Abstract
Abstract. Atmospheric brown carbon (BrC) plays significant roles in the light absorption and photochemistry of the atmosphere. Although the sources and occurrences of BrC have been studied extensively, its removal processes and optical characteristics in the atmosphere have been poorly understood. In this study, we examined the seasonal changes in sources and sinks of BrC and water-soluble organic carbon (WSOC) in the atmosphere of Seoul, South Korea. Our results showed that the concentrations of BrC and WSOC decreased by approximately 80 % and 30 %, respectively, from the cold season (October–January) to the warm season (June–September). Excitation–emission matrix (EEM) spectra showed that the humic-like substance (HULIS) was the dominant fraction of BrC as the other components were not measurable. The air mass back trajectories of fire burning practices and the variations in non-crustal potassium (K) and vanadium (V) contents in the water-soluble aerosols during all seasons showed no measurable decrease in input of biomass-burning sources in summer. However, there was a significant shift in photo-resistivity of light-absorbing organic aerosols in the summer, indicating larger removals of ultraviolet (UV) degradable BrC. This trend is supported by laboratory UV radiation experiments on the optical property changes of BrC and WSOC in aerosol samples. Thus, our results suggest that the photodegradation has dominant roles in controlling the quantity and quality of light-absorbing organic aerosols in the different seasons in the midlatitude atmosphere.
Highlights
Organic aerosols play a significant role in atmospheric chemistry and the global climate system (Ghan and Schwartz, 2007; Duarte and Duarte, 2013; Kirillova et al, 2014a)
We assumed that the humic-like substance (HULIS) (C1 and component 3 (C3)) was the dominant component of brown carbon (BrC) in these samples, and component 1 (C1) was used as a representative component of BrC in this study, as C1 and C3 exhibited a good correlation (r2 = 0.8; p < 0.05) (Fig. S3)
The seasonal variations in optical and chemical properties of the HULIS and water-soluble organic carbon (WSOC) were monitored in different seasons in the urban region
Summary
Organic aerosols play a significant role in atmospheric chemistry and the global climate system (Ghan and Schwartz, 2007; Duarte and Duarte, 2013; Kirillova et al, 2014a). Most organic aerosols consist of a significant fraction of carbonaceous organic aerosols absorbing radiation (Laskin et al, 2015) These light-absorbing organic aerosols, referred to as brown carbon (BrC), have attracted increasing attention owing to their significant roles in the radiative forcing of the global climate system by directly absorbing solar radiation (Andreae and Gelencsér, 2006; Graber and Rudich, 2006; Ramanathan et al, 2007; Kirillova et al, 2014a; Saleh et al, 2014) and indirectly acting as cloud condensation nuclei (CCN) for cloud formation (Kanakidou et al, 2005; Andreae and Gelencsér, 2006; Graber and Rudich, 2006; Kirillova et al, 2014a). Previous studies have estimated that the BrC contributes to approximately 19 % of the total atmospheric radiative forcing (Feng et al, 2013)
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