Abstract
The concentrations of atmospheric gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particle-bound mercury (particles with diameter smaller than 2.5 μm; PBM2.5) were continuously observed for a period of over 10 years at Cape Hedo, located on the north edge of Okinawa Island on the border of the East China Sea and the Pacific Ocean. Regional or global scale mercury (Hg) pollution affects their concentrations because no local stationary emission sources of Hg exist near the observation site. Their concentrations were lower than those at urban and suburban cities, as well as remote sites in East Asia, but were slightly higher than the background concentrations in the Northern Hemisphere. The GEM concentrations exhibited no diurnal variations and only weak seasonal variations, whereby concentrations were lower in the summer (June–August). An annual decreasing trend for GEM concentrations was observed between 2008 and 2018 at a rate of −0.0382 ± 0.0065 ng m−3 year−1 (−2.1% ± 0.36% year−1) that was the same as those in Europe and North America. Seasonal trend analysis based on daily median data at Cape Hedo showed significantly decreasing trends for all months. However, weaker decreasing trends were observed during the cold season from January to May, when air masses are easily transported from the Asian continent by westerlies and northwestern monsoons. Some GEM, GOM, and PBM2.5 pollution events were observed more frequently during the cold season. Back trajectory analysis showed that almost all these events occurred due to the substances transported from the Asian continent. These facts suggested that the decreasing trend observed at Cape Hedo was influenced by the global decreasing GEM trend, but the rates during the cold season were restrained by regional Asian outflows. On the other hand, GOM concentrations were moderately controlled by photochemical production in summer. Moreover, both GOM and PBM2.5 concentrations largely varied during the cold season due to the influence of regional transport rather than the trend of atmospheric Hg on a global scale.
Highlights
Toxic trace metals, including mercury (Hg), cadmium (Cd), and lead (Pb), that are emitted from anthropogenic and natural sources, exist in the natural environment for long periods of time without degradation
It was inferred that the concentrations of Hg at Cape Hedo were elevated due to regional pollution because there are no local Hg stationary emission sources around this site
The monitoring of atmospheric Hg concentrations started at several sites in Western Europe and North America in the 1980s; since the activity has expanded all over the world
Summary
Toxic trace metals, including mercury (Hg), cadmium (Cd), and lead (Pb), that are emitted from anthropogenic and natural sources, exist in the natural environment for long periods of time without degradation. GEM concentrations at Zeppelin station, Ny-Alesund, Svalbard showed no distinct annual trend during the period from 2000 to 2009 [22] These flat and/or decreasing trends of atmospheric Hg concentrations are inconsistent with the slightly increasing trend of anthropogenic emissions worldwide [23]. An improved global Hg suggested that the decline in atmospheric Hg concentrations, at least in the Northern hemisphere, was emission inventory for the period from 1990 to 2010 revealed that the observed decline in caused by the decreasing Hg concentrations in the subsurface of the North Atlantic Ocean. Long-term monitoring activities are necessary for evaluation of the from 1995 until now; Asia was responsible for more than 50% of the global anthropogenic emissions continuous influence of Asian emissions and to gauge the magnitude of the outflow of atmospheric in 2010 [27]. The Lulin Atmospheric influence of Asian emissions and to gauge the magnitude of the outflow of atmospheric Hg from
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