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

Abstract. 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 and background subtraction, the trends of ambient SO₂ related to ship emissions (Ship_related_SO₂) 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. 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.