Characteristics of water-soluble organic aerosols in Seoul, Korea

Abstract

Water-soluble organic matter (WSOM) in aerosols plays a significant role in atmospheric processes affecting on global climate system directly by absorbing and scattering solar radiation and indirectly by acting as cloud condensation nuclei (CCN) and as potential sources for further reactions. The light absorbing organic carbon in aerosol WSOM including brown carbon (BrC) and humic-like substances (HULIS) also has a significant impact on climate forcing by absorbing the ultraviolet (UV) wavelengths and visible spectrum. However, despite the potential importance of aerosol WSOM to global biogeochemical cycle and climate system, current understanding of its chemical compositions, optical properties, and processes are still poorly understood. Therefore, the objective of this study is to identify the characteristics of aerosol WSOM using the concentrations of water-soluble organic carbon (WSOC), water-soluble organic nitrogen (WSON), inorganic ion species, stable carbon isotope ratios (δ13CWSOC), and fluorescence techniques at an urban metropolitan city, Seoul, Korea from March 2015 to January 2016. A seasonal variation in the WSOC concentration ranged from 3.1 to 39.5 μg m-3 (mean: 16.1 μg m-3) with higher values during cold seasons (Mar–May and Oct–Jan) and lower values during warm seasons (Jun–Sep). The water-soluble organic nitrogen (WSON) concentrations ranged from 0.8 to 81.7 μg m-3 (mean: 14.5 μg m-3) with large variations over different seasons. The δ13CWSOC values ranging from –21.0 to –27.5‰ (mean: –24.0‰) had contrasting pattern in their seasonal variation compared to the WSOC concentration, which fall within the range of terrestrial C3 plants (–24.0 to –37.0‰) and anthropogenic fossil combustion sources (–25.0 to –32.0‰). In addition, a 10-day air mass back trajectory showed that most of air masses are transported from the East Asian continents except for summer indicating that continental outflow may become a powerful source influencing the organic carbon signature in Seoul. The fluorescence property of aerosol WSOM and WSOC concentration exhibited a similar seasonal variation indicating the greatest similarity in their atmospheric process through a year. As the UV radiation intensity increases more than two folds during the summer (Jun–Sep), the fluorescence intensities of humic compounds in aerosol WSOM decreased approximately by 80% and the total WSOC concentration decreased approximately by 30% suggesting that humic fraction is more susceptible to UV irradiation. A linear relationship between the fluorescence intensity of humic components and UV radiation rate (r2=0.4) indicates that atmospheric HULIS is predominantly controlled by photo-induced degradation. These results were also in good agreement with the experiment result. The results suggest that aerosol WSOM in Seoul was mainly derived from a local terrestrial and anthropogenic fossil combustion sources. A significant change in fluorescence property of chromophores in aerosol WSOM was predominantly influenced by photo-induced degradation. The photochemical degradation plays a significant role in atmospheric optical property and might be an important sink for light absorbing organic compounds in ambient urban aerosols.