Comparison of Phase States of PM_2.5 over Megacities, Seoul and Beijing, and Their Implications on Particle Size Distribution

Abstract

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.

Influence of Relative Humidity and Composition on PM2.5 Phases in Northeast Asia

“…44 PM 2.5 filter samples were collected during this study and 48 were used from previous studies. 35,43,45 PM 2.5 filter samples were collected daily from 10:00 a.m. to 09:00 a.m. the following day using high-volume air samplers in Ulaanbaatar, Beijing, Seoul, and Seosan. The samplers employed were the SIBATA HV-RW model (Japan) for Ulaanbaatar and the SIBATA HV-1000R model (Japan) for Beijing, Seoul, and Seosan.…”

“…52,53 The extraction procedure for filter samples has been described in earlier studies. 35,45,54 Briefly, the PM 2.5 extracts were nebulized onto a hydrophobic substrate (Hampton Research, Canada) using a nebulizer (MEIN-HARD, PerkinElmer, USA). Subsequently, the substrate was placed in a flow-cell capable of controlling the RH (and temperature) for optical observation and poke-and-flow experiments.…”

“…Numerous studies using laboratory-made aerosol particles have shown that they can exist in liquid, semisolid, and/or solid states depending on the closely related aerosol chemical composition, RH, and temperature. − However, only a few studies have focused on determining the phase states of real-world aerosol particles (such as within the PM 2.5 size range) due to technical limitations and the need to account for various atmospheric conditions. Recent studies have shown that PM 2.5 in near-surface air (such as within the planetary boundary layer) can exist in the liquid, semisolid, or solid state depending on the environmental thermodynamic conditions (primarily temperature and RH). − In Beijing, PM 2.5 occurs predominantly in the liquid state during heavy air pollution and associated SOA formation episodes . In contrast, in Taizhou, a rural area, POA-dominated PM 2.5 was present in the semisolid state during biomass burning events .…”

Predicting Atmospheric Particle Phase State Using an Explainable Machine Learning Approach Based on Particle Rebound Measurements

“…21,22 These methods were used to detect the viscosity of field particles. 23,24 Based on the laboratory measurements, several semiempirical methods have also been developed to estimate the glass-transition temperature (T g ), and the particle phase state was determined by comparing T g with ambient temperature. 14,25,26 The Aerosol Inorganic−Organic Mixtures Functional groups Activity Coefficients Viscosity (AIOMFAC-VISC) model was used to calculate the viscosity of particles by which the particle phase state can be further determined.…”

“…Over the past two decades, numerous methods have been developed to characterize the particle phase state or viscosity . The dynamic viscosity of laboratory-generated particles was quantified using a series of offline measurement techniques, such as bead-mobility technique, poke-and-flow technique, and measurement using optical tweezers. , These methods were used to detect the viscosity of field particles. , Based on the laboratory measurements, several semiempirical methods have also been developed to estimate the glass-transition temperature ( T g ), and the particle phase state was determined by comparing T g with ambient temperature. ,, The Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients Viscosity (AIOMFAC-VISC) model was used to calculate the viscosity of particles by which the particle phase state can be further determined . As a unique online method, particle rebound measurements are conducted to identify the phase state of atmospheric particles in forests, rural areas, and polluted megacities .…”