Particles consisting of secondary organic material (SOM) are abundant in the
atmosphere. To predict the role of these particles in climate, visibility and
atmospheric chemistry, information on particle phase state (i.e., single
liquid, two liquids and solid) is needed. This paper focuses on the phase
state of SOM particles free of inorganic salts produced by the ozonolysis of
Particles consisting of secondary organic material (SOM) can account for 20–80 % of the total submicron organic mass concentrations in the atmosphere (Zhang et al., 2007; Jimenez et al., 2009). SOM in the particle phase consists of the low volatility fraction of the oxidized products of biogenic or anthropogenic volatile organic compounds (Hallquist et al., 2009). To predict the role of SOM particles for climate, visibility and atmospheric chemistry, information on the phase state within individual SOM particles (e.g., one liquid, two liquids and one solid) is needed. Particle phase state influences the properties of particles such as cloud condensation nuclei (CCN) properties, optical properties and interactions with reactive and non-reactive gas-phase species (Martin et al., 2000; Raymond and Pandis, 2002; Bilde and Svenningsson, 2004; Zuend et al., 2010; Kuwata and Martin, 2012).
A possible phase transition of SOM particles during relative humidity (RH)
cycling is liquid–liquid phase separation (LLPS) (Pankow et al., 2003;
Petters et al., 2006). LLPS has been observed in the laboratory when SOM
produced by
This paper focuses on phase transitions of SOM produced by
Particles of secondary organic material were produced by
At the outlet of the flow tube reactor, particles were collected using one of
two different methods. In the first method, after charging in a bipolar
charger (TSI, model 3077), a portion of the flow (1.5 slpm) was sampled into
a Nanometer Aerosol Sampler (TSI, model 3089). The particles were collected
by electrostatic precipitation (
For the optical microscope experiments (see Sect. 2.1.2), supermicron
particles are needed, and in the case of method 1 the collected submicron
particles were exposed to water supersaturation (SS) conditions to grow and
coagulate the particles (Song et al., 2015). Specifically, the slides
containing the submicron particles were mounted to a temperature and
RH-controlled flow cell, which was coupled to a reflectance microscope, as
described previously (Koop et al., 2000; Parson et al., 2004; Pant et al.,
2006). The RH in the flow cell was initially set to > 100 %
by decreasing the cell temperature to below the dew point temperature. At the
initial RH (> 100 %) water condensed on the slides forming
large (150–300
Summary of experimental conditions for the production and collection
of
This hygroscopic cycling of method 1 did introduce the possibility for aqueous phase reactions to occur (e.g., simulating cloud water reactions) that might not be present under subsaturated conditions (e.g., aerosol water only). Furthermore, during the RH cycle some of the secondary organic material may have evaporated from the particles. However, the similarities in the results for two collection methods (see Table 1) suggest that neither of these possible processes, if present, changed the chemical composition enough to influence LLPS. Also, prior to collection with both methods, excess gas-phase components were removed with a carbon filter. During this process, some of the more volatile material in the SOM may have evaporated. If some evaporation of higher volatility species occurred, the SOM would likely be more similar to the chemical composition of SOM particles in the atmosphere, which are formed at lower particle mass concentrations compared to particles in the current laboratory experiments.
Hydrophobic substrates containing the supermicron particles were located
within a flow cell with temperature and RH control and coupled to a
reflectance microscope (Zeiss, AxioTech, 50
During the experiments used to determine SOM phase state the concentration of organic vapors in the flow cell was not controlled. Hence, some of the more volatile material in the SOM may have evaporated during these experiments. However, no visible change in the particle volume occurred during these experiments, suggesting evaporative loss was minimal. The SOM particle mass concentrations used when generating the SOM were similar to those used in Grayson et al. (2016), and the sample preparation methods were identical to those used in Grayson et al. (2016), who showed no visible volume change of the droplets over time periods of greater than 44 h. It is possible that condensed-phase reactions may have occurred that lowered the vapor pressure of the SOM.
Liquid–liquid equilibria and water uptake were calculated with the methods developed by Zuend et al. (2008, 2010, 2011) and Zuend and Seinfeld (2012, 2013). To calculate activity coefficients of the organic species as a function of the solution composition, the thermodynamic group-contribution model AIOMFAC (Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients) developed by Zuend et al. (2008, 2010, 2011) was utilized. To determine whether two liquid phases or a single liquid phase was the thermodynamic stable state, the Gibbs free energies of a two-liquid phase state and a one-liquid phase state were calculated (Zuend et al., 2010). If the two-liquid phase state had a lower Gibbs free energy compared to the one liquid phase state, then LLPS was predicted.
Molecular weights (M
To represent SOM from the ozonolysis of
The oxidation products and mole fractions used in the thermodynamic
modeling studies were used to (1) improve our understanding of the phase
state of multicomponent organic mixtures such as those generated during SOM
formation from
In addition to detecting the presence of LLPS, the thermodynamic model was
used to predict the hygroscopic growth factor (HGF), CCN activation and the
hygroscopicity parameter (
Calculated
properties of the mixtures SOM-high, SOM-low and SOM-ox:
average O : C elemental ratio, average molecular weight, simulated mass yields at 60% RH reported in Zuend and Seinfeld (2012), range of LLPS for
a 20
Effect of RH cycles on
Effect of RH cycles on
As the RH was scanned from high values (
Panel b of Fig. 1 and movie S2 in the Supplement show the same particle as panel a, but for
experiments using decreasing RH starting from close to 100 % RH. At
Figures 1–2 and movies S1–S4 in the Supplement show that there are differences in the process of LLPS and the resulting morphology depending on the direction of the RH change. For increasing RH, spinodal decomposition was identified as the mechanism of phase separation. For decreasing RH, disappearance of phase separation occurred by merging of the two phases.
