Significant seasonal changes in optical properties of brown carbon in the mid-latitude atmosphere

Abstract

<p><strong>Abstract.</strong> Atmospheric brown carbon (BrC) plays significant roles in the light absorption and photochemistry of the atmosphere. Although the occurrence and sources 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, Korea. Our results showed that the concentrations of BrC and WSOC decreased by approximately 80 % and 30 %, respectively, from the cold season (Oct–Jan) to the warm season (Jun–Sep). 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 K and V contents in the water-soluble aerosols during all seasons showed no measureable 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 significantly larger removals of ultraviolet (UV) degradable BrC. This was confirmed by laboratory UV radiation experiments on the optical property changes of BrC and WSOC in aerosol samples. Thus, our results suggest that the photo-degradation has dominant roles in controlling the quantity and quality of light-absorbing organic aerosols in the different seasons in the mid-latitude atmosphere.</p>