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It is imperative to understand the changes of the levels of air pollutants in Northeast Asia with respect to the changes of the emissions of air pollutants. In this study, we analyzed long-term trends of both the chemical composition of fine particles and gaseous species in Seoul, a megacity, and Baengnyeong Island, a background area located in the Yellow Sea of Republic of Korea (South Korea) from 2012 to 2019. Long-term changes of the concentrations of air pollutants were analyzed using the Mann–Kendall trend test and Sen’s slope. Since the SO₂ emissions in this region have been significantly reduced during last decade, NO₃⁻ has become the major species of fine particles instead of SO₄²⁻. Seoul and Baengnyeong Island are rich in NH₄⁺ in the atmosphere, and due to the SO₂ emission reduction, the balance of ammonia–nitric acid–sulfuric acid has been changed, and the concentration of NO₃⁻ has increased. This trend is more obvious in Baengnyeong Island than Seoul due to the lower local emissions in Baengnyeong Island. As a result of this study, it is possible to confirm that concentrations of air pollutants and the majority of aerosols affecting PM2.5 concentrations in Northeast Asia have changed according to the changes in emissions in this region.