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

Abstract

Abstract. Exhaust emissions from shipping are a major contributor to particle concentrations in coastal and marine areas. Previously, marine fuel sulfur content (FSC) was restricted globally to 4.5 m/m% but the limit was changed to 3.5 m/m% at the beginning of 2012 and further down to 0.5 m/m% in January 2020. In sulfur emission control areas (SECA), the limits are stricter; FSC restriction was originally 1.50 m/m% but it decreased first to 1.00 m/m% in July 2010 and again to 0.10 m/m% in January 2015. In this work, the effects of the FSC restrictions on particle number concentrations (PNC) and size distributions (NSD) are studied at the Baltic Sea SECA. Measurements were made on a small island (Utö, Finland; 59° 46’50N, 21° 22’23E) 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 total 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 PNC of the plumes. The latter restriction (to 0.10 m/m% in January 2015) decreased also the total 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 PNC of the plumes and ambient air was 27 and 32 %, respectively, for the total FSC change from 1.50 to 0.10 m/m%. The decrease in plume particle number concentration was caused mostly by a decrease in the concentration of particle sizes of ∼35–134 nm. The latter restriction also reduced the count median 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 in daytime compared to those measured in nighttime likely because of the photochemical aging of particles due to UV-light. The difference decreased with the reducing FSC indicating that lower FSC has also an impact on the atmospheric processing of ship plumes.

Comment on acp-2020-946

“…A steady decrease of the contributions of the ship plumes to PM0.144 and PM0.537 can be seen after the FSC restriction changes with the contributions reducing overall from 5.5 -14.0 % (31.6 -80.8 ng m-3) to 3.9 -8.9 % (12.9 -29.8 ng m3) for PM0.144 and from 2.8 -7.4 % (106.3 -283.3 ng m-3) to 2.4 -5.5 % (60.0 -136.2 ng m-3) for PM0.537. Similar values of 34 ± 19 ng m-3 (summer) and 18 ± 8 ng m-3 (winter) for PM0.15 during the FSC restriction of 0.10 m/m% have been reported by Ausmeel et al, (2020). Contribution to PM0.5 reported by Ausmeel et al, (2020) is slightly lower compared to this study, 37 ± 20 ng m-3 (summer) and 29 ± 13 ng m-3 (winter).…”

“…Similar values of 34 ± 19 ng m-3 (summer) and 18 ± 8 ng m-3 (winter) for PM0.15 during the FSC restriction of 0.10 m/m% have been reported by Ausmeel et al, (2020). Contribution to PM0.5 reported by Ausmeel et al, (2020) is slightly lower compared to this study, 37 ± 20 ng m-3 (summer) and 29 ± 13 ng m-3 (winter). An important thing to note is that these contributions are only the direct contributions of the detected plumes to ambient PM0.144 and PM0.537.…”

“…Printer-friendly version Discussion paper agree with the Referee that the shorter distance to the measured ships in Zanatta et al, (2020) and the sample size can both cause the difference in the reported concentrations. The text is modified and the following text is added to the manuscript: "Concentrations observed by Zanatta et al (2020) were higher likely because of the fresher plumes (plume age ∼200-800 s). Also, the small sample size (only 9 continuous periods of single or multiple exhaust plumes each lasting a couple of minutes at maximum) might cause statistical uncertainty to the results."…”

“…Your contribution to PM0.5 is about a factor 7-8 higher than at Falsterbo peninsula, which is likely due to the fact that ships in Falsterbo are smaller, and possibly due to the long average distance to the shipping lane in Falsterbo of around 10 km -however, I don't know the average distance to ships in your paper. AR: As suggested by the Referee, upper and lower estimates for the direct contribution of ship exhaust plumes to PM0.144 and PM0.537 during different FSC restrictions were calculated and compared to Ausmeel et al, (2020). A table including these values for valid plumes, all plumes, and ambient aerosol is added in the article (Table 1) with the following text at the end of section 3.5: "From the NSDpls and NSDtots, the masses of PM0.144 and PM0.537 (PM of particles smaller than 144 and 537 nm) were calculated.…”