Find in Library

<p>The photo-oxidation of myrcene, a monoterpene species emitted by plants, was investigated at atmospheric conditions in the outdoor simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a Large Reaction Chamber). The chemical structure of myrcene consists of one moiety that is a conjugated <span class="inline-formula"><i>π</i></span> system (similar to isoprene) and another moiety that is a triple-substituted olefinic unit (similar to 2-methyl-2-butene). Hydrogen shift reactions of organic peroxy radicals (<span class="inline-formula">RO₂</span>) formed in the reaction of isoprene with atmospheric <span class="inline-formula">OH</span> radicals are known to be of importance for the regeneration of <span class="inline-formula">OH</span>. Structure–activity relationships (SARs) suggest that similar hydrogen shift reactions like in isoprene may apply to the isoprenyl part of <span class="inline-formula">RO₂</span> radicals formed during the OH oxidation of myrcene. In addition, SAR predicts further isomerization reactions that would be competitive with bimolecular <span class="inline-formula">RO₂</span> reactions for chemical conditions that are typical for forested environments with low concentrations of nitric oxide. Assuming that <span class="inline-formula">OH</span> peroxy radicals can rapidly interconvert by addition and elimination of <span class="inline-formula">O₂</span> like in isoprene, bulk isomerization rate constants of 0.21 and 0.097 <span class="inline-formula">s⁻¹</span> (<span class="inline-formula"><i>T</i>=298</span> <span class="inline-formula">K</span>) for the three isomers resulting from the 3<span class="inline-formula">^′</span>-<span class="inline-formula">OH</span> and 1-<span class="inline-formula">OH</span> addition, respectively, can be derived from SAR. Measurements of radicals and trace gases in the experiments allowed us to calculate radical production and destruction rates, which are expected to be balanced. The largest discrepancies between production and destruction rates were found for <span class="inline-formula">RO₂</span>. Additional loss of organic peroxy radicals due to isomerization reactions could explain the observed discrepancies. The uncertainty of the total radical (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mrow><mrow class="chem"><msub><mi mathvariant="normal">RO</mi><mi>x</mi></msub></mrow><mo>=</mo><mrow class="chem"><mi mathvariant="normal">OH</mi><mo>+</mo><msub><mi mathvariant="normal">HO</mi><mn mathvariant="normal">2</mn></msub><mo>+</mo><msub><mi mathvariant="normal">RO</mi><mn mathvariant="normal">2</mn></msub></mrow></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="117pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="a66e00376045a9d898a434f5fb745e2e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-16067-2021-ie00001.svg" width="117pt" height="13pt" src="acp-21-16067-2021-ie00001.png"/></svg:svg></span></span>) production rates was high due to the uncertainty in the yield of radicals from myrcene ozonolysis. However, results indicate that radical production can only be balanced if the reaction rate constant of the reaction between hydroperoxy (<span class="inline-formula">HO₂</span>) and <span class="inline-formula">RO₂</span> radicals derived from myrcene is lower (0.9 to <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M20" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">1.6</mn><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">11</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="55pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="ba598779f702e61851fe1861e329dbed"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-16067-2021-ie00002.svg" width="55pt" height="14pt" src="acp-21-16067-2021-ie00002.png"/></svg:svg></span></span> <span class="inline-formula">cm³ s⁻¹</span>) than predicted by SAR. Another explanation of the discrepancies would be that a significant fraction of products (yield: 0.3 to 0.6) from these reactions include <span class="inline-formula">OH</span> and <span class="inline-formula">HO₂</span> radicals instead of radical-terminating organic peroxides. Experiments also allowed us to determine the yields of organic oxidation products acetone (yield: <span class="inline-formula">0.45±0.08</span>) and formaldehyde (yield: <span class="inline-formula">0.35±0.08</span>). Acetone and formaldehyde are produced from different oxidation pathways, so that yields of these compounds reflect the branching ratios of the initial <span class="inline-formula">OH</span> addition to myrcene. Yields determined in the experiments are consistent with branching ratios expected from SAR. The yield of organic nitrate was determined from the gas-phase budget analysis of reactive oxidized nitrogen in the chamber, giving a value of <span class="inline-formula">0.13±0.03</span>. In addition, the reaction rate constant for myrcene <span class="inline-formula">+</span> <span class="inline-formula">OH</span> was determined from<span id="page16068"/> the measured myrcene concentration, yielding a value of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M30" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>(</mo><mn mathvariant="normal">2.3</mn><mo>±</mo><mn mathvariant="normal">0.3</mn><mo>)</mo><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">10</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="89pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="d7bbd0f7c110cad1eee6faccb92f1781"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-16067-2021-ie00003.svg" width="89pt" height="15pt" src="acp-21-16067-2021-ie00003.png"/></svg:svg></span></span> <span class="inline-formula">cm³ s⁻¹</span>.</p>