Fig.1: Development of emission inventory.
These data are in agreement with those of the Lloyd's Register report [3] published in 1995 (1990 data), which concludes to an emission of 162 kt in the English Channel and the southern North Sea caused by international and national shipping, ports not included.
The 1990 Marintek study [4] concluded to an emission of 74 kt for the English Channel (international shipping only, ports not included) and 104 kt for the whole of the North Sea. The modelling of sulphur dispersion and deposition for the Concawe study was carried out by the Imperial College Centre for Environmental Technology (UK), using the Offshore Coastal and Port Dispersion Model (OCPD) which is a source-receptor model that estimates the air concentration and deposition of sulphur emitted by ships at-sea and in port. OCPD assumes that the vertical dispersion of material is characterised by a Gaussian distribution, while the horizontal dispersion, for a continuous release, is uniform across a sector of fixed angle. The model also assumes different dispersion characteristics as the plume moves over land and sea surfaces. The mixing layer depth is fixed over the sea at a depth consistent with neutral meteorological conditions. Continental European land areas are modelled on a 25 × 25 km gridscale, the gridscale for the UK is 20 × 20 km. The major ports are modelled on a 1 × 1 km gridscale to enable a more derailed assessment of the contribution of ships in port to overall air quality.
The model differentiates between the characteristics of sulphur dioxide and sulphate. Sulphur dioxide is oxidised to sulphate at a rate of 1% per hour. Unlike SO2, sulphate is subject to significant dry deposition.
Studies undertaken for the modelling showed that the exhaust gas temperatures for ships vary between 280 and 370 ℃, whilst the average exhaust gas velocity was around 30 m/s. The OCPD model incorporates a plume rise module. Meteorological conditions within the relevant region were interpolated from data supplied by the UK Meteorological Office. The data were then averaged over a 10 year period.
3. IMMISSION MODELLING RESULTS
Contributions of ships to sulphur deposition in the areas bordering the southern North Sea and the Channel were investigated in 4 different ways:
- immissions from ships outside the territorial waters onto land
- immissions from ships inside the territorial waters onto land
- immissions from ships in port onto land
- immission from ships at sea and in port, onto land and Sea.
The respective immissions are represented in fig. 3 Analysis of the results shows that in port emissions are responsible for the highest rate of deposition. Deposition from emissions from busy shipping lanes within territorial waters is also substantial in a number of modelling grid squares, although not reaching the peak values seen from ships in port. The contribution from ships outside the territorial waters to the sulphur deposition in the countries adjacent to the considered sea-area, is much less significant.
In fig. 4 a comparison is made between the OCPD estimated depositions and data which reflect the UN-ECE targets for the same areas after the implementation of the second UN-ECE sulphur protocol aimed at reducing SO2 emissions from land based sources. The peak depositions are mostly due to in port emissions, and to a lesser degree, to emissions from inside the territorial waters.
