Interactions between aerosol organic components and liquid water content during haze episodes in Beijing

• Main Authors: X. Li, S. Song, W. Zhou, J. Hao, D. R. Worsnop, J. Jiang
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-10-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/19/12163/2019/acp-19-12163-2019.pdf
• ISSN: 1680-7316; 1680-7324

<p>Aerosol liquid water (ALW) is ubiquitous in ambient aerosol and plays an important role in the formation of both aerosol organics and inorganics. To investigate the interactions between ALW and aerosol organics during haze formation and evolution, ALW was modelled based on long-term measurement of submicron aerosol composition in different seasons in Beijing. ALW contributed by aerosol inorganics (ALW<span class="inline-formula">_inorg</span>) was modelled by ISORROPIA II, and ALW contributed by organics (ALW<span class="inline-formula">_org</span>) was estimated with <span class="inline-formula"><i>κ</i></span>-Köhler theory, where the real-time hygroscopicity parameter of the organics (<span class="inline-formula"><i>κ</i>_org</span>) was calculated from the real-time organic oxygen-to-carbon ratio (<span class="inline-formula">O∕C</span>). Overall particle hygroscopicity (<span class="inline-formula"><i>κ</i>_total</span>) was computed by weighting component hygroscopicity parameters based on their volume fractions in the mixture. We found that ALW<span class="inline-formula">_org</span>, which is often neglected in traditional ALW modelling, contributes a significant fraction (18&thinsp;%–32&thinsp;%) to the total ALW in Beijing. The ALW<span class="inline-formula">_org</span> fraction is largest on the cleanest days when both the organic fraction and <span class="inline-formula"><i>κ</i>_org</span> are relatively high. The large variation in <span class="inline-formula">O∕C</span>, from 0.2 to 1.3, indicates the wide variety of organic components. This emphasizes the necessity of using real-time <span class="inline-formula"><i>κ</i>_org</span>, instead of fixed <span class="inline-formula"><i>κ</i>_org</span>, to calculate ALW<span class="inline-formula">_org</span> in Beijing. The significant variation in <span class="inline-formula"><i>κ</i>_org</span> (calculated from <span class="inline-formula">O∕C</span>), together with highly variable organic or inorganic volume fractions, leads to a wide range of <span class="inline-formula"><i>κ</i>_total</span> (between 0.20 and 0.45), which has a great impact on water uptake. The variation in organic <span class="inline-formula">O∕C</span>, or derived <span class="inline-formula"><i>κ</i>_org</span>, was found to be influenced by temperature (<span class="inline-formula"><i>T</i></span>), ALW, and aerosol mass concentrations, among which <span class="inline-formula"><i>T</i></span> and ALW both have promoting effects on <span class="inline-formula">O∕C</span>. During high-ALW haze episodes, although the organic fraction decreases rapidly, <span class="inline-formula">O∕C</span> and derived <span class="inline-formula"><i>κ</i>_org</span> increase with the increase in ALW, suggesting the formation of more soluble organics via heterogeneous uptake or aqueous processes. A positive feedback loop is thus formed: during high-ALW episodes, increasing <span class="inline-formula"><i>κ</i>_org</span>, together with decreasing particle organic fraction (or increasing particle inorganic fraction), increases <span class="inline-formula"><i>κ</i>_total</span>, and thus further promotes the ability of particles to uptake water.</p>