# Effects of marine fuel sulfur restrictions on particle number concentrations and size distributions in ship plumes in the Baltic Sea

Exhaust emissions from shipping are a major contributor to particle concentrations in coastal and marine areas. Previously, the marine fuel sulfur content (FSC) was restricted globally to 4.5 inline-formula $M1inlinescrollmathmlm/m\mathrm{italic %}$ 37pt14ptsvg-formulamathimgde291dba9c17654b59b65ed50c878159 acp-21-3215-2021-ie00001.svg37pt14ptacp-21-3215-2021-ie00001.png , but the limit was changed to 3.5 inline-formula $M2inlinescrollmathmlm/m\mathrm{italic %}$ 37pt14ptsvg-formulamathimg3969ef12f91258f7b3a89480b37dd106 acp-21-3215-2021-ie00002.svg37pt14ptacp-21-3215-2021-ie00002.png at the beginning of 2012 and further down to 0.5 inline-formula $M3inlinescrollmathmlm/m\mathrm{italic %}$ 37pt14ptsvg-formulamathimg3ef0ae0c4ff2b2268085dfc8ea7064e1 acp-21-3215-2021-ie00003.svg37pt14ptacp-21-3215-2021-ie00003.png in January 2020. In sulfur emission control areas (SECA), the limits are stricter: the FSC restriction was originally 1.50 inline-formula $M4inlinescrollmathmlm/m\mathrm{italic %}$ 37pt14ptsvg-formulamathimg1e3cecade7b558bb88aa08ff21caa790 acp-21-3215-2021-ie00004.svg37pt14ptacp-21-3215-2021-ie00004.png , but it decreased to 1.00 inline-formula $M5inlinescrollmathmlm/m\mathrm{italic %}$ 37pt14ptsvg-formulamathimgd7b8eb3155f9c17193c411ba23bf57e9 acp-21-3215-2021-ie00005.svg37pt14ptacp-21-3215-2021-ie00005.png in July 2010 and again to 0.10 inline-formula $M6inlinescrollmathmlm/m\mathrm{italic %}$ 37pt14ptsvg-formulamathimgb5e8b92fa7366cf1682ebb26e3889cf9 acp-21-3215-2021-ie00006.svg37pt14ptacp-21-3215-2021-ie00006.png in January 2015. In this work, the effects of the FSC restrictions on particle number concentrations (PNCs) and particle number size distributions (NSDs) are studied in the Baltic Sea SECA. Measurements were made on a small island (Utö, Finland; inline-formula $M7inlinescrollmathmlnormal 59{}^{\circ }{normal 46}^{\prime }normal 50$ 45pt11ptsvg-formulamathimg361f20c8fea6feb4cadec56e32b32c7e acp-21-3215-2021-ie00007.svg45pt11ptacp-21-3215-2021-ie00007.png  N, inline-formula $M8inlinescrollmathmlnormal 21{}^{\circ }{normal 22}^{\prime }normal 23$ 45pt11ptsvg-formulamathimg909ba35c84c6ed1dfcd1cdccba6baf67 acp-21-3215-2021-ie00008.svg45pt11ptacp-21-3215-2021-ie00008.png  E) between 2007 and 2016. Ship plumes were extracted from the particle number size distribution data, and the effects of the FSC restrictions on the observed plumes as well as on the ambient concentrations were investigated.

Altogether, 42 322 analyzable plumes were identified during the 10-year measurement period. The results showed that both changes in the FSC restrictions reduced the PNCs of the plumes. The latter restriction (to 0.10 inline-formula $M9inlinescrollmathmlm/m\mathrm{italic %}$ 37pt14ptsvg-formulamathimgeba4411a719b777ebc14eb6e64579718 acp-21-3215-2021-ie00009.svg37pt14ptacp-21-3215-2021-ie00009.png in January 2015) also decreased the ambient particle number concentrations, as a significant portion of particles in the area originated from ship plumes that were diluted beyond the plume detection limits. The overall change in the PNCs of the plumes and ambient air was 27 and 32 %, respectively, for the total FSC change from 1.50 inline-formula $M10inlinescrollmathmlm/m\mathrm{italic %}$ 37pt14ptsvg-formulamathimgcb5d3f272146d5ca83511492f9c758c3 acp-21-3215-2021-ie00010.svg37pt14ptacp-21-3215-2021-ie00010.png to 0.10 inline-formula $M11inlinescrollmathmlm/m\mathrm{italic %}$ 37pt14ptsvg-formulamathimg7d0b91af68789ab5e41ac295c43f1b5d acp-21-3215-2021-ie00011.svg37pt14ptacp-21-3215-2021-ie00011.png . The decrease in the plume particle number concentration was caused mostly by a decrease in the concentration of particle sizes of between approximately 33 and 144 nm. The latter restriction also reduced the geometric mean diameter of the particles, which was probably caused by the fuel type change from residual oil to distillates during the latter restriction. The PNC was larger for the plumes measured at daytime than for those measured at nighttime, likely because of the photochemical aging of particles due to UV light. The difference decreased with decreasing FSC, indicating that a lower FSC also has an impact on the atmospheric processing of ship plumes.

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Seppälä, Sami D. / Kuula, Joel / Hyvärinen, Antti-Pekka / et al: Effects of marine fuel sulfur restrictions on particle number concentrations and size distributions in ship plumes in the Baltic Sea. 2021. Copernicus Publications.

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