Experiments were also carried out to determine whether the lowest RH at which two
phases existed depended on the direction of RH change. Values for the lowest
RH at which two phases were observed when increasing and decreasing RH are
listed in Table 1 and shown in Fig. 3 (black circles correspond to increasing
RH and red circles correspond to decreasing RH). Table 1 and Fig. 3
illustrate that the lowest RH at which two phases were observed did not
depend significantly on the direction of RH change. Figure 3 and Table 1 also
show that within uncertainties of the measurements, there is no effect of the
SOM particle mass concentrations in the flow tube reactor on the lowest RH at
which two liquid phases were observed for the range of 75 to
11 000
Relative humidity (RH) at which phase transition between one phase
and two liquid phases were observed for
Simulated hygroscopic growth factors HGF
Assuming the surface tension of water, Köhler curves (panel
The behavior observed here for SOM is consistent with bulk thermodynamics. Consider, for example, a mixture of a relatively hydrophobic organic with a less hydrophobic organic, such as a mixture containing equal mole ratios of heptanol and propanol (Stoicescu et al., 2011). Under dry conditions this mixture exists as a single phase. As water is added to the system, the mixture exists as a single (organic-rich) phase until the water content is approximately 0.3 mole fraction. At this point, the mixture separates into an organic-rich phase and a water-rich phase. As water is further added to the system, the two phases coexist until a large amount of water has been added, at which point all the organic material dissolves into the water-rich phase. The formation of two phases is due to the non-ideality of the mixture; i.e., if the mixture was ideal, LLPS would not be observed. Examples of other organic mixtures that exhibit this type of behavior include mixtures of hexanol and acetic acid (Senol et al., 2004) and mixtures of octanol and acetone (Tiryaki et al., 1994). For a long list of organic mixtures that undergo liquid–liquid phase separation when mixed with water, see Table 1 in Ganbavale et al. (2015).
Shown in panel a of Fig. 4 are the simulated hygroscopic growth factors for
the three different SOM mixtures (SOM-high, SOM-low, SOM-ox) with a dry
diameter of 20
Shown in panel b of Fig. 4 are the simulated hygroscopic growth factors of a 100 nm dry particle for the three different SOM mixtures (SOM-high, SOM-low, SOM-ox), again assuming a surface tension of water. This figure illustrates that LLPS can shift to RH > 100 % in small particles due to the Kelvin effect. In 100 nm particles, the SOM took up little water at RH < 100 %, and LLPS is predicted above 100 % RH.
The presence of a miscibility gap at RH > 95 % has
consequences for the CCN activity of particles as suggested previously
(Petters et al., 2006). Shown in panel a of Fig. 5 are simulated Köhler
curves for SOM particles with dry diameters of 100 nm and using the surface
tension of water. The Köhler curves show a sharp increase in the
equilibrium water vapor SS above the particles as the size of the particles
increases from 100 to roughly 110 nm due to the Kelvin effect when they are
still in their organic-rich phase (i.e., low water content state). As the
particle size increases from 110 to 200 nm there is a steep decrease in SS
as the particles switch from the organic-rich phase to two phases by taking
up water from the gas phase. This gives rise to the first maximum in the
Köhler curve, which occurs at a wet particle diameter of
Shown in panel a of Fig. 6 are simulated Köhler curves for SOM particles
with dry diameters of 100 nm and using the surface tension of
40 mN m
Assuming a surface tension of 40 mN m
The non-ideality of SOM also has consequences for the applicability of the
single parameter
Recently researchers have observed inconsistencies between measured CCN
properties of SOM particles and hygroscopic growth measured below water
saturation. In other words, hygroscopic parameters measured below water
saturation were inconsistent with hygroscopic parameters measured above
water saturation. Several reasons have been put forward to explain these
discrepancies (Petters et al., 2006, 2009; Prenni et al., 2007;
Juranyi et al., 2009; Good et al., 2010; Massoli et al., 2010; Hersey
et al., 2013; Pajunoja et al., 2015). The results shown in panel b of Figs. 5–6
illustrate that such inconsistencies are expected for systems with LLPS
when the water uptake at subsaturated conditions represents the
hygroscopicity of the organic-rich phase while the barrier for CCN
activation is determined by the second maximum in the Köhler curve when
the particles are water rich. Additional laboratory studies are needed to
determine whether LLPS occurs in
This work was supported by the Natural Sciences and Engineering Research Council of Canada. Support from the US National Science Foundation and the US Department of Energy is also acknowledged. Claudia Marcolli acknowledges the Competence Center Environment and Sustainability of the ETH Domain (CCES) project OPTIWARES for financial support and Andreas Zuend for providing the program to perform the model calculations. The authors would also like to thank Doug Worsnop for enthusiastic and motivating discussions related to the current manuscript.Edited by: A. Virtanen