Non-equilibrium interplay between gas–particle partitioning and multiphase chemical reactions of semi-volatile compounds: mechanistic insights and practical implications for atmospheric modeling ofpolycyclic aromatic hydrocarbons

• Main Authors: J. Wilson, U. Pöschl, M. Shiraiwa, T. Berkemeier
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-04-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://acp.copernicus.org/articles/21/6175/2021/acp-21-6175-2021.pdf

<p>Polycyclic aromatic hydrocarbons (PAHs) are carcinogenic air pollutants. The dispersion of PAHs in the atmosphere is influenced by gas–particle partitioning and chemical loss. These processes are closely interlinked and may occur at vastly differing timescales, which complicates their mathematical description in chemical transport models. Here, we use a kinetic model that explicitly resolves mass transport and chemical reactions in the gas and particle phases to describe and explore the dynamic and non-equilibrium interplay of gas–particle partitioning and chemical losses of PAHs on soot particles. We define the equilibration timescale <span class="inline-formula"><i>τ</i>_eq</span> of gas–particle partitioning as the <span class="inline-formula"><i>e</i></span>-folding time for relaxation of the system to the partitioning equilibrium. We find this metric to span from seconds to hours depending on temperature, particle surface area, and the type of PAH. The equilibration time can be approximated using a time-independent equation, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">τ</mi><mi mathvariant="normal">eq</mi></msub><mo>≈</mo><mstyle displaystyle="false"><mfrac style="text"><mn mathvariant="normal">1</mn><mrow><msub><mi>k</mi><mi mathvariant="normal">des</mi></msub><mo>+</mo><msub><mi>k</mi><mi mathvariant="normal">ads</mi></msub></mrow></mfrac></mstyle></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="65pt" height="18pt" class="svg-formula" dspmath="mathimg" md5hash="b681b7b384ef8f18e7e80349db2fa316"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-6175-2021-ie00001.svg" width="65pt" height="18pt" src="acp-21-6175-2021-ie00001.png"/></svg:svg></span></span>, which depends on the desorption rate coefficient <span class="inline-formula"><i>k</i>_des</span> and adsorption rate coefficient <span class="inline-formula"><i>k</i>_ads</span>, both of which can be calculated from experimentally accessible parameters. The model reveals two regimes in which different physical processes control the equilibration timescale: a <i>desorption-controlled</i> and an <i>adsorption-controlled</i> regime. In a case study with the PAH pyrene, we illustrate how chemical loss can perturb the equilibrium particulate fraction at typical atmospheric concentrations of <span class="inline-formula">O₃</span> and <span class="inline-formula">OH</span>. For the surface reaction with <span class="inline-formula">O₃</span>, the perturbation is significant and increases with the gas-phase concentration of <span class="inline-formula">O₃</span>. Conversely, perturbations are smaller for reaction with the <span class="inline-formula">OH</span> radical, which reacts with pyrene on both the surface of particles and in the gas phase. Global and regional chemical transport models typically approximate gas–particle partitioning with instantaneous-equilibration approaches. We highlight scenarios in which these approximations deviate from the explicitly coupled treatment of gas–particle partitioning and chemistry presented in this study. We find that the discrepancy between solutions depends on the operator-splitting time step and the choice of time step can help to minimize the discrepancy. The findings and techniques presented in this work not only are relevant for PAHs but can also be applied to other semi-volatile substances that undergo chemical reactions and mass transport between the gas and particle phase.</p>