Remote monitoring of gaseous ship emissions using UV/visible solar occultation spectroscopy

The shipping sector will contribute to a substantial fraction of the total European emissions of SO2 and NOx in the next decade, according to a recent EU assessment. This has already been recognized by the EU and IMO (International Maritime Organization) and various strategiesto reduce these emissions are discussed and partly implemented, such as fuel sulfur capping,new engine nitrogen oxides emission limits, environmentally differentiated fairway dues, and emission trading schemes. One tool for reducing, controlling and getting a good overview of the distribution of the emissions from the shipping sector, and making it possible to enforce new regulatory schemes, would be the ability to conduct "road side checks" of ships in real traffic, in the same manner as one today conduct such measurements on cars. A feasibility study has therefore been conducted on a new measurement method by which it is possible to remotely monitor the emissions of by-passing ships from another platform, such as a small ship. The remote sensing technique, named the Solar Occultation Flux (SOF) method, is based on measuring UV/visible spectra of the sun. From these spectra the amount of absorbing species(e.g. SO2 and NO2) can be derived between the measurement point and the sun. By conducting the observations on a small ship positioned leeward of the by-passing ship to be measured, and simultaneously measure the windspeed, it is possible to derive the emissions from the ship in mass per traveled length unit (i.e. kg S km-1) or mass per time unit (kg S s-1).The anticipated accuracy for the remote measurements is 20-30%.The aim in this study was to investigate whether one could identify "gross" polluting ships, running on high sulfur fuel and not having NOx catalysts installed. An additional aim was to investigate the feasibility of using this technique to assess the total emissions from the shipping sector. Measurements of ship emissions in real traffic were therefore made in the inlet channel to Goteborg, Sweden, during a field campaign in June 2001. Emissions of SO2 (10-100 kg h-1) were detected from about 15 ships during 3 days, and these ships are believed to have used fuel with a sulfur content of 2% and above. For NO2 no good emission measurements were obtained even though this species was detected in the ship plumes on several occasions. The retrieved NO2 values had too high variability, which was caused by acombination of problems with the equipment, non-ideal optical design and difficultspectroscopy. Further feasibility studies are therefore needed for NO2 after having conducted improvements of the hardware and optics.