Multi-isotope constraints on PM_2.5 source fingerprints across contrasting urban environments in South Korea under the...

In monsoon-influenced East Asia, PM_2.5 reflects the interplay of emission mixtures, seasonal meteorology, and atmospheric processing, yet these controls differ among urban environments. We analyzed ¹⁴C fraction modern (F¹⁴C) and δ¹³C of total carbon (TC), and δ¹⁵N of total nitrogen (TN) in 237 PM_2.5 samples collected from January 2021 to June 2022 across six contrasting urban environments in South Korea. Using a regime-based seasonal framework, we evaluate how seasonal changes in meteorology and chemistry alter aerosol composition, the fossil versus non-fossil split of carbon, and the separability of urban-specific source fingerprints. δ¹⁵N declined during nitrate-rich winter high-PM_2.5 episodes, providing a clear isotopic marker of episodic nitrate enhancement. Radiocarbon apportionment showed that fossil carbon increased from cold to warm season at the coastal port city Busan, whereas non-fossil carbon increased at the megacity Seoul, highlighting season-specific and urban-specific control priorities. δ¹³C converged near -25‰ in the cold season, but northern cities remained relatively ¹³C-enriched, consistent with stronger regional transport and heating-related influence. In the warm season, Busan showed the lowest F¹⁴C (0.5 ± 0.1) and δ¹³C values consistent with liquid fossil-fuel influence, whereas the inland basin city Chuncheon showed high F¹⁴C (0.7 ± 0.1) and more C₃-plant-like signatures. Together, these results show that winter observations are well suited for capturing high-PM_2.5 regional episodes but often compress local source contrasts through synoptic mixing, whereas warm-season observations more effectively resolve place-specific source signatures. The findings provide a regime-based framework for season- and location-targeted PM_2.5 mitigation in monsoon-affected East Asian cities.