Onshore winds amplify shipping PM_2.5 exposure in a coastal megaport city: Insights from source apportionment and...

Port-related air pollution and its associated health risks remain a global concern. Aerosol loading over the Bohai Sea ranks among the highest of the world's major inland seas. Facing this region, Tianjin Port, located on its coast, is northern China's busiest port with intensive shipping and cargo-handling operations. To quantify shipping influence on coastal urban PM_2.5, we conducted an almost year-long PM_2.5 speciation measurement campaign at a downtown site ∼9 km west of the port. Source apportionment indicates that shipping emissions contributed 3.30 μg/m³ to PM_2.5, and population-weighted exposure placed this area among the most impacted port cities reported. Distinct monthly and diurnal patterns were observed, with contributions peaking in April-July and during afternoon-nighttime periods. Machine learning (ML)-based meteorological normalization revealed that the underlying emission strength of shipping PM_2.5 remained relatively stable, while most temporal variability was controlled by meteorology. Shipping impacts were highly sensitive to onshore transport driven by sea-land breeze circulations and boundary layer dynamics. The strongest enhancements occurred under E-SE winds at moderate speeds (2-6 m/s), during which shipping PM_2.5 reached 6.40 μg/m³, nearly twice the annual mean. The post-hoc explanation technique identified that meteorological factors associated with onshore transport jointly increased PM_2.5 mass by up to 3.82 μg/m³ relative to average conditions. These findings demonstrate that onshore winds substantially amplify shipping contributions to PM_2.5 in coastal cities. From a public health perspective, port emission-control strategies should explicitly account for the timing and frequency of onshore wind shifts to better mitigate population exposure.