Although the particle phase state is an important property, there is scant information on it, especially, for real-world aerosols. To explore the phase state of fine mode aerosols (PM2.5) in two megacities, Seoul and Beijing, we collected PM2.5 filter samples daily from Dec 2020 to Jan 2021. Using optical microscopy combined with the poke-and-flow technique, the phase states of the bulk of PM2.5 as a function of relative humidity (RH) were determined and compared to the ambient RH ranges in the two cities. PM2.5 was found to be liquid to semisolid in Seoul but mostly semisolid to solid in Beijing. The liquid state was dominant on polluted days, while a semisolid state was dominant on clean days in Seoul. These findings can be explained by the aerosol liquid water content related to the chemical compositions of the aerosols at ambient RH; the water content of PM2.5 was much higher in Seoul than in Beijing. Furthermore, the overall phase states of PM2.5 observed in Seoul and Beijing were interrelated with the particle size distribution. The results of this study aid in a better understanding of the fundamental physical properties of aerosols and in examining how these are linked to PM2.5 in polluted urban atmospheres.

Abstract. Although knowledge of the physical state of aerosol particles is essential to understand atmospheric chemistry model and measurements, information on the viscosity and physical state of aerosol particles consisting of organic and inorganic salts is still rare. Herein, we quantified viscosities at 293 ± 1 K upon dehydration for the binary systems, sucrose–H2O and ammonium sulfate (AS)–H2O, and the ternary systems, sucrose–AS–H2O for organic-to-inorganic dry mass ratios (OIRs) = 4:1, 1:1, and 1:4 using bead-mobility and poke-and-flow techniques. Based on the viscosity value of the aerosol particles, we defined the physical states of the total aerosol particles studied in this work. For binary systems, the viscosity of sucrose–H2O particles gradually increased from ∼ 4 × 10−1 to > ∼ 1 × 108 Pa s when the relative humidity (RH) decreased from ∼ 81 % to ∼ 24 %, ranging from liquid to semisolid or solid state, which agrees with previous studies. The viscosity of AS–H2O particles remained in the liquid state (< 102 Pa s) for RH > ∼ 50 %, while for RH ≤∼ 50 %, the particles showed a viscosity of > ∼ 1 × 1012 Pa s, corresponding to a solid state. In case of the ternary systems, the viscosity of organic-rich particles (OIR = 4:1) gradually increased from ∼ 1 × 10−1 to ∼ 1 × 108 Pa s for a RH decrease from ∼ 81 % to ∼ 18 %, similar to the binary sucrose–H2O particles. This indicates that the sucrose–AS–H2O particles range from liquid to semisolid or solid across the RH. In the ternary particles for OIR = 1:1, the viscosities ranged from less than ∼ 1 × 102 for RH > 34 % to > ∼ 1 × 108 Pa s at ∼ 27 % RH. The viscosities correspond to liquid for RH > ∼ 34 %, semisolid for ∼ 34 % < RH < ∼ 27 %, and semisolid or solid for RH < ∼ 27 %. Compared to the organic-rich particles, in the inorganic-rich particles (OIR = 1:4), drastic enhancement in viscosity was observed as RH decreased; the viscosity increased by approximately 8 orders of magnitude during a decrease in RH from 43 % to 25 %, resulting in liquid to semisolid or solid in the RH range. Overall, all particles studied in this work were observed to exist as a liquid, semisolid, or solid depending on the RH. Furthermore, we compared the measured viscosities of ternary systems with OIRs of 4:1, 1:1, and 1:4 to the predicted viscosities using the Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients Viscosity model (AIOMFAC-VISC) predictions with the Zdanovskii–Stokes–Robinson (ZSR) organic–inorganic mixing model, with excellent model–measurement agreement for all OIRs.

