To predict important secondary organic aerosol (SOA) properties, information on viscosity or diffusion rates within SOA is needed. Ozonolysis of β-caryophyllene is an important SOA source; however, very few viscosity or diffusion rate measurements have been performed for this SOA type and none as a function of relative humidity (RH). In this study, we measured viscosity as a function of RH for SOA generated from the ozonolysis of β-caryophyllene using the poke-flow technique. At an RH of 0 and 48%, the viscosity was between 6.9 × 105 and 2.4 × 107 Pa s, and between 1.3 × 103 and 5.6 × 104 Pa s, respectively. Based on these viscosities and the fractional Stokes–Einstein equation, characteristic mixing timescales of organics within 200 nm β-caryophyllene SOA particles range from ∼0.2 h at 0% RH to <3 s at 48% RH, suggesting that these particles should be well-mixed under most conditions in the lower atmosphere. The chemical composition of the SOA was also determined using nano-desorption electrospray ionization mass spectrometry. The measured chemical composition and the method of DeRieux et al. (ACP, 2018) were used to predict the viscosity of β-caryophyllene SOA. If the mass spectra peak abundances were adjusted to account for the sensitivity of the electrospray ionization to larger molecular weight components, the predicted viscosity overlapped with the measured viscosity at 0% RH, while the predicted viscosities at 15–48% RH were slightly higher than the measured viscosities. The measured viscosities also overlapped with viscosity predictions based on a simple mole-fraction based Arrhenius mixing rule.

Individual atmospheric particles can contain mixtures of primary organic aerosol (POA), secondary organic aerosol (SOA), and secondary inorganic aerosol (SIA). To predict the role of such complex multicomponent particles in air quality and climate, information on the number and types of phases present in the particles is needed. However, the phase behavior of such particles has not been studied in the laboratory, and as a result, remains poorly constrained. Here, we show that POA+SOA+SIA particles can contain three distinct liquid phases: a low-polarity organic-rich phase, a higher-polarity organic-rich phase, and an aqueous inorganic-rich phase. Based on our results, when the elemental oxygen-to-carbon (O:C) ratio of the SOA is less than 0.8, three liquid phases can coexist within the same particle over a wide relative humidity range. In contrast, when the O:C ratio of the SOA is greater than 0.8, three phases will not form. We also demonstrate, using thermodynamic and kinetic modeling, that the presence of three liquid phases in such particles impacts their equilibration timescale with the surrounding gas phase. Three phases will likely also impact their ability to act as nuclei for liquid cloud droplets, the reactivity of these particles, and the mechanism of SOA formation and growth in the atmosphere. These observations provide fundamental information necessary for improved predictions of air quality and aerosol indirect effects on climate.

Biomass burning organic aerosol (BBOA) in the atmosphere contains many compounds that absorb solar radiation, called brown carbon (BrC). While BBOA is in the atmosphere, BrC can undergo reactions with oxidants such as ozone which decrease absorbance, or whiten. The effect of temperature and relative humidity (RH) on whitening has not been well constrained, leading to uncertainties when predicting the direct radiative effect of BrC on climate. Using an aerosol flow-tube reactor, we show that the whitening of BBOA by oxidation with ozone is strongly dependent on RH and temperature. Using a poke-flow technique, we show that the viscosity of BBOA also depends strongly on these conditions. The measured whitening rate of BrC is described well with the viscosity data, assuming that the whitening is due to oxidation occurring in the bulk of the BBOA, within a thin shell beneath the surface. Using our combined datasets, we developed a kinetic model of this whitening process, and we show that the lifetime of BrC is 1 d or less below ∼1 km in altitude in the atmosphere but is often much longer than 1 d above this altitude. Including this altitude dependence of the whitening rate in a chemical transport model causes a large change in the predicted warming effect of BBOA on climate. Overall, the results illustrate that RH and temperature need to be considered to understand the role of BBOA in the atmosphere.

Molecular composition, viscosity, and phase state were investigated for secondary organic aerosol derived from synthetic mixtures of volatile organic compounds representing emissions from healthy and aphid-stressed Scots pine trees.

