Long-term continuous observations and trend analyses of transboundary air pollutants from the Asian continent at Suzu on the Noto Peninsula, Japan

Long-term continuous observations of the concentrations of total odd nitrogen (NO_y), total nitrate (T.NO₃), CO, and O₃ were performed at Suzu on the Noto Peninsula in Japan for almost 14 years (November 2009 to April 2023). The long-term trends and seasonal variations of these concentrations were analyzed. Concentrations of NO_y, T.NO₃, CO, and O₃ showed decreasing trends throughout the observation period, which reflects recent reductions in emissions of air pollutants on the Asian continent. The average concentrations and number of high concentration outliers extracted by the trend analyses for NO_y, T.NO₃, CO, and O₃ were high in spring, whereas those in winter were high for CO and low for NO_y, T.NO₃, and O₃. These seasonal variations occurred because of wet removal of T.NO₃ by high precipitation on the Sea of Japan coast for NO_y and T.NO₃ and low photochemical activity for O₃ in winter. The percentage contributions of air masses of different origins observed for the high concentration outliers showed concentration variations of NO_y, T.NO₃, and O₃ at Suzu were influenced by air masses from Japan and the Asian continent.