Individual particles that on a mass basis consist dominantly of the components ammonium sulfate, oxygenated organic material, and water are a common class of submicron particles found in today's atmosphere. Here we use (1) the organic-to-sulfate (org:sulf) mass ratio of the overall particle and (2) the oxygen-to-carbon (O:C) elemental ratio of the organic component as input variables in parameterisations that predict the critical relative humidity of several different types of particle phase transitions. Specifically these variables were used to predict the critical relative humidity of liquid-liquid phase separation (SRH), efflorescence (ERH), and deliquescence (DRH). Experiments were conducted by optical microscopy for 11 different oxygenated organic-ammonium sulfate systems covering the range 0.1< org:sulf <12.8 and 0.29 < O:C < 1.33. These new data, in conjunction with other data already available in the literature, were used to develop the parameterisations SRH(org:sulf, O:C), ERH(org:sulf, O:C), and DRH(org:sulf, O:C). The parameterisations correctly predicted SRH within 15% RH for 88% of the measurements, ERH within 5% for 84% of the measurements, and DRH within 5% for 94% of the measurements. The applicability of the derived parameterisations beyond the training data set was tested against observations for organic-sulfate particles produced in an environmental chamber. The organic component consisted of secondary organic material produced by the oxidation of isoprene, α-pinene, and β-caryophyllene. The predictions of the parameterisations were also tested against data from the Southern Great Plains, Oklahoma, USA. The observed ERH and DRH values for both the chamber and field data agreed within 5% RH with the values predicted by the parameterisations using the measured org:sulf and O:C ratios as the input variables.

A large fraction of submicron atmospheric aerosol particles contains both organic material and inorganic salts. As the relative humidity cycles in the atmosphere and the water content of the particles correspondingly changes, these mixed particles can undergo a range of phase transitions, possibly including liquid–liquid phase separation. If liquid–liquid phase separation occurs, the gas-particle partitioning of atmospheric semivolatile organic compounds, the scattering and absorption of solar radiation, and the reactive uptake of gas species on atmospheric particles may be affected, with important implications for climate predictions. The actual occurrence of liquid–liquid phase separation within individual atmospheric particles has been considered uncertain, in large part because of the absence of observations for real-world samples. Here, using optical and fluorescence microscopy, we present images that show the coexistence of two noncrystalline phases for real-world samples collected on multiple days in Atlanta, GA as well as for laboratory-generated samples under simulated atmospheric conditions. These results reveal that atmospheric particles can undergo liquid–liquid phase separations. To explore the implications of these findings, we carried out simulations of the Atlanta urban environment and found that liquid–liquid phase separation can result in increased concentrations of gas-phase NO₃and N₂O₅due to decreased particle uptake of N₂O₅.

Laboratory studies of liquid–liquid phase separation in particles containing organic species and inorganic salts of atmospheric relevance are reviewed. The oxygen-to-carbon elemental ratio (O:C) of the organic component appears to be the most useful parameter for estimating, to a first approximation, the occurrence of liquid–liquid phase separation and the separation relative humidity (SRH) in these particles. A trend of decreasing SRH for increasing O:C was found for simple organic–inorganic mixtures (<11 species). Phase separation in particles composed of laboratory-produced secondary organic material and sulphate species and in ambient particles is generally consistent with this trend. A further constraint is that liquid–liquid phase separation was always observed for O:C < 0.5 and was never observed for O:C ≥ 0.8. For organic materials of intermediate O:C ranging from 0.5 to 0.8, phase separation in simple organic–inorganic mixtures was influenced by the organic functional groups represented. The organic-to-inorganic mass ratio (OIR) affected the occurrence of liquid–liquid phase separation in a small number of cases. A dependence on salt type was observed with 87% of the studied organics exhibiting the following trend in SRH values: (NH₄)₂SO₄≥ NH₄HSO₄≥ NaCl ≥ NH₄NO₃, consistent with previous salting-out studies and the Hofmeister series. Liquid–liquid phase separation does not appear to be strongly influenced by the number of species making up the organic material. The morphology of phase separated particles appears to depend on composition, including O:C of the organic material, the inorganic salt and the OIR.

As the relative humidity varies from high to low values in the atmosphere, particles containing organic species and inorganic salts may undergo liquid–liquid phase separation. The majority of the laboratory work on this subject has used ammonium sulfate as the inorganic salt. In the following we studied liquid–liquid phase separation in particles containing organics mixed with the following salts: ammonium sulfate, ammonium bisulfate, ammonium nitrate and sodium chloride. In each experiment one organic was mixed with one inorganic salt and the liquid–liquid phase separation relative humidity (SRH) was determined. Since we studied 23 different organics mixed with four different salts, a total of 92 different particle types were investigated. Out of the 92 types, 49 underwent liquid–liquid phase separation. For all the inorganic salts, liquid–liquid phase separation was never observed when the oxygen-to-carbon elemental ratio (O : C) ≥ 0.8 and was always observed for O : C < 0.5. For 0.5 ≤ O : C < 0.8, the results depended on the salt type. Out of the 23 organic species investigated, the SRH of 20 organics followed the trend: (NH₄)₂SO₄ ≥ NH₄HSO₄ ≥ NaCl ≥ NH₄NO₃. This trend is consistent with previous salting out studies and the Hofmeister series. Based on the range of O : C values found in the atmosphere and the current results, liquid–liquid phase separation is likely a frequent occurrence in both marine and non-marine environments.

To improve predictions of air quality, visibility, and climate change, knowledge of the viscosities and diffusion rates within organic particulate matter consisting of secondary organic material (SOM) is required. Most qualitative and quantitative measurements of viscosity and diffusion rates within organic particulate matter have focused on SOM particles generated from biogenic volatile organic compounds (VOCs) such as α-pinene and isoprene. In this study, we quantify the relative humidity (RH)-dependent viscosities at 295 ± 1 K of SOM produced by photo-oxidation of toluene, an anthropogenic VOC. The viscosities of toluene-derived SOM were 2 × 10⁻¹ to ∼ 6 × 10⁶ Pa s from 30 to 90 % RH, and greater than ∼ 2 × 10⁸ Pa s (similar to or greater than the viscosity of tar pitch) for RH ≤ 17 %. These viscosities correspond to Stokes–Einstein-equivalent diffusion coefficients for large organic molecules of ∼ 2 × 10⁻¹⁵ cm² s⁻¹ for 30 % RH, and lower than ∼ 3 × 10⁻¹⁷ cm² s⁻¹ for RH ≤ 17 %. Based on these estimated diffusion coefficients, the mixing time of large organic molecules within 200 nm toluene-derived SOM particles is 0.1–5 h for 30 % RH, and higher than ∼ 100 h for RH ≤ 17 %. As a starting point for understanding the mixing times of large organic molecules in organic particulate matter over cities, we applied the mixing times determined for toluene-derived SOM particles to the world's top 15 most populous megacities. If the organic particulate matter in these megacities is similar to the toluene-derived SOM in this study, in Istanbul, Tokyo, Shanghai, and São Paulo, mixing times in organic particulate matter during certain periods of the year may be very short, and the particles may be well-mixed. On the other hand, the mixing times of large organic molecules in organic particulate matter in Beijing, Mexico City, Cairo, and Karachi may be long and the particles may not be well-mixed in the afternoon (15:00–17:00 LT) during certain times of the year.