Temporal changes in atmospheric mercury concentrations at a background mountain site downwind of the East Asia continent in 2006–2016

Atmospheric mercury (Hg) has been monitored at the Lulin Atmospheric Background Station (LABS) in Taiwan since April 2006 and is still continuing. Here we reported the trend in gaseous elemental Hg (GEM) concentrations at LABS between April 2006 and December 2016, before the Minamata Convention on Mercury entered into force in 2017. Previous research indicated nighttime (0–8 am) data collected at LABS are better representative of regional influence. Therefore, only nighttime GEM data were used for trend analysis. A significant decreasing trend in GEM at a rate of −1.5% yr⁻¹ (−0.022 ng m⁻³ yr⁻¹, p < 0.01) was found, comparable to the decreasing trends observed in Europe, North America, South Africa, and over the North Atlantic Ocean. Five major GEM source regions to the LABS were identified, including northern Indochina Peninsula, China, Northeast Asia, the Pacific Ocean, and South China Sea. Significant decreasing trends in GEM were found for air masses coming from northern Indochina Peninsula (−0.042 ng m⁻³ yr⁻¹, −2.6% yr⁻¹, p < 0.01), China (−0.041 ng m⁻³ yr⁻¹, −2.4% yr⁻¹, p < 0.01), Northeast Asia (−0.031 ng m⁻³ yr⁻¹, −2.0% yr⁻¹, p < 0.05), and the Pacific Ocean (−0.022 ng m⁻³ yr⁻¹, −1.7% yr⁻¹, p < 0.05). Decreasing GEM trend (−0.020 ng m⁻³ yr⁻¹, −1.5% yr⁻¹), but insignificant (p > 0.05), was also found for air masses coming from South China Sea. The decreasing trends observed with air from the Pacific Ocean and South China Sea indicated declining background GEM concentrations in Northern Hemisphere. Decrease in GEM concentrations at the LABS was in agreement with the reduction in atmospheric Hg export from the East Asia continent caused by changes in Hg emission quantity and speciation, and temporal and spatial distribution in emission sources that have been suggested by recent research. Additionally, changes in the frequency distribution of air mass origins and transport paths may also contribute to the changes in GEM concentrations at LABS.