Toward closure between predicted and observed particle viscosity over a wide range of temperatures and relative humidity

<p>Atmospheric aerosols can exist in amorphous semi-solid or glassy phase states whose viscosity varies with atmospheric temperature and relative humidity. The temperature and humidity dependence of viscosity has been hypothesized to be predictable from the combination of a water–organic binary mixing rule of the glass transition temperature, a glass-transition-temperature-scaled viscosity fragility parameterization, and a water uptake parameterization. This work presents a closure study between predicted and observed viscosity for sucrose and citric acid. Viscosity and glass transition temperature as a function of water content are compiled from literature data and used to constrain the fragility parameterization. New measurements characterizing viscosity of sub-100 <span class="inline-formula">nm</span> particles using the dimer relaxation method are presented. These measurements extend the available data of temperature- and humidity-dependent viscosity to <span class="inline-formula">−28</span> <span class="inline-formula">^∘C</span>. Predicted relationships agree well with observations at room temperature and with measured isopleths of constant viscosity at <span class="inline-formula">∼10⁷</span> <span class="inline-formula">Pa s</span> at temperatures warmer than <span class="inline-formula">−28</span> <span class="inline-formula">^∘C</span>. Discrepancies at colder temperatures are observed for sucrose particles. Simulations with the kinetic multi-layer model of gas–particle interactions suggest that the observed deviations at colder temperature for sucrose can be attributed to kinetic limitations associated with water uptake at the timescales of the dimer relaxation experiments. Using the available information, updated equilibrium phase-state diagrams (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">80</mn><mspace linebreak="nobreak" width="0.125em"/><mrow class="unit"><msup><mi/><mo>∘</mo></msup><mi mathvariant="normal">C</mi></mrow><mo>&lt;</mo><mi>T</mi><mo>&lt;</mo><mn mathvariant="normal">40</mn><mspace linebreak="nobreak" width="0.125em"/><mrow class="unit"><msup><mi/><mo>∘</mo></msup><mi mathvariant="normal">C</mi></mrow></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="92pt" height="11pt" class="svg-formula" dspmath="mathimg" md5hash="a8a5c5d34161296918d48810867ab97f"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-1127-2021-ie00001.svg" width="92pt" height="11pt" src="acp-21-1127-2021-ie00001.png"/></svg:svg></span></span>, temperature, and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">0</mn><mspace width="0.125em" linebreak="nobreak"/><mrow class="unit"><mi mathvariant="normal">%</mi></mrow><mo>&lt;</mo><mtext>RH</mtext><mo>&lt;</mo><mn mathvariant="normal">100</mn><mspace linebreak="nobreak" width="0.125em"/><mrow class="unit"><mi mathvariant="normal">%</mi></mrow></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="89pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="e604a1ecdeb3f856b232be1639e45ed7"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-1127-2021-ie00002.svg" width="89pt" height="10pt" src="acp-21-1127-2021-ie00002.png"/></svg:svg></span></span>, relative humidity) for sucrose and citric acid are constructed and associated equilibration timescales are identified.</p>

Abstract licence: CC BY

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