Shipping remains a crucial airborne particle source

Niina Kuittinen,
PhD Researcher,
Tampere University,
Finland

Freshly emitted airborne particles from ship engines are born during combustion in the marine engine cylinders or during dilution of the hot exhaust gases into the surrounding atmosphere. These particles can include various compounds – light absorbing black carbon, metals, as well as semi-volatile sulfuric and organic compounds.

The amount of airborne particles can be expressed by different quantities – the most common measure being the particle mass. The mass of the particles is highly dependent of their size, and for that reason, air quality measures such as PM2.5, indicating the mass of particles smaller than 2.5 micrometers are commonly used.

Particles emitted from modern combustion sources are generally significantly smaller and thus their mass may be low, but number high. In our latest study considering six different marine fuel options, it was found that, in all cases, the majority of the particles were in the ultrafine particle (UFP) size range, having diameters below 0.1 micrometers. Particles of this size can penetrate deep in the alveolar region of human lungs and evidence exists of their ability to translocate to other organs.

From the climate change perspective, information of particle number emissions rather than mass is needed for detailed climatic models, considering for example, the effects of particles to the formation of clouds and their radiation balance.

Also, as particle mass is widely measured in the air quality observation stations of the world, it has so far been in the focus of the epidemiological studies that connect particle concentrations in air to human morbidity and mortality. However, new studies suggest that particles’ number or surface area may be more relevant for understanding their health effects. In the recent air quality guideline update by the World Health Organization, a good practice recommendation for ultrafine particle number was introduced for the first time. High particle number concentration in outdoor or indoor air is considered to be more than 10 000 particles in one cubic centimeter of air, while less than 1000 is a low value.

Our recent study focused on quantifying the particle number emission factors for freshly emitted exhaust in the case of six different marine fuel options. The particle numbers corresponding the freshly emitted exhaust were in the order of 16-50 billiard particles for each kilogram of fuel burned. Applying a scrubber could bring these levels down to 3-7 billiard particles per kilogram of fuel. Despite the extremely high numbers of particles being produced, due to dilution to ambient air, the observed concentrations in the plumes of the ships followed by aircraft were in the order of 10 000 to 60 000 particles in one cubic centimeter of air, 7 to 10 minutes downwind from the stack.

When combining the information of particle number emission factors to STEAM ship emission assessment model, which uses satellite data for modelling of the global ship traffic, the global particle number emission from shipping could be assessed. The global emission was estimated at 1.2×10²⁸ in the year 2016, which is of similar magnitude with an earlier estimate of total anthropogenic particle number emissions in the continental areas. While bringing significant environmental and health benefits on its own, the global sulfur cap of 0.5% introduced in 2020 doesn’t significantly reduce the emission if use of residual fuels continues. Replacing high sulfur heavy fuel oils with cleaner marine diesel or gasoline oils could reduce the global particle number emission by 25-44% and introduction of natural gas or scrubbers by 61-67%. In the Baltic and North Sea emission control areas, fuel sulfur content is limited to 0.1%, favoring the use of cleaner fuel types, but desulfurized residual fuels can also still be used.

Our study also presented the global distribution of particle number emissions from shipping around the world, where emission levels were elevated on the main oceanic shipping lanes but, importantly, also near densely populated port cities. The importance of particle number emissions from shipping is emphasized by the trend in continental emissions that are dominated by on-road vehicles; for these, exhaust particle filters, leading to a decrease of particle number by orders of magnitude, are becoming mandatory. Therefore, the relative contribution of particle number from ship emissions may become higher in the future. At the moment, particle number emissions are not limited by the International Maritime Organization, but regulation has been discussed for specific particles such as black carbon.

Question also remains whether the new emerging fuel technologies such as methanol and ammonia can be used to reduce particle emissions in addition to helping the shipping sector to reach the carbon dioxide targets. Ships are currently undergoing a technology shift and it would be important to make sure that in addition to reaching the carbon targets, the climate and air quality effects of the co-emitted species are continuously considered.

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