the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Jul 2024
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
Abstract. 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.
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Status: open (until 21 Aug 2024)
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RC1: 'Comment on egusphere-2024-1703', Mingxi Yang, 12 Jul 2024
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This paper reports on the long term (2003-2020) measurements of atmospheric trace gases and aerosols from an island in Finland, with a particular focus on the impact of shipping regulation on SO2. The authors show that the SO2 concentrations have decreased significantly, especially since 2015, while NOx, PM2.5 haven’t really changed. This is clearly a very valuable timeseries dataset, thanks to its length. However at present I don’t feel like I’ve learned much new from reading this paper. There have been earlier studies that report on the reductions in atmospheric SO2 in coastal areas following the IMO regulations, which this paper fails to acknowledge. Other questions that could be (but not currently) addressed this paper include:
- Fuel sulfur content and the rate of compliance by ships
- Does the observed trend in SO2 reflect what one expects (e.g. based on atmospheric transport modelling with STEAM emission and terrestrial S sources)?
- Atmospheric processing of trace gases
Specific comments
Line 66. This paragraph is ok in isolation. But it’s not something this work will address. It’s probably better to remove or significantly shorten it.
Instead, before line 79 it would be useful to review previous work on ship SO2 emissions (including time series measurements similar to this study, e.g. doi:10.5194/acp-15-5229-2015 and http://www.atmos-chem-phys.net/16/4771/2016/). What are the knowledge gaps that this paper can fill? Is it e.g.
- impact of regulation on atmospheric pollutant level?
- scrubber vs. low sulfur fuel?
- rate of compliance from ships?
- attribution of emission to different ship types?
- atmospheric processing and transformation of trace gases
4.1 there is nothing wrong with this section on its own. However I don’t feel like it contributes very much to the paper at the moment. I guess the key message is that S emission from non-shipping sectors has been declining gradually, while S emission from shipping has decreased in step wise fashion following the IMO regulations (which we would’ve expected even without STEAM model)?
I think this section can be made more powerful if the authors implement these emissions in an atmospheric transport model, see what the predicted change in SO2 is, and then compare the model with the observations.
Line 158. SO2 lifetime was estimated to be only 0.5 day to the west of the UK. http://www.atmos-chem-phys.net/16/4771/2016/ in cloud oxidation is probably the largest sink.
Fig.2 how come the SOx emission from shipping sector hasn’t decreased in step-wise fashion, corresponding to the regulations? Is it because the emission also includes outside of SECA?
Table 1. a bit more detail on the SO2 measurements would be useful (even if this info had been reported previously elsewhere). E.g. how was it blanked and calibrated?
Line 169-170 this is a repeat
Line 176. ‘until’
Table 2. what’s N(%)?
Figure 5. how come percentiles and median are not shown for data over the first few years? Is it because the data were hourly, not minutely? In general, I don’t really see the value of presenting/analyzing minutely data for this section. Hourly data would be perfectly fine for looking at long term trends. Minutely data contain much more measurement noise, especially for SO2.
Line 238. How were ‘peaks’ identified/defined? What’s the minimum NO concentration in this calculation? Have you accounted for any possible lag between the SO2 and NO data due to imperfect time synchronization or different instrument response times?
Figure 6. NO reacts rapidly with O3 and has a strong diurnal cycle and SO2:NO will too. Was CO2 not measured at the site, which would’ve enabled the estimation of the fuel sulfur content? If not, SO2:NOx still seems better than SO2:NO, though NOx emissions can vary significantly depending on the ship/weather conditions.
Section 4.1 it’s a bit odd to have this section here, when you just chosen the western sector (180-360) for the SO2:NO analysis above. Wouldn’t be better to do the wind sector analysis first, and then apply the according wind sector to SO2:NO?
Figure 7. similar to figure 5, the SO2 axis is cut off at zero. Are you discarding all negative SO2 data? I don’t think that is the best approach. The negative numbers (due to measurement noise) need to be kept in in order for the stats to be representative.
Figure 9. I don’t doubt that the SECA regulation has been effective. However here the SO2 and NOx concentrations were not evaluated with consideration of plume dilution. Would’ve been best to normalize both gases to CO2 plume. If that’s not possible, at least look at SO2:NOx ratio (which does seem to be lower after 2015).
In general, I don’t find that the case study has added much to the paper. Are there more information that can be teased out? E.g. fuel sulfur content before vs after 2015? Did the ship install a scrubber?
Citation: https://doi.org/10.5194/egusphere-2024-1703-RC1
Data sets
Meteorological and air quality data Finnish Meteorological Institute https://en.ilmatieteenlaitos.fi/open-data
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