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Reporting period: 2021-06-01 to 2022-11-30

Poor urban air quality is caused, among other pollutants, by particles that partly originate from the transport sector. This includes exhausts from the tailpipe and none-exhaust sources from, for example, mechanical brakes. Emissions in the urban air are measured as PM10, i.e. Particulate Matter with an aerodynamic diameter of less than 10 µm. Recently, the attention has focused on smaller fractions, less than 100 nanometres, called ultrafine particles (UFP). The European air quality directives regulate concentrations of PM10 but not UFP. The nPETS project aims to understand and mitigate the effects of the non-regulated UFP emissions from the transport sector.

In 2021, the WHO recommended integrating UFP monitoring, measured as the total number of particles, into the existing air quality monitoring stations. In the EURO 6 regulations, the exhaust UFP emissions down to 23 nm are regulated. In the recent Euro 7 proposal, also, the non-exhaust UFP emissions from mechanical brakes are regulated.

With state-of-the-art particle instruments, the nPETS consortium aims to monitor and sample the UFP transport-generated emissions from shipping, road, rail, and aviation in the field and controlled laboratory environments. This includes aged and fresh aerosols and primary and secondary volatile and non-volatile components. Characterisation of the emissions includes linking their sizes, chemical compositions, and morphologies to their specific emission sources, such as engines and wear components (e.g. brakes, clutches, and tyres). Quantification and monitoring are performed with the same type of instruments to ensure comparable measurement results. The characterisation also includes toxicity studies based on sampled particles from filters but, most importantly, in situ cell exposure with an Air Liquid Interface (ALI) system that can mimic the interactions of airborne particles with lung cells as well as a zebrafish embryo model.
Two portable Air Liquid Interface (ALI) systems have been built within the project to ensure repeatability and comparability in cell exposure studies. The ALI systems have so far been used in Stockholm, Thessaloniki, and Barcelona. In addition, during these first 18 months, detailed protocols for measuring the UFP number concentrations, sampling on filters, and using the ALI system in field and laboratory studies have been established.

A first version of the nPETS database for storage and future open-access communication of the data has been developed within the project. In addition, discussions with the parallel project ULTRHAS on data storage and sharing have also been initiated.

UFP emissions have been identified in all field studies, i.e. street canyons, urban backgrounds, the harbour, the airport, and train stations. The new 2021 WHO Air Quality Guidelines have a recommended low and high limit. Most measured UFP number concentration levels are above the high level limit, and all are above the low limit. All studied non-exhaust sources produced UFP emissions, i.e. mechanical brakes, clutches, tyre to road, railway wheel to rail, and electric power systems for trains. Hotspot UFP-generating conditions have been identified for the studied mechanical brakes in which the UFP emission levels can increase drastically by more than a factor of 1000 as compared to non-hot-spot conditions.

Lab-generated nanoparticles show that both exhaust and non-exhaust sub-100 nm particles have clear DNA damage potential.

So far in the project, eight articles have been published in scientific journals, 12 conference presentations were given, and three student thesis projects have been performed. In addition, the nPETS project has been presented in national TV news programmes, and the nPETS webpage has more than 1000 unique visitors. nPETS LinkedIn has almost 300 followers.
A potential mitigation result from the nPETs project is that one can re-design the non-exhaust sources to minimise or avoid UFP emissions by limiting the amount of adhesive wear on the contacting wearing surfaces. This has been exemplified both for the mechanical brakes and the wheel-rail contact.

A novel result is also the identification of UFP emissions from clutches.

Further development and fine-tuning of the movable ALI systems are planned for the second half of the project.

Due to the delays in the DGI impactors, further testing in the field and laboratory studies are planned for the continuation of the project.

Laboratory studies of aged and fresh aerosols will be performed in a laboratory environment using aging chambers in the second half of the project. This is planned to be completed for both exhaust and non-exhaust emissions sources.

Based on the detailed chemical and toxicological characterisation from laboratory and field studies, source scores of potential toxicity will be developed. Combining epidemiological studies on exposure-response relationships will enable the assessment of impacts on public health in the project’s core cities. This work has just been initiated in the project, and further discussions with the parallel ULTRHAS project will follow.

The UFP characterisation, including toxicity studies, will progress beyond state of the art and provide air quality monitoring of UFP with source scores of potential toxicity. This will provide the possibility of risk analysis of measured UFP concentration levels according to the new WHO recommendations. Similarly, UFP emission levels for mechanical brakes, measured with the EURO 7 test cycle, can be accomplished and feed-in to risk analysis of the influence of different brake materials.
Karine Elihn at an underground metro platform making sure the movable ALI system assembly is correct