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 PM2.5 in coastal regions. Globally, the Bohai Sea is recognized as the most aerosol-polluted inland sea. Here, we quantify the influence of shipping-related PM2.5 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 PM2.5 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 PM2.5 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 PM2.5 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.