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<p>Research on the viscosity and phase state of aerosol particles is essential because of their significant influence on the particle growth rate, equilibration times, and related evolution of mass concentration as well as heterogeneous reactions. So far, most studies of viscosity and phase state have been focused on organic aerosol particles, yet data on how viscosity can vary when the organic materials are mixed with inorganic salts remain scarce. Herein, using bead-mobility and poke-and-flow techniques, we quantified viscosities at 293 <span class="inline-formula">±</span> 1 <span class="inline-formula">K</span> for binary mixtures of organic material <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="e653eaf840568ee76bb20ba3bf368ae0"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-10215-2021-ie00001.svg" width="8pt" height="14pt" src="acp-21-10215-2021-ie00001.png"/></svg:svg></span></span> <span class="inline-formula">H₂O</span> and inorganic salts <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="93e47eb16cb371fe6916d3191efc4f1d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-10215-2021-ie00002.svg" width="8pt" height="14pt" src="acp-21-10215-2021-ie00002.png"/></svg:svg></span></span> <span class="inline-formula">H₂O</span>, as well as ternary mixtures of organic material <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="36bd7baae116a5efc17e692d563c2b51"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-10215-2021-ie00003.svg" width="8pt" height="14pt" src="acp-21-10215-2021-ie00003.png"/></svg:svg></span></span> inorganic salts <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="fb147fccdcf98a9911cf3d26a8f6dc33"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-10215-2021-ie00004.svg" width="8pt" height="14pt" src="acp-21-10215-2021-ie00004.png"/></svg:svg></span></span> <span class="inline-formula">H₂O</span> over the atmospheric relative humidity (RH) range. Sucrose as the organic species and calcium nitrate (<span class="inline-formula">Ca(NO₃)₂</span>) or magnesium nitrate (<span class="inline-formula">Mg(NO₃)₂</span>) as the inorganic salts were examined. For binary sucrose <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="6bfc4ae3491d603d986b6e1d0e6866cf"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-10215-2021-ie00005.svg" width="8pt" height="14pt" src="acp-21-10215-2021-ie00005.png"/></svg:svg></span></span> <span class="inline-formula">H₂O</span> particles, the viscosities gradually increased from <span class="inline-formula">∼</span> 3 <span class="inline-formula">×</span> 10<span class="inline-formula">⁻²</span> to <span class="inline-formula"><i>≳</i></span>1 <span class="inline-formula">×</span> 10<span class="inline-formula">⁸</span> <span class="inline-formula">Pa s</span> as RH decreased from <span class="inline-formula">∼</span> 75 % to <span class="inline-formula">∼</span> 25 %. Compared with the results for the sucrose <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M23" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="265e2a7d42d09da6c1e252e5649f9787"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-10215-2021-ie00006.svg" width="8pt" height="14pt" src="acp-21-10215-2021-ie00006.png"/></svg:svg></span></span> <span class="inline-formula">H₂O</span> particles, binary <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M25" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Ca</mi><mo>(</mo><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn></msub><msub><mo>)</mo><mn mathvariant="normal">2</mn></msub><mspace width="0.125em" linebreak="nobreak"/><mo>/</mo><mspace width="0.125em" linebreak="nobreak"/><msub><mi mathvariant="normal">H</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">O</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="80pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="118f4f7d3db2e8a1fe6d09647eaa61e1"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-10215-2021-ie00007.svg" width="80pt" height="14pt" src="acp-21-10215-2021-ie00007.png"/></svg:svg></span></span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M26" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Mg</mi><mo>(</mo><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn></msub><msub><mo>)</mo><mn mathvariant="normal">2</mn></msub><mspace linebreak="nobreak" width="0.125em"/><mo>/</mo><mspace linebreak="nobreak" width="0.125em"/><msub><mi mathvariant="normal">H</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">O</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="81pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="5a4c94f4ca9db574937042e977021423"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-10215-2021-ie00008.svg" width="81pt" height="14pt" src="acp-21-10215-2021-ie00008.png"/></svg:svg></span></span> particles showed drastic enhancements to <span class="inline-formula"><i>≳</i></span>1 <span class="inline-formula">×</span> 10<span class="inline-formula">⁸</span> <span class="inline-formula">Pa s</span> at low RH close to the