A 6-year study in a Shanghai port shows that during a low-sulfur fuel policies adjustment phase (2018–2020), ship pollution decreased by 43.47 % and 23.08 % yearly, but emissions rebounded by 19.5 % yearly post-2020 as shipping grew. Using air sensors and data analysis, researchers identified cargo ships as key polluters and created a cost-effective monitoring method for global ports. Findings warn that shipping expansion risks air quality progress, urging smarter policies while supporting trade.