Distance-dependent contributions of shipping emissions to PM2.5 along the western Bohai coast: insights from multiple site PMF modeling

Shipping emissions are an important contributor to PM_2.5 in coastal regions. Globally, the Bohai Sea is recognized as the most aerosol-polluted inland sea. Here, we quantify the influence of shipping-related PM_2.5 from the Bohai Sea along the western coastal region using synchronous observations across coastal and inland areas. A coordinated monitoring network was established with three sites located approximately 9, 30, and 50 km from the coastline, providing continuous measurements of PM_2.5 mass and chemical composition from April 2023 to February 2024. On average, PMF-resolved shipping emissions contributed 8.4%, 1.6%, and 1.2% of PM_2.5 at sites located ~9, ~30, and ~50 km from the coast, respectively, indicating a clear decrease with increasing distance inland. Seasonally, shipping contributions at the coastal site were 5.9–13.0 times higher than those observed at ~50 km inland, with the strongest spatial gradients occurring in summer and the weakest in winter. These results provide direct observational evidence that shipping emissions can measurably influence urban PM_2.5 concentrations up to ~50 km inland. Our findings underscore the importance of explicitly accounting for marine emission sources in coastal air quality management, particularly for semi-enclosed seas such as the Bohai Sea.