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

Abstract

Abstract. 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 presence of an organic-rich outer phase at high-RH values can lower the supersaturation with respect to water required for cloud droplet formation. At ≤10 % RH, the viscosity was ≥1×108 Pa s, which corresponds to roughly the viscosity of tar pitch. At 38 %–50 % RH, the viscosity was in the range of 1×108 to 3×105 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 ≤5.4×10-17 cm2 s−1 and the mixing time of organics within 200 nm diesel fuel SOA particles (τmixing) 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 timescales of minutes to hours. At 38 %–50 % RH, the calculated organic diffusion coefficients are in the range of 5.4×10-17 to 1.8×10-13 cm2 s−1 and calculated τmixing values are in the range of ∼0.01 h to ∼50 h. These values provide important constraints for the physicochemical properties of anthropogenic SOA.

Viscosity and liquid–liquid phase separation in healthy and stressed plant SOA

“…[66][67][68] The existence of LLPS in a purely organic SOA particle below 90% RH has only been observed previously for SOA generated by the photooxidation of diesel fuel vapors. 62 The presence of LLPS down to as low as 20% RH for sp-SOA, suggests that LLPS could be more common in organic SOA systems than previously thought.…”

“…The procedure for LLPS has been described by Song et al (2019). 62 In short, the hydrophobic glass slides containing the SOA samples were placed in a RH and temperature-controlled ow cell. The RH was monitored using a chilled-mirror dew point hygrometer (General Eastern M4/E4 dew point monitor, Canada).…”

“…63 More recently, it has been shown that LLPS also occurs in SOA particles in the absence of inorganic salts. 62,[66][67][68][69] The LLPS events in SOA particles can impact the extent to which gas-particle partitioning of semivolatile compounds may occur, [70][71][72] the reactive uptake of gasses, [73][74][75] as well as their ability to act as nuclei for cloud droplets and ice particles, 67,[76][77][78] making it an important process to study.…”

Hygroscopic behavior of aerosols generated from solutions of 3-methyl-1,2,3-butanetricarboxylic acid, its sodium salts, and its mixtures with NaCl

“…2, even though an overlapped C=O (from COOH) peak at 1660-1680 cm −1 appeared during the dehydration process, and the water peak became undetectable, as shown in the Raman spectra at RHs of 45 % and 3 %, suggesting an amorphous and solid state as well as the presence of an activation barrier or diffusional resistance to homogeneous nucleation required for the crystallization of MBTCA droplets as efflorescence is a kinetically controlled process (Martin, 2000;Freedman, 2017). Previous studies reported that αpinene SOAs were very likely to exist as a highly viscous semisolid or even glassy state at low humidity (Saukko et al, 2012;Renbaum-Wolff et al, 2013;Berkemeier et al, 2014;Dette et al, 2014;Kidd et al, 2014;Song et al, 2016;Lessmeier et al, 2018). In addition, many organic substances, such as carboxylic acids, carbohydrates, and proteins, tend to form amorphous rather than crystalline phases upon the drying of aqueous-solution droplets (Mikhailov et al, 2009).…”

Hygroscopic behavior of aerosols generated from solutions of 3-methyl-1,2,3-butanetricarboxylic acid, its sodium salts, and its mixtures with NaCl

“…fine aerosol mass that exists as liquid, amorphous solid, semi-solid, and phase-separated aerosol particles (Jang et al, 2002;Hallquist et al, 2009;Jimenez et al, 2009;Virtanen et al, 2010;Koop et al, 2011;Bateman et al, 2015b;Shrivastava et al, 2015;Bernard et al, 2016;Pajunoja et al, 2016;Freedman, 2017;Shrivastava et al, 2017;Kim et al, 2018;Srivastava et al, 2018;Liu et al, 2019;Slade et al, 2019;Song et al, 2019;Wu et al, 2019a). These aerosols are of critical importance because of their ability to scatter and absorb solar radiation directly, to affect the number of CCN (cloud condensation nuclei) through the formation of new particles and the growth of preexisting particles, and further impact the climate and human health (Haywood and Boucher, 2000;Topping et al, 2013;Poschl and Shiraiwa, 2015;Reid et al, 2018;Marsh et al, 2019).…”