Measurement Report: The effects of SECA regulations on the atmospheric SO₂ concentrations in the Baltic Sea, based on long-term observations at the Finnish Utö Island

The designation of the Baltic Sea as a Sulphur Emission Control Area (SECA) in May 2006, with subsequent tightening of regulations in 2010 and 2015 has reduced the sulphuric emission from shipping traffic. This study, focusing on impacts of SECA on observed SO2 concentrations, provides a long-term analysis of 1–minute time resolution air quality data from 2006 to 2020 at Utö island (Baltic Sea), supported by the predictions from the Ship Traffic Emission Assessment Model (STEAM). Additionally, hourly data from 2003 to 2005 is utilized to investigate changes due to the SECA limits set in 2006. The observed SO2 concentrations at 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. Three–year average SO2 concentrations dropped from pre–SECA (2003–2005) to post–SECA periods (2007–2009, 2011–2013, 2016–2018) by 38 %, 39 %, and 67 %, respectively. No clear trends were observed in the concentrations of other pollutants measured. In addition to time series analysis, we investigated wind direction resolved SO2 concentrations for two selected years and studied the changes in ship plumes of one vessel regularly passing by Utö. This study brings out the importance of long–term, high time-resolution air quality observations at remote marine research stations, in the vicinity of a heavily trafficked ship lane, providing possibility for both quantitative and qualitative analyses of the impacts of regulatory environmental legislation.