Trend and inter-annual variability of atmospheric mercury concentrations at a mountain site in Taiwan

Concentrations of gaseous elemental mercury (GEM) have been monitored at the Lulin Atmospheric Background Station (LABS; 120.87ºE, 23.47ºN, 2862 m a.s.l.), a high mountain forest site in central Taiwan, since April 2006. A significant decreasing trend in GEM at a rate of -1.1% yr^-1 (-0.018 ng m^-3 yr^-1) was found between 2007 and 2022, comparable to the decreasing trends observed in other regions. Multiscale temporal variations of GEM concentrations were studied and distinguished by the application of the Hilbert-Huang transformation (HHT). 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 seasonal variability is driven by regional air mass origins and transport paths. 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. The IAV cycles for the SOI were similar in frequency to the GEM IAV cycles but negatively correlated, revealing the dependency of GEM IAV on climatology variations (e.g., ENSO). Large-scale atmospheric circulation played an important role in modulating GEM IAV. In El Niño years, the westerly was enhanced with more air masses passing India, northern Indochina Peninsula, and southwest and southeast China before arriving LABS, causing elevated levels of atmospheric Hg. On the other hand, the westerly weakened and deviated from Taiwan in La Niña years. This caused more air masses from the Pacific Ocean, resulting in lower atmospheric Hg concentrations. 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 evaluation of the Minamata Convention on Mercury.