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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.

The physicochemical and optical properties of combustion-generated particles from various sources were investigated. Coal-fired power plants, charcoal kilns, automobiles, and ship engines were the major sources, representing combustions of coal, biomass, and two different types of diesel, respectively. Scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and energy-dispersive X-ray spectroscopy (EDX) equipped with both SEM and HRTEM were used for physicochemical analysis. Light-absorbing properties were assessed using a spectrometer equipped with an integrating sphere. Particles generated from different combustion sources and conditions demonstrate great variability in their morphology, structure, and composition. From coal-fired power plants, both fly ash and flue gas were mostly composed of heterogeneously mixed mineral ash spheres, suggesting that complete combustion released carbonaceous species at high temperatures (1200–1300 °C). Both automobile and ship exhaust from diesel combustion show the typical features of soot: concentric circles comprising closely packed graphene layers. However, heavy fuel oil (HFO)-combusted particles from ship exhaust demonstrate more complex compositions containing a different morphology of particles than soot; for example, spherically shaped char particles composed of minerals and carbon. Regarding soot aggregates, particles from HFO combustion have different chemical compositions; carbon dominates but Ca (29.8%), S (28.7%), Na (1%), and Mg (1%) are also present, which were not found in particles from automobile emission. This indicates that fuel properties and combustion conditions are important in determining the fate of particles. Finally, from biomass combustion, amorphous and droplet-like carbonaceous particles with no crystalline structure were observed, and are generally formed by the condensation of low-volatile species at low-temperature (∼300–800 °C) combustion conditions. Significant differences in optical properties depending on the combustion source were observed. Diesel combustion particles from automobile and ship showed wavelength-independent absorbing properties, whereas the particles from coal and charcoal kiln combustion showed enhanced absorption at shorter wavelengths, which is a characteristic of brown carbon. Main light absorbers at short wavelengths are proposed for organics and minerals (e.g., iron oxide) for biomass and coal combustion, respectively. Our findings suggest that source-dependent properties and distributions across the globe should be considered when their impacts on climate change and air qualities are discussed.