Determination of the absorption cross sections of higher-order iodine oxides at 355 and 532 nm
oleh: T. R. Lewis, T. R. Lewis, J. C. Gómez Martín, M. A. Blitz, C. A. Cuevas, J. M. C. Plane, A. Saiz-Lopez
| Format: | Article |
|---|---|
| Diterbitkan: | Copernicus Publications 2020-09-01 |
Deskripsi
<p>Iodine oxides (<span class="inline-formula">I<sub><i>x</i></sub>O<sub><i>y</i></sub></span>) play an important role in the atmospheric chemistry of iodine. They are initiators of new particle formation events in the coastal and polar boundary layers and act as iodine reservoirs in tropospheric ozone-depleting chemical cycles. Despite the importance of the aforementioned processes, the photochemistry of these molecules has not been studied in detail previously. Here, we report the first determination of the absorption cross sections of <span class="inline-formula">I<sub><i>x</i></sub>O<sub><i>y</i></sub></span>, <span class="inline-formula"><i>x</i>=2</span>, 3, 5, <span class="inline-formula"><i>y</i>=1</span>–12 at <span class="inline-formula"><i>λ</i>=355</span> nm by combining pulsed laser photolysis of <span class="inline-formula">I<sub>2</sub>∕O<sub>3</sub></span> gas mixtures in air with time-resolved photo-ionization time-of-flight mass spectrometry, using <span class="inline-formula">NO<sub>2</sub></span> actinometry for signal calibration. The oxides selected for absorption cross-section determinations are those presenting the strongest signals in the mass spectra, where signals containing four iodine atoms are absent. The method is validated by measuring the absorption cross section of IO at 355 nm, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">σ</mi><mrow><mn mathvariant="normal">355</mn><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="normal">nm</mi><mo>,</mo><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="normal">IO</mi></mrow></msub><mo>=</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="57pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="be0b89e71de67d168ed90f8a2b661021"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-10865-2020-ie00001.svg" width="57pt" height="12pt" src="acp-20-10865-2020-ie00001.png"/></svg:svg></span></span> (<span class="inline-formula">1.2±0.1</span>) <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">18</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="37pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="f6c81bdba2af4ac70869ecec47c18b8f"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-10865-2020-ie00002.svg" width="37pt" height="14pt" src="acp-20-10865-2020-ie00002.png"/></svg:svg></span></span> cm<span class="inline-formula"><sup>2</sup></span>, which is found to be in good agreement with the most recent literature. The results obtained are <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">σ</mi><mrow><mn mathvariant="normal">355</mn><mspace width="0.125em" linebreak="nobreak"/><mi mathvariant="normal">nm</mi><mo>,</mo><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="normal">I</mi><mn mathvariant="normal">2</mn><mi mathvariant="normal">O</mi><mn mathvariant="normal">3</mn></mrow></msub><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="italic"><</mi><mspace linebreak="nobreak" width="0.125em"/><mn mathvariant="normal">5</mn><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">19</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="110pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="89df5c5e5946ba7aeb89f39871240bc9"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-10865-2020-ie00003.svg" width="110pt" height="17pt" src="acp-20-10865-2020-ie00003.png"/></svg:svg></span></span> cm<span class="inline-formula"><sup>2</sup></span> molec.<span class="inline-formula"><sup>−1</sup></span>, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">σ</mi><mrow><mn mathvariant="normal">355</mn><mspace linebreak="nobreak" width="0.125em"/><mspace width="0.125em" linebreak="nobreak"/><mi mathvariant="normal">nm</mi><mo>,</mo><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="normal">I</mi><mn mathvariant="normal">2</mn><mi mathvariant="normal">O</mi><mn mathvariant="normal">4</mn></mrow></msub><mo>=</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="67pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="601b6317c4e4a7953820911945bbea5e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-10865-2020-ie00004.svg" width="67pt" height="12pt" src="acp-20-10865-2020-ie00004.png"/></svg:svg></span></span> (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">3.9</mn><mo>±</mo><mn mathvariant="normal">1.2</mn><mo>)</mo><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">18</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="85pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="647bec75b1120dbca7f13987dd1093ef"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-10865-2020-ie00005.svg" width="85pt" height="15pt" src="acp-20-10865-2020-ie00005.png"/></svg:svg></span></span> cm<span class="inline-formula"><sup>2</sup></span> molec.