Atmospheric Observations Reveal the Spatial Influence of Inland Shipping Channels and Enhanced Emission Signals from AIS-Unmatched Vessels

Inland shipping is an important but poorly constrained source of urban air pollution, particularly in dense waterway networks, where vessel activity records are incomplete. Here, we integrate high-time-resolution differential optical absorption spectroscopy (DOAS), national air-quality observations, meteorological normalization, spatial exposure analysis, automatic identification system (AIS) records, and field photographs to evaluate inland shipping impacts in two representative channel environments in eastern China. SO2 showed stronger spatial heterogeneity than nitrogen-related pollutants and was more closely associated with the nearby shipping channels. After meteorological normalization, inland-waterway configuration explained 44% of the interstation variability in urban SO2, with the strongest association at an influence distance of approximately 4 km. Event-scale analysis revealed frequent short-term pollutant enhancements associated with the vessel passages. However, only 54.4% of plume events at the open-channel station and 44.8% at the lock-controlled site could be matched to AIS-recorded vessels. AIS-unmatched plume events were not negligible and, in the lock-controlled environment, were associated with SO2 signals higher than those of AIS-matched events. These results demonstrate that atmospheric observations provide an independent constraint on underdetected inland shipping emissions and suggest that inventories relying solely on vessel activity records may underestimate or mischaracterize sulfur-related emissions from inland waterways.