sup>&#x2212;1</sup>, and the uptake coefficients of ozone on the surface of AME and DHOPA were (1.3&#x20;&#xb1; 0.8) &#xd7; 10<sup>&#x2212;8</sup> and (4.5&#x20;&#xb1; 2.7) &#xd7; 10<sup>&#x2212;8</sup>, respectively.</p>
<p>The oxidation rates of AME and DHOPA were found to be relatively slow compared with those of the primary organic tracers reported in previous literature, even exposed to relative high ozone concentrations. It indicates that AME and DHOPA could be reactive, but the oxidation processes would be so slow that the changes of AME and DHOPA due to ozone oxidation are negligible. Therefore, under the atmospheric conditions with ozone as the main oxidant, AME and DHOPA, the secondary organic tracers of isoprene and toluene respectively, can be considered the source contribution estimated on the basis of the tracer methods are reliable. In this paper, the heterogeneous oxidation of secondary organic tracers in ozone environment was discussed. The heterogeneous oxidation of organic tracers by different oxidants needs to be studied more comprehensively in the future.</p>
</sec>
</body>
<back>
<sec id="s5">
<title>Data Availability Statement</title>
<p>The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.</p>
</sec>
<sec id="s6">
<title>Author Contributions</title>
<p>RW Participate in experiments and write this paper YH Participate in experiments QH Participate in experiments GC Support and guide experiments RZ Guide experiments.</p>
</sec>
<sec id="s7">
<title>Funding</title>
<p>This study was financially supported by the National Natural Science Foundation of China (21876036) and the Fund for the Research and Development of Science and Technology in Shenzhen (JCYJ20150625142543472, ZDSYS201603301417588).</p>
</sec>
<sec sec-type="COI-statement" id="s8">
<title>Conflict of Interest</title>
<p>The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.</p>
</sec>
<sec sec-type="disclaimer" id="s9">
<title>Publisher&#x2019;s Note</title>
<p>All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.</p>
</sec>
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