Atmospheric Mercury Dynamics and Climatic Regulation at a Subtropical High-Altitude Background Station in East Asia

Dynamics and cycling of atmospheric mercury (Hg) are sensitive to meteorological factors and climate-driven changes. Concentrations and speciation of atmospheric Hg have been monitored since April 2006 at the Lulin Atmospheric Background Station (LABS; 120.87ºE, 23.47ºN, 2862 m a.s.l.), a subtropical high-altitude background site in central Taiwan. Multiple methods, including backward trajectory, concentration-weighted trajectory (CWT), generalized additive model (GAM), ensemble empirical mode decomposition (EEMD), Hilbert−Huang transform (HHT), and Hg isotope analysis, have been applied to explore the source-receptor relationship and multiscale temporal variations of gaseous elemental Hg (GEM) at LABS. East Asia continent and Peninsula Southeast Asia are identified as the major source regions of atmospheric Hg. Diurnal, monthly, annual, and inter-annual GEM cycles were identified. Daily GEM variability at the LABS is controlled by the local upslope movement of boundary layer air, whereas monthly/seasonal variability is driven by regional air mass origins and transport paths, i.e., monsoonal activity. The amplitude of the GEM concentration inter-annual variability (IAV) is greater than those of diurnal and seasonal variabilities, highlighting the importance of GEM IAV and the associated driving factors (e.g., ENSO). Large-scale atmospheric circulation plays an important role in modulating GEM IAV at LABS. Furthermore, the relationship between ENSO and GEM is sensitive to extreme events (e.g., 2015−2016 El Niño), resulting in perturbation of the long-term trend and atmospheric Hg cycling. Future climate change will likely increase the number of extreme El Niño events and, hence, could alter atmospheric Hg cycling and influence the effectiveness of the Minamata Convention on Mercury.