PAH and scrubbers – part one

When it comes down to it, it’s not actually PAH content that the EGCS Guidelines request monitoring of – but rather oil.[1] Looking back, the discharge water limits were negotiated prior to the 2009 EGCS Guidelines. There was discussion then as to whether the limit should be 15 ppm on open sea (as for oily bilge water) and 5 ppm in ports, etc. or if it should be even lower in general.[2]

However, the question at that time was how to continuously monitor oil content far lower than 15 ppm. Until then, IMO’s only experience with oil-in-water sensors had been in conjunction with oily bilge water treatment systems, which were regulated by a relatively new amendment to MARPOL Annex I (2003). The experience up to that point had not been positive (although today the sensors are considered reliable), for reasons beyond the inability to monitor low-range oil values. Given that oil-in-water sensors didn’t seem promising, someone came up with the idea of using PAH sensors instead.

Why monitoring PAH as a proxy for oil makes sense

PAH sensors have been used previously in connection with oil rigs and drinking water reservoirs. It’s known that there are PAH compounds in oil, which are described as being petrogenic. [3] PAH screening is used to fingerprint the offender in an oil spill, and it can also identify the type of oily substance, thanks to a statistically significant correlation between the substance type and the PAH constituents.[4] (Around 40 PAH species are used for this fingerprinting and identification, although approximately 660 parent PAH compounds have been identified. [7]) Thus, it is empirically possible to relate oil and PAH content.

In other words, PAH monitoring can be used as a proxy for oil monitoring. This is why it would make sense to use PAH sensors with scrubbers, as they can detect very low “oil” content.

Which PAH species to measure?

So far so good. However, there’s more to this than just correlating PAH content and oil, because there are so many PAH species – and they cannot all be monitored at once.

The different PAH species can be identified by their fluorescence emission when exposed to and excited by UV light.[5] The excitation wavelength and the emission wavelength, in combination with the intensity, will identify a given PAH species.[5] Looking at the PAH fingerprinting data, it’s apparent that naphthalene is the predominant constituent in HFO, followed by phenanthrene, acenaphthene and fluorene.

Since we want to monitor oil, it would seem logical to monitor the most significant PAH species for oil – those just mentioned. The obvious choice would be naphthalene. However, this isn’t the case. Why?

Now, this is me guessing. Naphthalene is the lightest of the PAH species. It’s also quite water-soluble, which makes it more vulnerable to degradation.[3] This might be the reason why phenanthrene, which is more persistent with regard to combustion and degradation, has been chosen as a reference. However, the choice – and I’m still speculating here – could also have something to do with the intensity with which the PAH species fluoresces. It’s easier to detect a substance that fluoresces with high intensity than with low. A third reason might be the spectral overlap between excitation and emission. If this overlap is too narrow (similar wavelengths), there could be interferences created. The excitation and emission wavelength for phenanthrene are further apart than they are for naphthalene.[5] But those are only my hypotheses, so it would be helpful if the brilliant mind that came up with the methodology also wrote a paper about it.

Finding the source of the PAH content

A scrubber will not capture all the PAH content that comes with the engine exhaust gas, but it will capture most of it. (For further elaboration, see my next blog post.) This content may be both pyrogenic and petrogenic,[6] although we don’t know the distribution of the two varieties. There is, however, a methodology for determining PAH origin described in “Source Characterization of Polycyclic Aromatic Hydrocarbons by Using Their Molecular Indices”.[7]

Apparently, the ratio between phenanthrene and anthracene is a reliable indicator of the source or origin.[8] Phenanthrene is thermodynamically very stable, whereas anthracene is not. [7] Therefore, comparing the ratio in the discharge water with the typical ratio for fuels will indicate whether the PAH content represents combusted PAH compounds (pyrogenic) or PAH compounds from the fuel oil (petrogenic).

If we look at the data available from the washwater sampling campaigns, for example the CSA 2020 washwater study,[9] and if we review the P0/A0 ratio based on the 99.7% mean (256 samples), we arrive at a P0/A0 ratio of 20. This would suggest that the PAH content in the exhaust gas is petrogenic. In other words, the PAH compounds we see in the discharge water represent unburned fuel oil. This strengthens the argument that oil is what we need to detect in the EGCS discharge water, not PAH content as an isolated matter.

Why don’t measurements match up?

Now that we’ve established the logic in monitoring phenanthrene as a proxy for oil, and that oil indeed is the substance to monitor, one further question remains. Why does reading of a PAH phenanthrene equivalence sensor differ from the phenanthrene analysis of your water sample by a laboratory using gas chromatography mass spectrometry (GC-MS), which is believed to be the most precise measurement?

The answer is cross-sensitivity. This may be caused by spectral bands overlapping[5]^,[10] (other PAH species than phenanthrene being detected as phenanthrene), but also by “contamination” from organic and/or biological matter already present in the marine environment.[5]^,[10]

GC-MS involves extraction and cleaning-up of the samples before testing, which isn’t possible for continuous monitoring on board. One simply has to accept that what is monitored continuously in-situ will never align with laboratory analytics. The methodologies just aren’t comparable. Furthermore, one needs to accept that a PAH phenanthrene equivalence measurement is an expression of phenanthrene-like content, so the PAH sensor will always overestimate the actual phenanthrene content – and thus the oil content.

Lastly, one needs to realize that a phenanthrene measurement is, in fact, an oil measurement. The PAH phenanthrene sensor will never be able to provide information on general PAH contamination by the EGCS. The sensor is not geared for it.

Watch for part two

In my next blog post, I’ll reflect on PAH content as a substance in the marine environment. Already now, I can say that scientists don’t align with each other in their conclusions about the impacts, toxicity effects or accumulation aspects. I’m not an expert in marine biology, but it strikes me that their conclusions may depend on the exact species and micro-environments involved in their particular experiments and tests.

I hope you’ll check back to read it.