Investigation of long-range transported PM_2.5 events over Northern Taiwan during 2005–2015 winter seasons

In-situ PM_2.5 observations and the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) aerosol reanalysis were analyzed to characterize long-range transported high PM_2.5 events over Northern Taiwan during winter seasons. MERRA-2 aerosol composition was evaluated using independent in-situ observations at the Cape Fuguei (CAFE) site. Results showed that MERRA-2 was able to distinguish the contribution of different species within complicated aerosol mixtures. Fifty transported high PM_2.5 events were identified during the winters of 2005–2015. To investigate the transport characteristics associated with Asian continental outflow, these events were further classified into sulfate-dominated, dust-dominated and mixed-composition events. More than 80% of transported events were influenced by Asian dust and 20% of them were dust-dominated. Both sulfate-dominated and dust-dominated events showed similar average PM_2.5 concentrations (~44 μg m⁻³), while dust-dominated events showed a higher average PM₁₀ concentration (114 μg m⁻³) and thus a lower PM_2.5/PM₁₀ ratio (0.41) compared to sulfate-dominated events (94 μg m⁻³ and 0.46, respectively). Results indicated that the low-level trough at 700 hPa plays a critical role in determining the transport paths of dust aerosols and their impact on local air quality. Therefore, the influence of Asian dust over Northern Taiwan cannot be neglected during winter seasons, and could potentially offset the effectiveness of emission controls and pollutant reductions. Such potential impacts could occur over vast areas affected by Asian continental outflow. The utilization of aerosol reanalysis provided valuable insights for long-range transport studies in the regions with complicated aerosol compositions.