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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. Unlabelled Image • The first long-term (2006–2016) GEM trend reported from a mountain site downwind of the East Asia continent • A significant decreasing GEM trend was found, comparable to decreasing trends observed in Europe and North America. • Reduction in atmospheric Hg export from East Asia is suggested to contribute to the decreasing GEM trend at LABS. [ABSTRACT FROM AUTHOR]

Concentrations of gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particle-bound mercury (PBM) were measured at a suburban site in northern Taiwan between October 2017 and September 2018. Mean concentrations of GEM, GOM, and PBM were 2.61 ± 6.47 ng m−3, 12.1 ± 34.3 pg m−3, and 18.7 ± 86.8 pg m−3, respectively. On 35% of the days, elevated atmospheric mercury (Hg) concentration occurred between midnight and early morning when local wind direction shifted and came from sectors between SSW and S, indicating influence of local anthropogenic emission sources. As such, a diurnal pattern with nighttime peaks was observed for GEM, GOM, and PBM. However, elevated GOM values were also observed around noon on 87 occasions, coinciding with peak O 3 and solar radiation, suggesting additional contribution from in situ photochemical reactions. Seasonally, atmospheric Hg concentrations were higher in spring than in the other seasons. Besides the contribution from local emissions and the East Asian outflow, lower wind speed, cumulative rainfall, and days of rain in spring suggesting poor air dispersion and inefficient pollutant removal, hence leading to the accumulation of atmospheric Hg and thus higher concentrations. Two trans-boundary haze events from China were encountered during this study. Concentrations of GEM and PBM were enhanced on haze days, with particularly significant enhancement in PBM. Comparison of the maximum PBM values during these haze events to respective pre-event PBM values showed enhancements of 378% and 1438%, whereas enhancements of 100% and 147% were observed for GEM. GOM remained relatively low and stable during haze events, which could be due to the favored partitioning of GOM toward solid phase under high PM 2.5 concentrations and the lack of sunlight to fuel the photochemical production of GOM. Backward trajectories indicated that Hg emissions in east China likely contributed to the enhancements in atmospheric Hg concentrations measured at the suburban site in northern Taiwan during these two trans-boundary haze events. • Both local sources and regional transport contributed to the measured atmospheric Hg. • Two trans-boundary haze events from China were encountered. • Concentrations of GEM and PBM were enhanced on haze days, but GOM remained low. • Yangtze River Delta and east China were the Hg source regions for these haze events. [ABSTRACT FROM AUTHOR]