I). 
0.2 
EA 
X 
m 
u 
” 
LU 
Ship 1 
8 5Ship 2 
@ Ship 3 
5 Ship 4 
B zero FSC 
7.1 
MM 
. Lower Detection Limit 
C 
SO, mean modelled concentration 
(ppb) 
5389 
590 
591 
Figure 13: Relationship between FSC and mean time-series concentration of SO; for the five ship case studies. The lower 
detection limit of the SO, instrument is illustrated as a black dashed line 
592 4 Discussion 
593 
594 
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500 
501 
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516 
The CFD methodology developed can also be useful in other applications. The method 
developed can be used in a number of cases, when examining the optimum location of a 
measurement station. Moreover, the model can provide a detailed picture of the dispersion of a 
ship plume in the vicinity of a vessel. This might become important e.g. when vessels operate in 
port areas to quantify their effects on air pollution. Moreover, the method could be used to screen 
out faulty (e.g., Ship 2) and unrealistic recordings (e.g., Ship 5), 1.e., when peaksums are affected 
by nearby sources, by comparing modelled vs recorded concentration signals. This could be 
fundamental in further establishing the reliability of such remote measurement techniques for the 
enforcement of sulphur and other pollutant regulations in the future. 
Despite the benefits of this methodology, the reduction of uncertainties will further 
improve the simulation results. Uncertainties can be separated into three categories. The first one 
considers the ambient conditions in the whole area of interest. Most importantly, turbulence 
variables that are affected both by local atmospheric and water conditions, cannot be easily 
defined. Also, the exact temperature of the boundary layer above water may also have an effect on 
the plume rise conditions. The second category of uncertainties is related to the conditions 
considered at the exit of the funnel. The main parameter is the calculation of the mass flow that is 
used for the estimation of the exhaust gas exit velocity. The mass flow is affected by the engine 
operation point, the use of auxiliary engines, and the air fuel ratio A, while the exhaust temperature 
may slightly differ per ship type. In particular, use of a scrubber for SO2 compliance will 
significantly affect exhaust gas temperature and this has to be separately modelled. The third 
source of uncertainty is the actual measurement of SO2 and CO»: and the method used to estimate 
FSC from these two values. According to (Mellqvist et al., 2022), there are uncertainties 
originating from the gases’ imperfect distribution in the plume. Other uncertainties arise from the 
instrument’s calibration (uncertainty of 5%) and the relative humidity effects. Assumptions such