Long-term monitoring of ambient aerosols at a subtropical high-altitude mountain site in the western North Pacific

Aerosols exert a significant influence on the Earth's radiative balance. Black carbon (BC), a potent light-absorbing aerosol primarily generated from incomplete combustion of fossil fuels, biofuels, and biomass, has garnered substantial global research attention due to its substantial impact on regional and global climate change. However, long-term trends in aerosols in the western North Pacific remain poorly understood. Located at 2862 meters above sea level on Lulin Mountain in central Taiwan (23.47°N, 120.87°E), the Lulin Atmospheric Background Station (LABS) stands as the sole high-altitude background station in this region. Operational since the spring of 2006, LABS has been continuously monitoring the impact of various air pollutants through long-range transport. This study utilized continuous real-time measurements of PM₁₀ (2006-2016), PM_2.5 (2013-2020), and BC (2008-2020) collected at LABS using two tapered element oscillating microbalances (TEOM 1405) and an aethalometer (AE-31) to investigate their temporal variations, characteristics, and key controlling factors. Correlation analysis was employed to assess the influence of meteorological parameters on their monthly/seasonal burdens. The multi-year annual mean mass concentrations of PM₁₀, PM_2.5, and BC were determined to be 9.2, 7.2, and 0.4 µg m^-3, respectively. Concentration-weighted trajectory analyses identified northern peninsular Southeast Asia and mainland China as major long-distance source regions for all aerosols at LABS, particularly during spring (March-May) and the northeast monsoon season (October-November), respectively. A slight downward trend in the mass concentrations of ambient aerosols was observed at LABS. This decline may be attributed to a decrease in biomass burning emissions from peninsular Southeast Asia, recent energy policy changes in China, and alterations in regional atmospheric boundary layer dynamics.