Atmospheric aerosol particles are complex mixtures having various physicochemical properties. To predict the role and characteristics of such complex aerosol particles in air pollution and related atmospheric chemistry, our knowledge of the number and types of phases in complex aerosol particles should be improved. However, most studies on the phase behavior of aerosol particles have been conducted in the laboratory and have not used real-world aerosol particles. In this study, using a combination of optical microscopy and poke-and-flow technique, we investigated the number and types of phases of actual aerosol particles of particulate matter &lt; 2.5 µm (PM2.5) collected on heavily polluted days in Seosan, South Korea in winter 2020–2021. From the microscopic observations at 293 K, it showed that the PM2.5 particles exist in a single liquid phase at relative humidity (RH) &gt;∼85%, a liquid-liquid phase at ∼70% &lt; RH &lt;∼85%, a liquid-liquid-(semi)solid phase at ∼30% &lt; RH &lt;∼70%, and a (semi)solid phase at RH &lt;∼30% upon dehydration. This reveals that three phases of atmospheric aerosol particles coexisting as liquid-liquid and liquid-liquid-(semi)solid would be the most common phases in the atmosphere considering ambient RH ranges. These observations provide fundamental properties necessary for improved predictions of air quality and aerosol chemistry such as reactive uptake of N2O5, size distributions, and mass concentrations of aerosol particles.

Abstract. Although knowledge of the physical state of aerosol particles is essential to understand atmospheric chemistry model and measurements, information on the viscosity and physical state of aerosol particles consisting of organic and inorganic salts are still rare. Herein, we quantified viscosities at 293 ± 1 K upon dehydration for the binary systems, sucrose/H2O and ammonium sulfate (AS)/H2O, and the ternary systems, sucrose/AS/H2O for organic-to-inorganic dry mass ratios (OIRs) = 4 : 1, 1 : 1, and 1 : 4. For binary systems, the viscosity of sucrose/H2O particles gradually increased from ~6 × 10−1 to > ~1 × 108 Pa‧s when the relative humidity (RH) decreased from ~83 % to ~24 %, which agrees with previous studies. The viscosity of AS/H2O particles remained in the liquid state (< 102 Pa‧s) for RH > ~50 %, and for RH ≤ ~50 %, the particles showed viscosity of > ~1 × 1012 Pa‧s, corresponding to a solid state. For ternary systems, the viscosity of organic-rich particles (OIR = 4 : 1) gradually increased from ~4 × 10−2 to ~1 × 108 Pa‧s for a RH decrease from ~80 % to ~18 %, similar to the sucrose/H2O particles. In the particles for OIR = 1 : 1, the viscosities ranged smaller than ~4 × 101 for RH > 34 %, and > ~1 × 108 Pa‧s at ~27 % RH. Compared to the organic-rich particles, in the inorganic-rich particles (OIR = 1 : 4), drastic enhancement in viscosity was observed as RH decreased; the viscosity enhanced approximately 8 orders of magnitude in RH from 43 % to 25 %. Based on viscosity data, all particles studied in this work were observed to exist as a liquid, semi-solid or solid depending on the RH. Furthermore, we compared the measured viscosities of ternary systems with OIRs of 4 : 1, 1 : 1, and 1 : 4 to the predicted viscosities using the Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients Viscosity model (AIOMFAC-VISC) predictions with the Zdanovskii–Stokes–Robinson (ZSR)-style organic–inorganic mixing model, with excellent model–measurement agreement for all OIRs.

The morphology and phase state are critical physical properties of aerosol particles. However, studies related to the analysis of these properties primarily focus on laboratory experiments, and studies on real aerosol particles are limited. Herein, fine particulate matter (PM2.5) filter samples were obtained to investigate and compare the morphology and phase state of ambient aerosol particles in South Korea. The PM2.5 samples were collected in the summer of June 2021 from two different environments: Seoul (urban) and Seosan (coastal-rural). Optical microscopy was combined with the poke-and-flow technique to determine the morphology and phase state of the PM2.5 as a function of relative humidity (RH) at 293 ± 1 K. At both sites, the PM2.5 droplets, which were extracted in purified water, showed a multiphase nature that was dependent on the RH and chemical composition. Based on the results and ambient average RH in Seoul, most of the PM2.5 was observed in a liquid state on polluted days under an inorganic-dominant condition, but in a semisolid state on clean days under an organic carbon-rich condition. In Seosan, the PM2.5 predominantly existed in a liquid state, due to the high RH caused by proximity to the Yellow Sea. Our study provides fundamental physical properties of PM2.5 for both urban and coastal-rural environments. The results have strong applications for atmospheric chemistry and predicting particle size distributions.