<span class="inline-formula"><sup>−1</sup></span>, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M19" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">σ</mi><mrow><mn mathvariant="normal">355</mn><mspace width="0.125em" linebreak="nobreak"/><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="normal">nm</mi><mo>,</mo><mspace width="0.125em" linebreak="nobreak"/><mi mathvariant="normal">I</mi><mn mathvariant="normal">3</mn><mi mathvariant="normal">O</mi><mn mathvariant="normal">6</mn></mrow></msub><mo>=</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="67pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="d1771de8d0c9f2e5dfec77138fe18f40"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-10865-2020-ie00006.svg" width="67pt" height="12pt" src="acp-20-10865-2020-ie00006.png"/></svg:svg></span></span> (<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">6.1</mn><mo>±</mo><mn mathvariant="normal">1.6</mn><mo>)</mo><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">18</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="85pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="52c0333e9ad35cfc53e3929dba0ac936"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-10865-2020-ie00007.svg" width="85pt" height="15pt" src="acp-20-10865-2020-ie00007.png"/></svg:svg></span></span> cm<span class="inline-formula"><sup>2</sup></span> molec.<span class="inline-formula"><sup>−1</sup></span>, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M23" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">σ</mi><mrow><mn mathvariant="normal">355</mn><mspace width="0.125em" linebreak="nobreak"/><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="normal">nm</mi><mo>,</mo><mspace width="0.125em" linebreak="nobreak"/><mi mathvariant="normal">I</mi><mn mathvariant="normal">3</mn><mi mathvariant="normal">O</mi><mn mathvariant="normal">7</mn></mrow></msub><mo>=</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="67pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="1ee4752f6836cab5b34ac39907257efc"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-10865-2020-ie00008.svg" width="67pt" height="12pt" src="acp-20-10865-2020-ie00008.png"/></svg:svg></span></span> (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M24" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">5.3</mn><mo>±</mo><mn mathvariant="normal">1.4</mn><mo>)</mo><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">18</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="85pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="bd2c1223e60ca150f383536a9b77231f"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-10865-2020-ie00009.svg" width="85pt" height="15pt" src="acp-20-10865-2020-ie00009.png"/></svg:svg></span></span> cm<span class="inline-formula"><sup>2</sup></span> molec.<span class="inline-formula"><sup>−1</sup></span>, and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M27" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">σ</mi><mrow><mn mathvariant="normal">355</mn><mspace linebreak="nobreak" width="0.125em"/><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="normal">nm</mi><mo>,</mo><mspace width="0.125em" linebreak="nobreak"/><mi mathvariant="normal">I</mi><mn mathvariant="normal">5</mn><mi mathvariant="normal">O</mi><mn mathvariant="normal">12</mn></mrow></msub><mo>=</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="72pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="0066eb2199b028de142478a940638fbf"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-10865-2020-ie00010.svg" width="72pt" height="12pt" src="acp-20-10865-2020-ie00010.png"/></svg:svg></span></span> (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M28" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">9.8</mn><mo>±</mo><mn mathvariant="normal">1.0</mn><mo>)</mo><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">18</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="85pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="5035400845f2b18d7999be66f56115a8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-10865-2020-ie00011.svg" width="85pt" height="15pt" src="acp-20-10865-2020-ie00011.png"/></svg:svg></span></span> cm<span class="inline-formula"><sup>2</sup></span> molec.<span class="inline-formula"><sup>−1</sup></span>. Photodepletion at <span class="inline-formula"><i>λ</i>=532</span> nm was only observed for OIO, which enabled determination of upper limits for the absorption cross sections of <span class="inline-formula">I<sub><i>x</i></sub>O<sub><i>y</i></sub></span> at 532 nm using OIO as an actinometer. These measurements are supplemented with ab initio calculations of electronic spectra in order to estimate atmospheric photolysis rates <span class="inline-formula"><i>J</i></span>(<span class="inline-formula">I<sub><i>x</i></sub>O<sub><i>y</i></sub></span>). Our results confirm a high <span class="inline-formula"><i>J</i></span>(<span class="inline-formula">I<sub><i>x</i></sub>O<sub><i>y</i></sub></span>) scenario where <span class="inline-formula">I<sub><i>x</i></sub>O<sub><i>y</i></sub></span> is efficiently removed during daytime, implying enhanced iodine-driven ozone depletion and hindering iodine particle formation. Possible <span class="inline-formula">I<sub>2</sub>O<sub>3</sub></span> and <span class="inline-formula">I<sub>2</sub>O<sub>4</sub></span> photolysis products are discussed, including <span class="inline-formula">IO<sub>3</sub></span>, which may be a precursor to iodic acid (<span class="inline-formula">HIO<sub>3</sub></span>) in the presence of <span class="inline-formula">HO<sub>2</sub></span>.</p>