Abstract: The designation of the Baltic Sea as a sulfur emission control area (SECA) in May 2006, with subsequent tightening of regulations in 2010 and 2015, has reduced the sulfuric emission from shipping traffic. This study assesses the impacts of SECA on observed SO2 concentrations by providing a long-term analysis of 1 min time-resolution air quality data from 2006 to 2020 on Utö (an island in the Baltic Sea) supported by the predictions from the Ship Traffic Emissions Assessment Model (STEAM). Additionally, 1 h resolution data from 2003 to 2005 are utilized to investigate changes due to the SECA limits set in 2006. The observed SO2 concentrations on Utö have continuously decreased since 2003 due to an overall decrease in SO2 emissions in northern Europe, combined with reduced emissions from shipping traffic due to SECA regulations. The 3-year average SO2 concentration dropped from pre-SECA (2003–2005) to post-SECA (2007–2009, 2011–2013, 2016–2018) periods by 38 %, 39 %, and 67 %, respectively. No clear trends were observed in the concentrations of other measured pollutants. Furthermore, we investigated wind-direction-resolved SO2 concentrations for 2 selected years (2014 and 2019), and the results showed a significant decrease in high-SO2-concentration shipping plumes due to the implementation of SECA in 2015. Our study brings out the importance of long-term, high-time-resolution air quality observations at a remote marine research station by providing means for both quantitative and qualitative analyses of the impacts of regulatory environmental legislation.
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