Comparison to direct emission data

From the direct emission data (Kuittinen, 2016) MDO emissions measured were chosen for the comparison. In the MDO used by Kuittinen (2016) there was less than 0.10 % sulfur which best resembled the regulations after the implementation of the sulfur re-strictions of 0.10 % in January 1^st, 2015. In Figure 31 the NSDs of the emissions from the engine loads of 25 % and 75 % have been plotted in the same figure with the average NSDpl of the nighttime plumes from the sector 1 during the latest sulfur restriction of sulfur content less than 0.10 %.The maximums of the NSDs have been normalized to the con-centration of 1. The nighttime plumes were chosen as they are expected to be composed mostly from unaged emissions as discussed in the chapter 6.5 of this thesis. The sector 1 was chosen because the distance to ships is on average the shortest.

Figure 31 The comparison of the average NSDpl of the plumes from the sector 1 during the nighttime and the sulfur restriction of 0.10 % to the NSDs from the direct emission measurements from Kuittinen (2016). The NSDs for the direct emission measurements have been drawn for the engine loads of 25 % and 75 %. The max-imums of the NSDs are normalized to 1.

The load of 25 % is expected to resemble ship engines load conditions in maneuvering and nearing the harbor and the load of 75 % is expected to resemble the load conditions of the ship engines in open waters. The maximum of the NSDpl of the nighttime plumes of the sector 1 is found to be slightly smaller than either of these two load conditions. In the plot on the right, where the both axes are set as logarithmic, all the NSDs can be seen to have two distinct modes, one at the maximum of particle sizes of 20-40 nm and another in larger particle sizes. This mode is seen as a shoulder in all the three size distributions and is expected to be composed of soot.

Notable in Figure 31 is that the shapes of the NSDs are almost identical in particle sizes larger than 100 nm. These particles can be assumed to be composed mostly from soot.

Anderson et al. (2015) stated that particles with a diameter over 50 nm from the com-bustion of marine fuels are solid primary particles and are associated with the quality of the fuel and not the sulfur content. They also stated that the nanoparticles smaller than 50 nm can be related to both sulfur content and the other properties of the fuels and consist both primary and secondary particles. They also found that the volatile particles were in the size class of the nanoparticles. If the NSDs are normalized according to the concentrations of the particles larger than 100 nm the NSDs are assumed having ap-proximately the same shapes as they have the same sulfur fuel content. To further study

this, another normalization for the NSDs was made. In Figure 32 the NSDs from the direct emission measurements of Kuittinen (2016) and the nighttime sector 1 NSDpl have been normalized to have equal particle concentrations of 100 #/cm³ in particle sizes of approximately 18-402 nm.

Figure 32 The comparison of the average NSDpl of the plumes from the sector 1 during the nighttime and the sulfur restriction of 0.10 % and the NSDs from the direct emission measured by Kuittinen, (2016). The NSDs of the direct emission measurements have been drawn for the engine loads of 25 % and 75 %. The NSDs have been normalized to have the same number of particles in the particle sizes of 108-402 nm.

From Figure 32 it is clearly visible that the NSDs in particle the sizes larger than 100 nm have almost identical shapes. In the NSDs from the direct emission measurements the mode in smaller particle sizes is seen to have many orders of magnitude higher concen-trations than the mode in larger particle sizes. In the case of NSDpl the difference of the concentrations is much smaller.

When the large soot modes of the three NSDs are normalized to the same concentra-tions, the normalized concentrations for the smaller mode are found to be approximately two orders of magnitude lower for the NSDpl in comparison to the NSDs of the direct emission measurements. The difference in the NSDs might be partly caused by the dif-ferent shapes of the emission NSDs form the difdif-ferent kind of engines producing the

emissions. This means that that the NSDs from Kuittinen (2016) and the average NSDpl

of the harbor nighttime plumes might have had different shapes from the start. However, it is unlikely that this alone would explain the difference of the two orders of magnitude in the concentrations. Robinson et al. (2007) found that primary emissions evaporate substantially upon dilution to ambient conditions. They stated that up to 75 % of the POA might evaporate in the atmospheric conditions. This is well in line with the results attained in this study. As the nanoparticles smaller than 50 nm are assumed being partly volatile (Anderson et al., 2015) and the concentrations in this study are reduced in particle sizes of less than 70 nm.