Six-year DOAS observations reveal post-2020 rebound of ship SO2 emissions in a Shanghai port despite low-sulfur fuel policies

The expansion of maritime trade has made ship emissions a significant target for SO₂ reduction policies. However, there is still a lack of observational data to reflect the long-term changes in SO₂ emission from ships. This study conducted continuous observational experiments using Differential Optical Absorption Spectroscopy (DOAS) from 2018 to 2023 in a shipping channel in Shanghai, China. By employing machine learning for gap filling and meteorological normalization, the trends of ambient SO₂ related to ship emissions over the six-year period were revealed. Furthermore, whether ships in the channel were using low-sulfur fuels was determined by a decomposition of SO₂-rich plumes signals (which reflect high-emission ships) and baseline variations. The findings indicate that ship activities increased ambient SO₂ concentrations in the channel by 0.48 ± 0.25 ppbv (43.24% of urban background levels). During the policy adjustment phase (2018 to 2020), Ship related SO₂ levels declined steadily due to low-sulfur fuel regulations. While from 2021 to 2023 (the policy stabilization phase), increased ship activity became the dominant driver of rising ship related SO₂ levels. Despite policy effectiveness, excessive emissions from cargo ships persisted throughout the study period, suggesting that the emission inventory could be overestimating the actual abatement effectiveness of the policy. This study quantified the contribution of ship emissions to ambient SO₂ during 2018–2023 based on observations, evaluating the effectiveness of low-sulfur policies and supporting ongoing efforts to mitigate SO₂ pollution from maritime activities. The methodology developed here can be adapted to other global shipping channels, providing a framework for monitoring and regulating ship emissions worldwide.