Oxidation of low-molecular-weight organic compounds in cloud droplets: global impact on tropospheric oxidants

• Main Authors: S. Rosanka, R. Sander, B. Franco, C. Wespes, A. Wahner, D. Taraborrelli
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-07-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://acp.copernicus.org/articles/21/9909/2021/acp-21-9909-2021.pdf
• ISSN: 1680-7316; 1680-7324

<p>In liquid cloud droplets, superoxide anion (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">O</mi><mrow><mn mathvariant="normal">2</mn><mo>(</mo><mi mathvariant="normal">aq</mi><mo>)</mo></mrow><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="ebd69070c0270c567c00619ccbc69cbd"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-9909-2021-ie00001.svg" width="29pt" height="17pt" src="acp-21-9909-2021-ie00001.png"/></svg:svg></span></span>) is known to quickly consume ozone (<span class="inline-formula">O_3(aq)</span>), which is relatively insoluble. The significance of this reaction as a tropospheric <span class="inline-formula">O₃</span> sink is sensitive to the abundance of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">O</mi><mrow><mn mathvariant="normal">2</mn><mo>(</mo><mi mathvariant="normal">aq</mi><mo>)</mo></mrow><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="723c90d096eb941d30f52b92232e72ca"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-9909-2021-ie00002.svg" width="29pt" height="17pt" src="acp-21-9909-2021-ie00002.png"/></svg:svg></span></span> and therefore to the production of its main precursor, the hydroperoxyl radical (<span class="inline-formula">HO_2(aq)</span>). The aqueous-phase oxidation of oxygenated volatile organic compounds (OVOCs) is the major source of <span class="inline-formula">HO_2(aq)</span> in cloud droplets. Hence, the lack of explicit aqueous-phase chemical kinetics in global atmospheric models leads to a general underestimation of clouds as <span class="inline-formula">O₃</span> sinks. In this study, the importance of in-cloud OVOC oxidation for tropospheric composition is assessed by using the Chemistry As A Boxmodel Application (CAABA) and the global ECHAM/MESSy Atmospheric Chemistry (EMAC) model, which are both capable of explicitly representing the relevant chemical transformations. For this analysis, three different in-cloud oxidation mechanisms are employed: (1) one including the basic oxidation of <span class="inline-formula">SO_2(aq)</span> by <span class="inline-formula">O_3(aq)</span> and <span class="inline-formula">H₂O_2(aq)</span>, which thus represents the capabilities of most global models; (2) the more advanced standard EMAC mechanism, which includes inorganic chemistry and simplified degradation of methane oxidation products; and (3) the detailed in-cloud OVOC oxidation scheme Jülich Aqueous-phase Mechanism of Organic Chemistry (JAMOC). By using EMAC, the global impact of each mechanism is assessed focusing mainly on tropospheric volatile organic compounds (VOCs), <span class="inline-formula">HO_<i>x</i></span> (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow><mrow class="chem"><msub><mi mathvariant="normal">HO</mi><mi>x</mi></msub></mrow><mo>=</mo><mrow class="chem"><mi mathvariant="normal">OH</mi></mrow><mo>+</mo><mrow class="chem"><msub><mi mathvariant="normal">HO</mi><mn mathvariant="normal">2</mn></msub></mrow></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="85pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="516048bc576ae945bca2297bebedcb0c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-9909-2021-ie00003.svg" width="85pt" height="13pt" src="acp-21-9909-2021-ie00003.png"/></svg:svg></span></span>), and <span class="inline-formula">O₃</span>. This is achieved by performing a detailed <span class="inline-formula">HO_<i>x</i></span> and <span class="inline-formula">O₃</span> budget analysis in the gas and aqueous phase. The resulting changes are evaluated against <span class="inline-formula">O₃</span> and methanol (<span class="inline-formula">CH₃OH</span>) satellite observations from the Infrared Atmospheric Sounding Interferometer (IASI) for 2015. In general, the explicit in-cloud oxidation leads to an overall reduction in predicted OVOC levels and reduces EMAC's overestimation of some OVOCs in the tropics. The in-cloud OVOC oxidation shifts the <span class="inline-formula">HO₂</span> production from the gas to the aqueous phase. As a result, the <span class="inline-formula">O₃</span> budget is perturbed with scavenging being enhanced and the gas-phase chemical losses being reduced. With the simplified in-cloud chemistry, about 13 <span class="inline-formula">Tg yr⁻¹</span> of <span class="inline-formula">O₃</span> is scavenged, which increases to 336 <span class="inline-formula">Tg yr⁻¹</span> when JAMOC is used. The highest <span class="inline-formula">O₃</span> reduction of 12 % is predicted in the upper troposphere–lower stratosphere (UTLS). These changes in the free troposphere significantly reduce the modelled tropospheric ozone columns, which are known to be generally overestimated by EMAC and other global atmospheric models.</p>