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

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.
