Understanding the mechanisms of coastal PM_2.5 formation driven by land-sea breeze recirculation and ship emissions

Coastal cities experience persistent air pollution episodes driven by mesoscale circulation such as land-sea breezes, which can recirculate pollutants and intensify near-surface concentrations. This study quantitatively evaluated the impact of pollutant recirculation influenced by land-sea breeze circulation and ship emissions on high PM_2.5 episodes in Busan, South Korea. The Recirculation Index (RI) from 2016 to 2024 was analyzed, and April had the highest values, indicating a high potential for recirculation-related air pollution. April 2019 was selected, when recirculation events coincided with high PM_2.5 concentrations. Weather Research and Forecasting (WRF) and Community Multiscale Air Quality (CMAQ) simulations for two high pollution cases (April 7 and 21) revealed that PM_2.5 was recirculated between land and sea by diurnal land-sea breeze circulation, sustaining high concentrations in both cases. However, differences in boundary layer development, sea breeze direction, and vertical mixing intensity influenced by synoptic conditions and Busan's coastal geography with southern and eastern coastlines altered the transport pathways and spatial patterns of elevated PM_2.5. Integrated Process Rate (IPR) analysis showed that a deeper boundary layer (April 7) enhanced vertical mixing, whereas a shallower boundary layer (April 21) trapped pollutants near the surface, prolonging high concentrations. Source apportionment indicated that ship emissions contributed 0.2-14 % to PM_2.5, and recirculation enhanced PM_2.5 and sulfate concentrations by 1.4 and 3.6 %, respectively. The RI is practical for evaluating the recirculation potential. These findings highlight the role of land-sea breeze recirculation in amplifying ship emissions and underscore the need for targeted coastal emission-control strategies.