Relationships between atmospheric mercury and optical properties of spring outflow aerosols from Southeast Asia

Southeast Asia is a major biomass burning (BB) source area and BB has been identified as an important source of mercury (Hg) and aerosols to the atmosphere. Previous studies found correlations between atmospheric Hg and Delta-C (BC₃₇₀–BC₈₈₀) during the BB episodes in low-elevation BB source areas; however, whether this relationship persists during long-range transport remains unknown. Lulin Atmospheric Background Station (LABS), a high mountain background site in central Taiwan, has monitored the impact of BB long-range transports from Southeast Asia on atmospheric Hg and aerosols in downwind areas since 2006 and 2008, respectively. As such, these data provide an opportunity to investigate the relationship between atmospheric Hg (PBM and GOM) and aerosol optical properties, such as Delta-C and absorption Ångström exponent (AAE_370-880). In this study, three long-range transport BB events were observed at LABS in the springtime during 2014–2016. Significant positive correlations with PBM/GOM were observed for Delta-C and (R² = 0.34–0.51) and AAE_370-880 (R² = 0.41–0.54) during the BB events, while poor correlations were shown under non-BB episodes. Additionally, the positive correlation between PBM and AAE_370-880 suggests that the Hg(II) may be preferably absorbed onto organic aerosols or originated from the same emission sources. This study demonstrated that AAE_370-880 can be used to identify the source of Hg(II) from BB long-range transport. In addition, our results also indicated the influences of aerosol chemical compositions on the partitioning of Hg(II), which is important to further the understanding of atmospheric Hg cycling.