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Particles containing secondary organic material (SOM) are ubiquitous in the atmosphere and play a role in climate and air quality. Recently, research has shown that liquid–liquid phase separation (LLPS) occurs at high relative humidity (RH) (greater than  ∼  95 %) in α-pinene-derived SOM particles free of inorganic salts, while LLPS does not occur in isoprene-derived SOM particles free of inorganic salts. We expand on these findings by investigating LLPS at 290 ± 1 K in SOM particles free of inorganic salts produced from ozonolysis of β-caryophyllene, ozonolysis of limonene, and photo-oxidation of toluene. LLPS was observed at greater than  ∼  95 % RH in the biogenic SOM particles derived from β-caryophyllene and limonene while LLPS was not observed in the anthropogenic SOM particles derived from toluene. This work combined with the earlier work on LLPS in SOM particles free of inorganic salts suggests that the occurrence of LLPS in SOM particles free of inorganic salts is related to the oxygen-to-carbon elemental ratio (O : C) of the organic material. These results help explain the difference between the hygroscopic parameter κ of SOM particles measured above and below water saturation in the laboratory and field, and have implications for predicting the cloud condensation nucleation properties of SOM particles. 

Abstract Hygroscopic growth and cloud condensation nuclei activation are key processes for accurately modeling the climate impacts of organic particulate matter. Nevertheless, the microphysical mechanisms of these processes remain unresolved. Here we report complex thermodynamic behaviors, including humidity-dependent hygroscopicity, diameter-dependent cloud condensation nuclei activity, and liquid–liquid phase separation in the laboratory for biogenically derived secondary organic material representative of similar atmospheric organic particulate matter. These behaviors can be explained by the non-ideal mixing of water with hydrophobic and hydrophilic organic components. The non-ideality-driven liquid–liquid phase separation further enhances water uptake and induces lowered surface tension at high relative humidity, which result in a lower barrier to cloud condensation nuclei activation. By comparison, secondary organic material representing anthropogenic sources does not exhibit complex thermodynamic behavior. The combined results highlight the importance of detailed thermodynamic representations of the hygroscopicity and cloud condensation nuclei activity in models of the Earth’s climate system. 

Abstract. Information on liquid–liquid phase separation (LLPS) and viscosity (ordiffusion) within secondary organic aerosol (SOA) is needed to improvepredictions of particle size, mass, reactivity, and cloud nucleatingproperties in the atmosphere. Here we report on LLPS and viscosities withinSOA generated by the photooxidation of diesel fuel vapors. Diesel fuelcontains a wide range of volatile organic compounds, and SOA generated bythe photooxidation of diesel fuel vapors may be a good proxy for SOA fromanthropogenic emissions. In our experiments, LLPS occurred over the relativehumidity (RH) range of ∼70 % to ∼100 %,resulting in an organic-rich outer phase and a water-rich inner phase. Theseresults may have implications for predicting the cloud nucleating propertiesof anthropogenic SOA since the presence of an organic-rich outer phase athigh-RH values can lower the supersaturation with respect to water requiredfor cloud droplet formation. At ≤10 % RH, the viscosity was ≥1×108 Pa s, which corresponds to roughly the viscosity of tarpitch. At 38 %–50 % RH, the viscosity was in the range of 1×108 to 3×105 Pa s. These measured viscosities areconsistent 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 theStokes–Einstein relation, at ≤10 % RH diffusion coefficients oforganics within diesel fuel SOA is ≤5.4×10-17 cm2 s−1 and the mixing time of organics within 200 nm diesel fuel SOAparticles (τmixing) is 50 h. These small diffusion coefficientsand large mixing times may be important in laboratory experiments, where SOAis often generated and studied using low-RH conditions and on timescales ofminutes to hours. At 38 %–50 % RH, the calculated organic diffusioncoefficients are in the range of 5.4×10-17 to 1.8×10-13 cm2 s−1 and calculated τmixing values arein the range of ∼0.01 h to ∼50 h. These valuesprovide important constraints for the physicochemical properties ofanthropogenic SOA.