Heterogeneous OH oxidation of isoprene-epoxydiol-derived organosulfates: kinetics, chemistry and formation of inorganic sulfate

• Main Authors: H. K. Lam, K. C. Kwong, H. Y. Poon, J. F. Davies, Z. Zhang, A. Gold, J. D. Surratt, M. N. Chan
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-02-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/19/2433/2019/acp-19-2433-2019.pdf
• ISSN: 1680-7316; 1680-7324

<p>Acid-catalyzed multiphase chemistry of epoxydiols formed from isoprene oxidation yields the most abundant organosulfates (i.e., methyltetrol sulfates) detected in atmospheric fine aerosols in the boundary layer. This potentially determines the physicochemical properties of fine aerosols in isoprene-rich regions. However, chemical stability of these organosulfates remains unclear. As a result, we investigate the heterogeneous oxidation of aerosols consisting of potassium 3-methyltetrol sulfate ester (<span class="inline-formula">C₅H₁₁SO₇K</span>) by gas-phase hydroxyl (OH) radicals at a relative humidity (RH) of 70.8&thinsp;%. Real-time molecular composition of the aerosols is obtained by using a Direct Analysis in Real Time (DART) ionization source coupled to a high-resolution mass spectrometer. Aerosol mass spectra reveal that 3-methyltetrol sulfate ester can be detected as its anionic form (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M2" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">5</mn></msub><msub><mi mathvariant="normal">H</mi><mn mathvariant="normal">11</mn></msub><msubsup><mi mathvariant="normal">SO</mi><mn mathvariant="normal">7</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="55pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="fb4b437cf9d2b769ec586f66c1e3f65d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-2433-2019-ie00001.svg" width="55pt" height="15pt" src="acp-19-2433-2019-ie00001.png"/></svg:svg></span></span>) via direct ionization in the negative ionization mode. Kinetic measurements reveal that the effective heterogeneous OH rate constant is measured to be <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">4.74</mn><mo>±</mo><mn mathvariant="normal">0.2</mn><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">13</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="88pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="0022baee6c4d85fd5615a7f70ede3d1c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-2433-2019-ie00002.svg" width="88pt" height="14pt" src="acp-19-2433-2019-ie00002.png"/></svg:svg></span></span>&thinsp;cm<span class="inline-formula">³</span>&thinsp;molecule<span class="inline-formula">⁻¹</span>&thinsp;s<span class="inline-formula">⁻¹</span> with a chemical lifetime against OH oxidation of <span class="inline-formula">16.2±0.3</span> days, assuming an OH radical concentration of <span class="inline-formula">1.5×10⁶</span>&thinsp;molecules&thinsp;cm<span class="inline-formula">⁻³</span>. Comparison of this lifetime with those against other aerosol removal processes, such as dry and wet deposition, suggests that 3-methyltetrol sulfate ester is likely to be chemically stable over atmospheric timescales. Aerosol mass spectra only show an increase in the intensity of bisulfate ion (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">HSO</mi><mn mathvariant="normal">4</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="33pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="6e1e76b79502b5044660e5d5b035b6d6"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-2433-2019-ie00003.svg" width="33pt" height="16pt" src="acp-19-2433-2019-ie00003.png"/></svg:svg></span></span>) after oxidation, suggesting the importance of fragmentation processes. Overall, potassium 3-methyltetrol sulfate ester likely decomposes to form volatile fragmentation products and aqueous-phase sulfate radial anion (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">SO</mi><mn mathvariant="normal">4</mn><mrow><mi mathvariant="normal" class="Radical">⚫</mi><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="28pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="4e2ae2305388842d7e4cbf13666d4b76"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-2433-2019-ie00004.svg" width="28pt" height="16pt" src="acp-19-2433-2019-ie00004.png"/></svg:svg></span></span>). <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">SO</mi><mn mathvariant="normal">4</mn><mrow><mi class="Radical" mathvariant="normal">⚫</mi><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="28pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="543654ec2c60df6c4d1a1bfde98e45a6"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-2433-2019-ie00005.svg" width="28pt" height="16pt" src="acp-19-2433-2019-ie00005.png"/></svg:svg></span></span> subsequently undergoes intermolecular hydrogen abstraction to form <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M13" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">HSO</mi><mn mathvariant="normal">4</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="33pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="f88652e579276fb2e0946dceb43aed22"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-2433-2019-ie00006.svg" width="33pt" height="16pt" src="acp-19-2433-2019-ie00006.png"/></svg:svg></span></span>. These processes appear to explain the compositional evolution of 3-methyltetrol sulfate ester during heterogeneous OH oxidation.</p>