Measurements of emission factors of nitrogen oxides of seagoing vessels in the port of Rotterdam

In order to improve air quality and to lower nitrogen deposition, nitrogen oxide
emissions from vessel engines must respect certain Tier norms to obtain the
required Engine International Air Pollution Prevention (EIAPP) certificate for sailing.
In this study emissions factors of nitrogen oxides (NOx) of vessels in the port of
Rotterdam region were determined, using a so-called sniffer system. Over the time
period 10-10-2019 till 1-11-2021 the data of the sniffer system were taken into
account in this study. The NOx emissions in gNOx/kg fuel, were calculated from the
concentrations of nitric oxide (NO) (based on the assumption that 80% of the NOx is
NO and 20% is NO2) and carbon dioxide (CO2). To compare to IMO Tier norms the
ships keeling date, rated engine speed and service speed have to be known. The
fuel specific oil consumption (SFOC) is used to convert the NOx emission in g/kg
fuel to g/kWhr (the unit used in the IMO norm). In the period considered, the
emission factors of 13 000 individual plumes were determined from almost 3 500
different ships. General statistics of the NOx emission (assuming a constant SFOC)
showed that, as expected, the higher the speed over ground, the higher the NOx
emissions. The 23 vessels which passed by the station most often (we excluded
dredgers and tuggers for the analysis) were analysed in more detail using a load
specific SFOC given the speed over ground of the vessel at the time of the
measurement. This analysis showed that 22 ships, using the average of the
observed plume, lie within the IMO norm (when the uncertainty in the average NOx
emission is taken into account). Only one Tier I vessel seems outside the IMO
norm, though direct non-compliancy monitoring is not possible since not all engine
loads can be considered. The average uncertainty in the NOx estimate of these
vessels is in general low with only 4.2% on average. Taken into account the effect
of the tides on load can further improve the SFOC estimate and thus the NOx
emission estimate. On average Tier II had higher NOx emission factors than Tier I
ships. This is probably, because close to the port the power on the engine is low
and Tier II engines are tuned to have low NOx emissions at high powers.