Two-decade study of monsoon-driven transport and source signatures of polycyclic aromatic hydrocarbons at a Japanese background site

Long-range transport of particle-bound polycyclic aromatic hydrocarbons (PAHs) under the East Asian monsoon regime poses significant challenges to regional air quality management, yet research on how human activities influence the spatiotemporal dynamics of PAHs remains limited. Through 20 years (2004–2024) of monitoring at a background site within the East Asian monsoon region, this study analyzed the temporal variations of particulate PAHs and their sources using backward trajectory and positive matrix factorization models. The mean PAHs concentration (426.1 ± 399.5 pg/m³) reflected regional atmospheric background values, exhibiting distinct characteristics of winter maxima and summer minima. This seasonality was driven primarily by monsoon-mediated transport of pollution plumes from East Asian hotspots. Coal combustion (CC; 41.1 %) and vehicle emissions (VE; 29.9 %) emerged as dominant sources, displaying inverse seasonal patterns: CC contributions peaked during winter under continental outflow driven by the East Asian Winter Monsoon, while VE dominated summer months coinciding with strengthened East Asian Summer Monsoon transporting Japanese emissions. Significant reductions in PAHs concentration between 2012 and 2024 tracked policy-driven declines in CC and VE contributions across East Asia. Nevertheless, contributions of biomass burning and non-road mobile machinery and coking emissions to PAHs underscore unresolved regional challenges. This work demonstrates the cumulative benefits of regional air quality governance in mitigating transboundary PAHs pollution, delivering a benchmark dataset and important insight for optimizing multinational mitigation strategies in East Asia.