Halogen activation and radical cycling initiated by imidazole-2-carboxaldehyde photochemistry

Pablo Corral Arroyo, Raffael Aellig, Peter A. Alpert, Rainer Volkamer, Markus Ammann
2019 Atmospheric Chemistry and Physics  
<p><strong>Abstract.</strong> Atmospheric aerosol particles can contain light-absorbing organic compounds, also referred to as brown carbon (BrC). The ocean surface and sea spray aerosol particles can also contain light-absorbing organic species referred to as chromophoric dissolved organic matter (CDOM). Many BrC and CDOM species can contain carbonyls, dicarbonyls or aromatic carbonyls such as imidazole-2-carboxaldehyde (IC), which may act as photosensitizers because they form triplet excited
more » ... rm triplet excited states upon UV–VIS light absorption. These triplet excited states are strong oxidants and may initiate catalytic radical reaction cycles within and at the surface of atmospheric aerosol particles, thereby increasing the production of condensed-phase reactive oxygen species (ROS). Triplet states or ROS can also react with halides, generating halogen radicals and molecular halogen compounds. In particular, molecular halogens can be released into the gas phase, which is one halogen activation pathway. In this work, we studied the influence of bromide and iodide on the photosensitized production and release of hydroperoxy radicals (<span class="inline-formula">HO<sub>2</sub></span>) upon UV irradiation of films in a coated wall flow tube (CWFT) containing IC in a matrix of citric acid (CA) irradiated with UV light. In addition, we measured the iodine release upon irradiation of IC&amp;thinsp;<span class="inline-formula">∕</span>&amp;thinsp;CA films in the CWFT. We developed a kinetic model coupling photosensitized CA oxidation with condensed-phase halogen chemistry to support data analysis and assessment of atmospheric implications in terms of <span class="inline-formula">HO<sub>2</sub></span> production and halogen release in sea spray particles. As indicated by the experimental results and confirmed by the model, significant recycling of halogen species occurred via scavenging reactions with <span class="inline-formula">HO<sub>2</sub></span>. These prevented the full and immediate release of the molecular halogen (bromine and iodine) produced. Recycling was stronger at low relative humidity, attributed to diffusion limitations. Our findings also show that the <span class="inline-formula">HO<sub>2</sub></span> production from BrC or CDOM photosensitized reactions can increase due to the presence of halides, leading to high <span class="inline-formula">HO<sub>2</sub></span> turnover, in spite of low release due to the scavenging reactions. We estimated the iodine production within sea salt aerosol particles due to iodide oxidation by ozone (<span class="inline-formula">O<sub>3</sub></span>) at <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">5.0</mn><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">6</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="51pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="678fbc42c1097b6908f0a111f2705d59"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-10817-2019-ie00001.svg" width="51pt" height="14pt" src="acp-19-10817-2019-ie00001.png"/></svg:svg></span></span>&amp;thinsp;M&amp;thinsp;s<span class="inline-formula"><sup>−1</sup></span> assuming <span class="inline-formula">O<sub>3</sub></span> was in Henry's law equilibrium with the particle. However, using an <span class="inline-formula">O<sub>3</sub></span> diffusion coefficient of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">1</mn><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">12</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="46pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="b43e05a8fe66c9c7bfc9d55a33002e29"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-10817-2019-ie00002.svg" width="46pt" height="14pt" src="acp-19-10817-2019-ie00002.png"/></svg:svg></span></span>&amp;thinsp;cm<span class="inline-formula"><sup>2</sup></span>&amp;thinsp;s<span class="inline-formula"><sup>−1</sup></span>, iodine activation in an aged, organic-rich sea spray is estimated to be <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">5.5</mn><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">8</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="51pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="4265db4c3d91105a194c5373da0d7363"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-10817-2019-ie00003.svg" width="51pt" height="14pt" src="acp-19-10817-2019-ie00003.png"/></svg:svg></span></span>&amp;thinsp;M&amp;thinsp;s<span class="inline-formula"><sup>−1</sup></span>. The estimated iodine production from BrC photochemistry based on the results reported here amounts to <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">4.1</mn><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">7</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="51pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="62eacd7870e1eca87f008a5fd7c2f12d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-10817-2019-ie00004.svg" width="51pt" height="13pt" src="acp-19-10817-2019-ie00004.png"/></svg:svg></span></span>&amp;thinsp;M&amp;thinsp;s<span class="inline-formula"><sup>−1</sup></span> and indicates that BrC photochemistry can exceed <span class="inline-formula">O<sub>3</sub></span> reactive uptake in controlling the rates of iodine activation from sea spray particles under dry or cold conditions where diffusion is slow within particles.</p>
doi:10.5194/acp-19-10817-2019 fatcat:7dfhme2vkbawnglx47dwonz6vm