efflorescence RH. For ternary mixtures of sucrose <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M31" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="cde794723cf5af95a282a8ea7937b694"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-10215-2021-ie00009.svg" width="8pt" height="14pt" src="acp-21-10215-2021-ie00009.png"/></svg:svg></span></span> <span class="inline-formula">Ca(NO₃)₂</span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M33" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="1f30da269e2118f293c445361b5afb08"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-10215-2021-ie00010.svg" width="8pt" height="14pt" src="acp-21-10215-2021-ie00010.png"/></svg:svg></span></span> <span class="inline-formula">H₂O</span> or sucrose <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M35" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="07d7a1dd05fd1823c5d4d7644fe5a26c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-10215-2021-ie00011.svg" width="8pt" height="14pt" src="acp-21-10215-2021-ie00011.png"/></svg:svg></span></span> <span class="inline-formula">Mg(NO₃)₂</span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M37" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="f77694ccc584f068782523cf5fd7b6a6"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-10215-2021-ie00012.svg" width="8pt" height="14pt" src="acp-21-10215-2021-ie00012.png"/></svg:svg></span></span> <span class="inline-formula">H₂O</span>, with organic-to-inorganic mass ratios of <span class="inline-formula">1:1</span>, the viscosities of the particles gradually increased from <span class="inline-formula">∼</span> 3 <span class="inline-formula">×</span> 10<span class="inline-formula">⁻²</span> to greater than <span class="inline-formula">∼</span> 1 <span class="inline-formula">×</span> 10<span class="inline-formula">⁸</span> <span class="inline-formula">Pa s</span> for RH values from <span class="inline-formula">∼</span> 75 % to <span class="inline-formula">∼</span> 5 %. Compared to the viscosities of the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M49" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Ca</mi><mo>(</mo><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn></msub><msub><mo>)</mo><mn mathvariant="normal">2</mn></msub><mspace linebreak="nobreak" width="0.125em"/><mo>/</mo><mspace width="0.125em" linebreak="nobreak"/><msub><mi mathvariant="normal">H</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">O</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="80pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="6a82dcf5c4e50ee7806032de728f50c2"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-10215-2021-ie00013.svg" width="80pt" height="14pt" src="acp-21-10215-2021-ie00013.png"/></svg:svg></span></span> particles, higher viscosities were observed for the ternary sucrose <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M50" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="9ca79db2e01f7db08f0222454febfbc8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-10215-2021-ie00014.svg" width="8pt" height="14pt" src="acp-21-10215-2021-ie00014.png"/></svg:svg></span></span> <span class="inline-formula">Ca(NO₃)₂</span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M52" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="035c567c8a5300b91dec04a48d19d85c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-10215-2021-ie00015.svg" width="8pt" height="14pt" src="acp-21-10215-2021-ie00015.png"/></svg:svg></span></span> <span class="inline-formula">H₂O</span> particles, with values increased by about 1 order of magnitude at 50 % RH and about 6 orders of magnitude at 35 % RH. Moreover, we applied a thermodynamics-based group-contribution model (AIOMFAC-VISC, Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients Viscosity) to predict aerosol viscosities for the studied systems. The model predictions and viscosity measurements show good agreement within <span class="inline-formula">∼</span> 1 order of magnitude in viscosity. The viscosity measurements indicate that the studied mixed organic–inorganic particles range in phase state from liquid to semi-solid or even solid across the atmospheric RH range at a temperature of 293 <span class="inline-formula">K</span>. These results support our understanding that organic <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M56" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="39e5179c5454bc9a18c509626ceec533"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-10215-2021-ie00016.svg" width="8pt" height="14pt" src="acp-21-10215-2021-ie00016.png"/></svg:svg></span></span> inorganic <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M57" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="61144182d0fae4def16d67c772e85617"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-10215-2021-ie00017.svg" width="8pt" height="14pt" src="acp-21-10215-2021-ie00017.png"/></svg:svg></span></span> <span class="inline-formula">H₂O</span> particles can exist in a liquid, semisolid, or even a solid state in the troposphere.</p>