Measurement report: Six-year DOAS observations reveal post-2020 rebound of ship SO ₂ 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 for gap filling and meteorological normalization, the trends of ambient SO2 related to ship emissions (Ship_related_SO2) over the 6-year period were revealed. Furthermore, whether ships in the channel were using low-sulfur fuels was determined by a decomposition of SO2-rich plume 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. Meanwhile, 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.