Liquid-liquid phase separation and viscosity within secondary organic aerosol generated from diesel fuel vapors

Abstract

<p><strong>Abstract.</strong> Information on liquid-liquid phase separation (LLPS) and viscosity (or diffusion) within secondary organic aerosol (SOA) is needed to improve predictions of particle size, mass, reactivity, and cloud nucleating properties in the atmosphere. Here we report on LLPS and viscosities within SOA generated by the photooxidation of diesel fuel vapors. Diesel fuel contains a wide range of volatile organic compounds, and SOA generated by the photooxidation of diesel fuel vapors may be a good proxy for SOA from anthropogenic emissions. In our experiments, LLPS occurred over the relative humidity (RH) range of ~ 70 % to ~ 100 %, resulting in an organic-rich outer phase and a water-rich inner phase. These results may have implications for predicting the cloud nucleating properties of anthropogenic SOA since the organic-rich outer phase can lower the kinetic barrier for activation to a cloud droplet. At ≤ 10 % RH, the viscosity was in the range of ≥ 1 × 10<sup>8</sup> Pa s, which corresponds to roughly the viscosity of tar pitch. At 38–50 % RH the viscosity was in the range of 1 × 10<sup>8</sup>–3 × 10<sup>5</sup> Pa s. These measured viscosities are consistent with predictions based on oxygen to carbon elemental ratio (O : C) and molar mass as well as predictions based on the number of carbon, hydrogen, and oxygen atoms. Based on the measured viscosities and the Stokes–Einstein relation, at ≤ 10 % RH diffusion coefficients of organics within diesel fuel SOA is ≤ 5.4 × 10<sup>−17</sup>cm<sup>2</sup> s<sup>−1</sup> and the mixing time of organics within 200 nm diesel fuel SOA particles (<i>τ</i><sub>mixing</sub>) is ≳ 50 h. These small diffusion coefficients and large mixing times may be important in laboratory experiments, where SOA is often generated and studied using low RH conditions and on time scales of minutes to hours. At 38–50 % RH, the calculated organic diffusion coefficients are in the range of 5.4 × 10<sup>−17</sup> to 1.8 × 10<sup>−13</sup> cm<sup>2</sup> s<sup>−1</sup> and calculated <i>τ</i><sub>mixing</sub> values are in the range of ~ 0.01 h to ~ 50 h. These values provide important constraints for the physicochemical properties of anthropogenic SOA.</p>

Supplementary material to "Viscosity and physical state of sucrose mixed with ammonium sulfate droplets"

“…In addition, (iv) the surface tension for sucrose and AS are available in the literature (Grayson et al, 2015;Dutcher et al, 2010). The lower limits of these physical parameters for all solutions, used in this study, fall within the range of the lower limits used in previous studies that used the same technique, i.e., poke-and-flow (Renbaum-Wolff et al, 2013b;Grayson et al, 2015;Song et al, 2016aSong et al, , 2019Song et a., 2021). Therefore, the lower limit of the viscosity was defined as ~10 8 Pa‧s when a particle cracked during poke-and-flow experiments.…”

Viscosity and physical state of sucrose mixed with ammonium sulfate droplets

“…Most studies of viscosities and the related physical states of aerosol particles have been carried out by means of laboratory studies, because there are almost no direct instruments to quantify the viscosities of ambient, airborne submicronand supermicron-sized aerosol particles. Numerous laboratory studies on viscosities and physical states undertaken so far have focused on organic aerosols with different types of secondary organic aerosols investigated (Virtanen et al, 2010;Kuwata and Martin, 2012;Perraud et al, 2012;Renbaum-Wolff et al, 2013a;O'brien et al, 2014;Bateman et al, 2015;Song et al, 2015;Athanasiadis et al, 2016;Grayson et al, 2016;Hosny et al, 2016;Song et al, 2016a;Yli-Juuti et al, 2017;Ham et al, 2019;Petters et al, 2019;Song et al, 2019;Gervasi et al, 2020;Song et al, 2021). These studies showed that the viscosities of organic aerosol particles can vary depending on RH and chemical composition leading to different physical states, including amorphous solid (glassy), (semi-)solid, as well as liquid-like phases.…”

Predictions of the glass transition temperature and viscosity of organic aerosols by volatility distributions

“…The predicted ambient viscosity is compared with the experimental observed viscosity of SOA formed from isoprene (Song et al, 2015), α-pinene (Abramson et al, 2013;Renbaum-Wolff et al, 2013;Kidd et al, 2014;Pajunoja et al, 2014;Zhang et al, 2015;Grayson et al, 2016;Petters et al, 2019), toluene (Song et al, 2016a) and diesel fuel (Song et al, 2019). The majority of experimental values are well bounded by the predicted viscosity of OOA, represented by the pink shaded area.…”

“…Predicted viscosity of (a) HOA, COA and BBOA and (b) LO-OOA, MO-OOA, IEPOX SOA and Isoprene OA in different locations at 298 K as a function of RH. Experimentally measured viscosity of laboratory-generated SOA formed from isoprene (Song et al, 2015), α-pinene (Abramson et al, 2013;Renbaum-Wolff et al, 2013;Kidd et al, 2014;Pajunoja et al, 2014;Bateman et al, 2015;Zhang et al, 2015;Grayson et al, 2016;Petters et al, 2019), toluene (Song et al, 2016), and diesel fuel (Song et al, 2019) are also shown. Predicted viscosity of IEPOX-derived OS mixtures (solid blue line) is from Riva et al (2019).…”

“…1a with multi-linear least squares analysis with 68% prediction and confidence intervals. however, this uncertainty may be much smaller for multicomponent SOA mixtures under ideal mixing conditions as indicated in the confidence band (dashed lines, almost overlapping with the 1:1 line) (Shiraiwa and Li et al, 2017;DeRieux and Li et al, 2018;Song et al, 2019).…”