Morphology and phase state of PM2.5 in urban and coastal-rural areas during summer

Abstract

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.

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

“…As the RH continued to decrease, the PM 2.5 droplets, previously phase separated into liquid–liquid droplets, underwent abrupt crystallization of a (inorganic) portion of their material, resulting in the coexistence of liquid–liquid-(semi)­solid PM 2.5 droplets (Figure ). This morphology was consistent with similar cases reported in previous studies. ,, Based on the previous studies, we defined the single-liquid and liquid–liquid PM 2.5 droplets as “liquid” in terms of the phase state and the liquid–liquid-(semi)­solid multiphase PM 2.5 droplets as “semisolid”., ,, …”

“…To expand and characterize the physical properties of PM 2.5 in Northeast Asia, we investigated a total of 92 PM 2.5 filter samples comprising previous and present samples from Beijing, Seoul, Seosan, and Ulaanbaatar during the winter, summer, and spring from 2020 to 2022. However, due to the low surface tension of some samples collected from Beijing and Ulaanbaatar in summer and spring, our optical microscopic approaches on a hydrophobic substrate were unable to analyze the phase state of these PM 2.5 droplets, as previously reported. , Tables S1 and S3 present information on data availability and the number of samples, respectively. During the entire measurement, the chemical compositions of PM 2.5 and meteorological parameters were also measured (see Section S1).…”

“…As a part of the Fine Particle Research Initiative in East Asia considering National Differences (FRIEND) project, which was launched in 2020 to investigate the mechanisms and characteristics of aerosol pollution in Northeast Asia, ,− we investigated the physicochemical properties of PM 2.5 in four different sites in Northeast Asia: Beijing (40.25° N, 116.19° E), Seoul (37.61° N, 126.93° E), Seosan (36.77° N, 126.49° E), and Ulaanbaatar (47.91° N, 106.94° E) (Figure S1). The measurement sites in Beijing, Seoul, and Ulaanbaatar are in urban environments, − whereas the site in Seosan is in a rural area. , Concurrent field measurements were conducted in these four cities with daily collection of PM 2.5 samples covering three seasons (winter, summer, and spring) between 2020 and 2022 (Tables S1 and S2, Figure S2).…”

“…In total, 92 PM 2.5 filter samples were analyzed to determine their phase states. 44 PM 2.5 filter samples were collected during this study and 48 were used from previous studies. ,, …”

“…Optical observations were conducted to investigate the liquid phase state of PM 2.5 droplets at 290 ± 1 K. ,, PM 2.5 filters were extracted in purified water (resistivity ≥18.2 MΩ·cm, Millipore, USA) to produce droplets. Previous studies have shown that water-soluble species constitute >∼70% of the PM 2.5 mass. , The extraction procedure for filter samples has been described in earlier studies. ,, Briefly, the PM 2.5 extracts were nebulized onto a hydrophobic substrate (Hampton Research, Canada) using a nebulizer (MEINHARD, 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.…”