A large fraction of atmospheric aerosols can be characterized as primary organic aerosol (POA) and secondary organic aerosol (SOA). Knowledge of the phase behavior, that is, the number and type of phases within internal POA + SOA mixtures, is crucial to predict their effect on climate and air quality. For example, if POA and SOA form a single phase, POA will enhance the formation of SOA by providing organic mass to absorb SOA precursors. Using microscopy, we studied the phase behavior of mixtures of SOA proxies and hydrocarbon-like POA proxies at relative humidity (RH) values of 90%, 45%, and below 5%. Internal mixtures of POA and SOA almost always formed two phases if the elemental oxygen-to-carbon ratio (O/C) of the POA was less than 0.11, which encompasses a large fraction of atmospheric hydrocarbon-like POA from fossil fuel combustion. SOA proxies mixed with POA proxies having 0.11 ≤ O/C ≤ 0.29 mostly resulted in particles with one liquid phase. However, two liquid phases were also observed, depending on the type of SOA and POA surrogates, and an increase in phase-separated particles was observed when increasing the RH in this O/C range. The results have implications for predicting atmospheric SOA formation and policy strategies to reduce SOA in urban environments.

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.

&lt;p&gt;&lt;strong&gt;Abstract.&lt;/strong&gt; 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 ~&amp;#8201;70&amp;#8201;% to ~&amp;#8201;100&amp;#8201;%, 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 &amp;#8804;&amp;#8201;10&amp;#8201;% RH, the viscosity was in the range of &amp;#8805;&amp;#8201;1&amp;#8201;&amp;#215;&amp;#8201;10&lt;sup&gt;8&lt;/sup&gt;&amp;#8201;Pa&amp;#8201;s, which corresponds to roughly the viscosity of tar pitch. At 38&amp;#8211;50&amp;#8201;% RH the viscosity was in the range of 1&amp;#8201;&amp;#215;&amp;#8201;10&lt;sup&gt;8&lt;/sup&gt;&amp;#8211;3&amp;#8201;&amp;#215;&amp;#8201;10&lt;sup&gt;5&lt;/sup&gt;&amp;#8201;Pa&amp;#8201;s. These measured viscosities are consistent with predictions based on oxygen to carbon elemental ratio (O&amp;#8201;:&amp;#8201;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&amp;#8211;Einstein relation, at &amp;#8804;&amp;#8201;10&amp;#8201;% RH diffusion coefficients of organics within diesel fuel SOA is &amp;#8804;&amp;#8201;5.4&amp;#8201;&amp;#215;&amp;#8201;10&lt;sup&gt;&amp;#8722;17&lt;/sup&gt;cm&lt;sup&gt;2&lt;/sup&gt;&amp;#8201;s&lt;sup&gt;&amp;#8722;1&lt;/sup&gt; and the mixing time of organics within 200&amp;#8201;nm diesel fuel SOA particles (&lt;i&gt;&amp;#964;&lt;/i&gt;&lt;sub&gt;mixing&lt;/sub&gt;) is &amp;#8819;&amp;#8201;50&amp;#8201;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&amp;#8211;50&amp;#8201;% RH, the calculated organic diffusion coefficients are in the range of 5.4&amp;#8201;&amp;#215;&amp;#8201;10&lt;sup&gt;&amp;#8722;17&lt;/sup&gt; to 1.8&amp;#8201;&amp;#215;&amp;#8201;10&lt;sup&gt;&amp;#8722;13&lt;/sup&gt;&amp;#8201;cm&lt;sup&gt;2&lt;/sup&gt;&amp;#8201;s&lt;sup&gt;&amp;#8722;1&lt;/sup&gt; and calculated &lt;i&gt;&amp;#964;&lt;/i&gt;&lt;sub&gt;mixing&lt;/sub&gt; values are in the range of ~&amp;#8201;0.01&amp;#8201;h to ~&amp;#8201;50&amp;#8201;h. These values provide important constraints for the physicochemical properties of anthropogenic SOA.&lt;/p&gt;

The phase behavior, the number and type of phases, in atmospheric particles containing mixtures of hydrocarbon-like organic aerosol (HOA) and secondary organic aerosol (SOA) is important for predicting their impacts on air pollution, human health, and climate. Using a solvatochromic dye and fluorescence microscopy, we determined the phase behavior of 11 HOA proxies (O/C = 0−0.29) each mixed with 7 different SOA materials generated in environmental chambers (O/C 0.4− 1.08), where O/C represents the average oxygen-to-carbon atomic ratio. Out of the 77 different HOA + SOA mixtures studied, we observed two phases in 88% of the cases. The phase behavior was independent of relative humidity over the range between 90% and <5%. A clear trend was observed between the number of phases and the difference between the average O/C ratios of the HOA and SOA components (ΔO/C). Using a threshold ΔO/C of 0.265, we were able to predict the phase behavior of 92% of the HOA + SOA mixtures studied here, with one-phase particles predicted for ΔO/C < 0.265 and twophase particles predicted for ΔO/C ≥ 0.265. The threshold ΔO/C value provides a relatively simple and computationally inexpensive framework for predicting the number of phases in internal SOA and HOA mixtures in atmospheric models.