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The air quality of the megacities in populated and industrialized regions like East Asia is affected by both local and regional emission sources. The combined effect of regional transport and local emissions on multiday haze was investigated through a synthetic analysis of PM2. 5 sampled at both an urban site in Seoul, South Korea and an upwind background site on Deokjeok Island over the Yellow Sea during a severe multiday haze episode in late February 2014. Inorganic components and carbonaceous species of daily PM2. 5 samples were measured, and gaseous pollutants, local meteorological factors, and synoptic meteorological conditions were also determined. A dominance of fine-mode particles (PM2. 5 ∕ PM10 ∼ 0.8), a large secondary inorganic fraction (76 %), high OC ∕ EC (> 7), and highly oxidized aerosols (oxygen-to-carbon ratio of ∼ 0.6 and organic-mass-to-carbon ratio of ∼ 1.9) under relatively warm, humid, and stagnant conditions characterize the multiday haze episode in Seoul; however, the early and late stages of the episode show different chemical compositions of PM2. 5. High concentrations of sulfate in both Seoul and the upwind background in the early stage suggest a significant regional influence on the onset of the multiday haze. At the same time, high concentrations of nitrate and organic compounds in Seoul, which are local and highly correlated with meteorological factors, suggest the contribution of local emissions and secondary formation under stagnant meteorological conditions to the haze. A slow eastward-moving high-pressure system from southern China to the East China Sea induces the regional transport of aerosols and potential gaseous precursors for secondary aerosols from the North China Plain in the early stage but provides stagnant conditions conducive to the accumulation and the local formation of aerosols in the late stage. A blocking ridge over Alaska that developed during the episode hinders the zonal propagation of synoptic-scale systems and extends the haze period to several days. This study provides chemical insights into haze development sequentially by regional transport and local sources, and shows that the synoptic condition plays an important role in the dynamical evolution of long-lasting haze in the Asian continental outflow region.

The spectral properties of light-absorbing organic aerosol extractions were investigated using 24-h average fine particulate matter (PM2.5) measurements from October 2012 to September 2013 in Seoul, Korea. The light-absorption spectra of water and methanol extracts exhibited strong evidence of brown carbon with Absorption Angstrom Exponent (AAE; fitted between 300 and 700 nm) ranges of 5.84–9.17 and 4.08–5.75, with averages of 7.23 ± 1.58 and 5.05 ± 0.67, respectively. The light absorption of both extracts at 365 nm (Abs365), which is typically used as a proxy for brown carbon (BrC), displayed strong seasonal variations and was well correlated with both water-soluble organic carbon (WSOC; r = 0.81) and organic carbon (OC; r = 0.85), indicating that both primary and secondary organics were sources of BrC in this region. Normalizing the Abs365 of water and methanol extracts to the mass of WSOC and OC yielded average solution mass absorption efficiency (MAE365) of 0.28–1.18 and 0.44–1.45 m2 g−1 C, respectively. MAE365 in Korea were in the same range or slightly lower than those in China, however, despite the same ranges, the seasonal variations were different, suggesting that the sources of light absorbers could be different. Combining the AAE, Abs365, and MAE365 of both extracts and a detailed chemical speciation of filter extracts identified the compounds responsible for the temporal variations of BrC in Korea. During summer, secondary organic aerosol (SOA), photochemically generated from anthropogenic emissions, was the major source; however, during winter, long range transported organics or transported BrC seem to be a source of BrC in Korea, a downwind site of China, where severe smog and BrC were observed during this season. Biomass burning was also an important source; however, unlike in previous studies, where it was identified as a major source during winter, here, it contributed during the whole year. Although many of its properties, sources, and potential impacts remain unknown, this is the first result showing that BrC was prevalent in the PM2.5 fraction in Seoul throughout the year. Also this suggest that BrC and its sources can be long range transported, emphasizing that the optical effect of BrC might be important in East Asia.

Particle-phase concentrations were measured at 10, 80, and 200 m from the roadside of a national highway near Seoul in January and May 2008. The highway has two lanes each way, with an average hourly traffic volume of 1,070 vehicles. In January 2008, PM 10 concentrations decreased from 10 to 80 m but increased at 200 m. Black carbon (BC) decreased only slightly with distance due to the influence of biomass burning and open burning from the surrounding areas. In May 2008, the effect of secondary formation on both PM 10 and PM 2.5 was significant due to high temperatures compared with January. Because on-road emissions had little effect on secondary formation for a short time, variations in PM 10 concentrations became smaller, and PM 2.5 concentrations increased with distance. The effects of fugitive dust on PM concentrations were greater in May than in January when the mean temperature was below freezing. In the composition variations, the amounts of primary ions, organic carbon (OC), and BC were larger in January, while those of secondary ions and others were larger in PM 10 , as well as PM 2.5 in May.

The physico-chemical characteristics and nanostructure of the aerosol samples from a coal-fired power plant, a charcoal kiln and diesel vehicles were investigated with focusing on black carbon (BC). Aerosols from the coal-fired power plant were mostly comprised of mineral ash spheres which are heterogeneously mixed. The main components of the aerosols from coal-fired power plant were calcium compounds, iron oxide, alumino-silicate without BC. The typical combustion-generated BC which shows the shape of bunch of grapes with 20~50 nm particles which were detected in aerosol particles from diesel vehicles. The nanostructure of each BC particle shows the shape of concentric circles which is comprised of closely-packed graphene layers. Aerosols from charcoal kiln were likely condensed organic carbon generated from the low-temperature combustion process.