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 (PM_2.5) in two megacities, Seoul and Beijing, we collected PM_2.5 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 PM_2.5 as a function of relative humidity (RH) were determined and compared to the ambient RH ranges in the two cities. PM_2.5 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 PM_2.5 was much higher in Seoul than in Beijing. Furthermore, the overall phase states of PM_2.5 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 PM_2.5 in polluted urban atmospheres.

Research on the viscosity and phase state of aerosol particles is essential because of their significant influence on the particle growth rate, equilibration times, and related evolution of mass concentration as well as heterogeneous reactions. So far, most studies of viscosity and phase state have been focused on organic aerosol particles, yet data on how viscosity can vary when the organic materials are mixed with inorganic salts remain scarce. Herein, using bead-mobility and poke-and-flow techniques, we quantified viscosities at 293 ± 1 K for binary mixtures of organic material $/$ H₂O and inorganic salts $/$ H₂O, as well as ternary mixtures of organic material $/$ inorganic salts $/$ H₂O over the atmospheric relative humidity (RH) range. Sucrose as the organic species and calcium nitrate (Ca(NO₃)₂) or magnesium nitrate (Mg(NO₃)₂) as the inorganic salts were examined. For binary sucrose $/$ H₂O particles, the viscosities gradually increased from ∼ 3 × 10⁻² to ≳1 × 10⁸ Pa s as RH decreased from ∼ 75 % to ∼ 25 %. Compared with the results for the sucrose $/$ H₂O particles, binary $\mathrm{Ca}({\mathrm{NO}}_{\mathrm{3}}{)}_{\mathrm{2}}/{\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ and $\mathrm{Mg}({\mathrm{NO}}_{\mathrm{3}}{)}_{\mathrm{2}}/{\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ particles showed drastic enhancements to ≳1 × 10⁸ Pa s at low RH close to the efflorescence RH. For ternary mixtures of sucrose $/$ Ca(NO₃)₂ $/$ H₂O or sucrose $/$ Mg(NO₃)₂ $/$ H₂O, with organic-to-inorganic mass ratios of 1:1, the viscosities of the particles gradually increased from ∼ 3 × 10⁻² to greater than ∼ 1 × 10⁸ Pa s for RH values from ∼ 75 % to ∼ 5 %. Compared to the viscosities of the $\mathrm{Ca}({\mathrm{NO}}_{\mathrm{3}}{)}_{\mathrm{2}}/{\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ particles, higher viscosities were observed for the ternary sucrose $/$ Ca(NO₃)₂ $/$ H₂O particles, with values increased by about 1 order of magnitude at 50 % RH and about 6 orders of magnitude at 35 % RH. Moreover, we applied a thermodynamics-based group-contribution model (AIOMFAC-VISC, Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients Viscosity) to predict aerosol viscosities for the studied systems. The model predictions and viscosity measurements show good agreement within ∼ 1 order of magnitude in viscosity. The viscosity measurements indicate that the studied mixed organic–inorganic particles range in phase state from liquid to semi-solid or even solid across the atmospheric RH range at a temperature of 293 K. These results support our understanding that organic $/$ inorganic $/$ H₂O particles can exist in a liquid, semisolid, or even a solid state in the troposphere.

Nitrate has become the primary inorganic compound in urban aerosol particles, but its effects on particle phase state, which is crucial in multiphase chemistry, remains largely unknown. Herein, particle rebound measurements were conducted to explore the relationship between the liquid–phase–transition threshold relative humidity (RH_threshold) and the inorganic compounds mass fraction in dry particles (F_inorg). Results revealed negative correlations between RH_threshold and F_inorg, with more nitrate leading to lower RH_threshold. Even with RH < 20%, particles with ∼50% nitrate mass fraction remained in non‐solid state. Taking Beijing as an example, decreases were observed in RH_threshold from 64% in 2015 to below 53% nowadays during the moderate‐pollution periods (PM_2.5 = 35–75 μg/m³) due to an enhanced nitrate fraction. This allowed urban aerosol particles to exist in liquid state at lower RH, and consequently, kinetic limitation by bulk diffusion in nitrate‐dominated particles might be negligible, making them key seeds for secondary aerosol formation through multiphase reactions.

Atmospheric ammonia (NH₃) plays a critical role in PM_2.5 pollution. Data on atmospheric NH₃ are scanty; thus, the role of NH₃ in the formation of ammonium ions (NH₄⁺) in various environments is understudied. Herein, we measured concentrations of NH₃, PM_2.5, and its water-soluble SO₄²⁻, NO₃⁻, and NH₄⁺ ions (SNA) at an urban site in Jeonju, South Korea from May 2019 to April 2020. During the measurement period, the average concentrations of NH₃ and PM_2.5 were 10.5 ± 4.8 ppb and 24.0 ± 12.8 μg/m³, respectively, and SNA amounted to 4.3 ± 3.1, 4.4 ± 4.9, and 1.6 ± 1.8 μg/m³, respectively. A three-dimensional photochemical model analysis revealed that a major portion of NH₃, more than 88%, originated from Korea. The enhancement of the ammonium-to-total ratio of NH₃, NH_X (NHR = [NH₄⁺]/[NH₄⁺] + [NH₃]) was observed up to ~0.61 during the increase of PM_2.5 concentration (PM_2.5 ≥ 25 μg/m³) under low temperature and high relative humidity conditions, particularly in winter. The PM_2.5 and SNA concentrations increased exponentially as NHR increased, indicating that NH₃ contributed significantly to SNA formation by gas-to-particle conversion. Our study provided experimental evidence that atmospheric NH₃ in the urban area significantly contributed to SNA formation through gas-to-particle conversion during PM_2.5 pollution episodes.