The concentrations of atmospheric gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particle-bound mercury (particles with diameter smaller than 2.5 μm; PBM_2.5) 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 PBM_2.5 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 PBM_2.5 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.

Ground‐based measurement of total reactive nitrogen (NO_y), NO_y(g) (gas phase NO_x + HNO₃), and particulate NO₃⁻ was carried out at the Cape Hedo Atmosphere and Aerosol Monitoring Station (CHAAMS) in Okinawa, Japan, from spring to winter in 2006. The concentrations of NO_y, NO_y(g), and particulate NO₃⁻ were simultaneously high in spring but low in summer. This difference was mainly caused by air mass history, which was strongly associated with the typical weather pattern observed in the east Asian region for each season. The chemical transformation process of particulate NO₃⁻ during transport was examined using the data measured at Qingdao, China, in spring 2006. As the transport time of air masses increased, particulate NO₃⁻ continuously shifted from fine mode to coarse mode. It was found that the chemical transformation of particulate NO₃⁻ was mainly associated with the transport time of air masses, the geographical position of CHAAMS, and the transition from NH₄NO₃ to gas phase HNO₃. In air masses from Qingdao, China, the ratio of NO_y concentration observed at CHAAMS to that at Qingdao was about 0.1, which was lower than that of SO_y (SO₂ + nss‐SO₄²⁻). Sulfate was found in fine particles at CHAAMS in contrast to particulate NO₃⁻. As the lifetimes of NO_y and SO_y depend on the particle size, the difference in chemical transformation process during transport largely influences the abundance of transported NO_y. The variations of NO_y(g) and particulate NO₃⁻ were analyzed when dust plumes reached CHAAMS. The presence of dust causes the formation of particulate NO₃⁻ in coarse mode from NO_y(g) and an increase of its fraction in NO_y. The effect of volcanic activity on particulate NO₃⁻ concentration was also analyzed. It is suggested that particulate NO₃⁻ escaped to gas phase HNO₃ through the uptake of abundant volcanic H₂SO₄ by aerosols.

Daily PM_2.5 (particulate matter with aerodynamic diameter ≤2.5 μm) samples were simultaneously collected at two background sites (Wajima Air Monitoring Station (WAMS) and Fukue-Jima Atmosphere and Aerosol Monitoring Station (FAMS)) in Japan in the East Asian winter and summer monsoon periods of 2017 and 2019, to compare the characteristics of air pollutants among different regions and to determine the possible variation during the long-range transport process. Polycyclic aromatic hydrocarbons (PAHs), nitro-PAHs (NPAHs), and water-soluble inorganic ions (WSIIs) were analyzed. Despite the PM_2.5 concentrations at FAMS (8.90–78.5 µg/m³) being higher than those at WAMS (2.33–21.2 µg/m³) in the winter monsoon period, the average concentrations of ∑PAHs, ∑NPAHs, and ∑WSIIs were similar between the two sites. Diagnostic ratios indicated PAHs mainly originated from traffic emissions and mostly aged, whereas NPAHs were mostly secondarily formed during long-range transport. WSIIs at WAMS were mainly formed via the combustion process and secondary reactions, whereas those at FAMS mainly originated from sea salt and dust. Backward trajectories revealed the air masses could not only come from Asian continental coastal regions but also distant landlocked areas in the winter monsoon period, whereas most came from the ocean in the summer monsoon period. These findings can provide basic data for the establishment of prediction models of transboundary air pollutants in East Asia.