Background number size distributions

NSDbgs were drawn for each of the sectors during all the three different sulfur restriction periods to examine if the changes of the sulfur restrictions had influenced the PNCbgs.

Differing from the NSDpls of the plumes, the NSDbgs were drawn from unnormalized dis-tributions. The NSDbgs have been calculated only for the valid time periods as during the invalid time periods the PNCpls and the PNCbgs get mixed to some degree because of the plume detection method used. In Figure 33 the NSDbgs from the sector 1 have been plotted during all three sulfur restriction periods.

Figure 33 The NSDbgs from the sector 1 during the different sulfur restrictions.

A clear difference in the NSDbgs between different sulfur restriction periods can be seen in Figure 33. As the sulfur content in the marine fuel decreases the maximum of NSDbg

shifts to smaller particle sizes and the total PNCbg decreases. During the sulfur restriction period of 1.50 % the maximum of the NSDbg was 56 nm decreasing to 48 nm during the sulfur restriction of 1.00 % and to 47 nm during the sulfur restriction of 0.10 %. These diameters are slightly larger than the maximums of the NSDpls measured in sector 1 during the same sulfur restrictions. These diameters of the maximum of the NSDpls are presented in Table 4.

Notable is that the shape and height of NSDbgs during different sulfur restriction periods are remarkably similar in particle sizes smaller than of approximately 22 nm. This is ex-pected as the shipping plumes have effect on the PNCbg when they are diluted to point that they are undetectable as individual plumes. This dilution takes time and the particles have time to grow during this time.

Figure 34 The NSDbgs from the sector 2 during the different sulfur restrictions.

In Figure 34 the NSDbgs have been presented for the sector 2. Restricting the sulfur content in the marine fuels seems to have effect on NSDbgs also in the sector 2. The decreased sulfur contents decrease the total PNCbgs and shift the maximum of the NSDbg

to smaller particle sizes. The maximum of the NSDbg during the first sulfur restriction is 56 nm as in case of the sector 1, but the shape of the peak is sharper, and the peak is higher. During the second sulfur restriction period the diameter of the NSDbg maximum stays unchanged, but the concentration overall is lower. During the third sulfur restriction period the diameter of the NSDbg drops to 47 nm, which again is same as in the sector 1. The similarity of the maximums of the NSDbgs during the different sulfur restriction periods with the sector 1 is expected as the nearby shipping activity should not have any major effect directly on the NSDbgs. The effect of shipping can still be seen in NSDbg

because when plumes are coming from very long distances, they have much time to dilute and will not be identified as individual plumes anymore but are just added to PNCbgs and NSDbgs. This effect might be a reason the total PNCbgs seem to be decreas-ing as the sulfur content in the marine fuels decreases.

Figure 35 The NSDbgs from the sector 3 during the different sulfur restrictions.

In Figure 35 the NSDbgs have been presented for the sector 3. In the sector 3 the different amounts of sulfur in the marine fuels also seem to influence the NSDbgs. Now only the PNCbgs rise after the first change of the sulfur restrictions from 1.50 % to 1.00 % and decrease after the second change from 1.00 % to 0.10 %. Notable is that in this sector the sulfur restrictions do not have an effect on the diameters of the maximums of the NSDbgs and all these maximums are larger than in the other two sectors. The maximums of the NSDbgs being 65 nm during the first, 64 nm during the second and 63 nm during the third sulfur restriction period. The number of large particles in total is also larger for this sector. In this sector there was a significant shipping activity behind the closest ship-ping lanes and many, if not all, the plumes arriving from these very long distances will be distillated to the point of being undetectable as individual plumes. Therefore, they only contribute to the PNCbgs as the increasing concentrations. The maximum of the NSDbg

is in good accordance with this as the maximum is at the larger particle size than in the other two sectors where the plumes are coming from shorter distances. In this sector there is also a visible decrease in the PNCbgs after the implementation of sulfur limit of 0.10 % in the marine fuels.

The NSDbgs were plotted also for all the measured background data during the valid time periods. To this plot all the NSDbgs with and without wind direction were included. These NSDbgs are presented in Figure 36.

Figure 36 The NSDbgs from all wind directions including undetermined wind di-rections, during the three different sulfur restriction periods.

In Figure 36 the NSDbgs during the sulfur restriction periods of 1.50 % and 1.00 % are seen to be almost identical. There are only minor differences in the largest and the small-est particle sizes. The NSDbg during sulfur restriction period of 0.10 % is different. The maximum of the NSDbg has shifted to the slightly smaller particle size of 55 nm from the previous 56 nm and the average PNCbg has decreased. This is in line with Figures 33-35. After the first change of sulfur restrictions from 1.50 % to 1.00 %, the background aerosol changes very little and almost all of the change happens after the second change of restrictions from 1.00 % to 0.10 %. This might be related to the uncertainty concerning the real sulfur content change of the marine fuels after the change of 1.50 % to 1.00 % in sulfur restriction seen in Pirjola et al (2014).