Ultrafine particles (UFPs) are tiny airborne particles with a diameter of less than 100 nanometers. They are produced during combustion processes—including in jet engines, vehicle engines, and heating systems—and are particularly critical due to their size: They can penetrate deep into the lungs, enter the bloodstream and potentially cause damage to health. Despite these risks, UFPs have seldomly been taken into account in legal limits or systematically measured. This will change with the new Ambient Air Quality Directive 2024/2881/EU. UFPs will be explicitly mentioned for the first time as a pollutant group relevant for monitoring.

The directive calls for:

• the monitoring of particle number concentrations (PNC),
• hotspot measurements at particularly polluted locations such as air- and seaports,
• the development and application of uniform measurement standards

Zurich as a Pioneer Location

Zurich Airport began addressing this challenge at an early stage. A comprehensive study on UFP pollution provides valuable insights into emission sources, measurement methods, and possible courses of action – and shows how the requirements of the EU directive were already implemented a long time ago.

The first measurement campaign was carried out at Zurich Airport between April 2019 and April 2020. The aim was to systematically record the concentration and composition of ultrafine particles, with a particular focus on distinguishing between volatile and non-volatile components. This differentiation is crucial, as volatile particles are predominantly formed from short-lived gaseous precursors and are strongly influenced by current emissions, while non-volatile particles are more persistent. Both are considered particularly relevant to health.

The measuring station was located on the roof of Passenger Pier A – centrally located in the airport area and directly influenced by aircraft taking off, landing, and taxiing. The study looked at both the time-of-day dependence of particle pollution and the effects of different wind directions and flight movement intensities. Particle sizes in the range of 7 to 470 nanometers were recorded.

Measurement Technology from Palas in Use

In order to be able to make the challenging distinction between volatile and non-volatile UFPs, high-precision measurement technology from Palas GmbH was used:

• The U-SMPS 2100X (Scanning Mobility Particle Sizer) analyzed the particle size distribution in the range from 4 to 1,200 nm with high temporal resolution.
• The Envi-CPC 100 (Condensation Particle Counter) counted the total number of particles between 7 and 5,000 nm.
• A catalytic stripper (350 °C) was integrated to thermally remove the volatile particle components. The volatile components could be determined indirectly by cyclically alternating between measurements with and without a catalytic stripper.

Air Traffic as a Dominant UFP Source

The evaluation of the measurement data clearly shows that air traffic is the dominant source of ultrafine particles at Zurich Airport – especially the volatile components.

• During the day (7:00 a.m. to 9:00 p.m.), the average UFP concentration was around 62,700 particles/cm³, with around 74% being volatile.
• At night (midnight to 5 a.m.), the concentration fell to 5,700 particles/cm³, with the proportion of non-volatile particles rising to 64%.

The volatile particles were mainly concentrated in the size range of 7 to 20 nanometers – a typical characteristic of engine exhaust gases. As the number of flight movements increased, not only did the absolute concentration rise, but also the relative proportion of volatile particles – from 70% during moderate activity to 80% during intense activity.

Size and Wind Effects

The wind direction also had a significant influence on the measured values:

• When the wind was blowing from the west, carrying exhaust plumes away from the measuring station, the concentrations were significantly lower.
• With northeast winds, with busy handling and takeoff areas downwind, the values rose significantly.

The study impressively shows that not only the time of day and flight intensity, but also local meteorological conditions play a decisive role. A clear source assignment is therefore only possible to a limited extent with single-point measurements.

Model for Other Airports and Cities

The UFP study at Zurich Airport provides a valuable practical example far before the revision and the implementation of the new EU directive on air quality. The set up used not only enable precise and size-resolved particle measurements, but also differentiate between volatile and non-volatile fractions – a decisive criterion for future-oriented air quality assessment.

At the same time, the study makes it clear that single-point measurements alone are not sufficient to clearly identify emission sources. Future monitoring concepts should therefore be implemented in a spatially differentiated, meteorologically embedded, and standards-based manner – in line with the new EU requirements.

With the Air Quality Directive 2024, the European Union is sending a clear signal: ultrafine particles must be systematically measured, understood, and regulated. The study at Zurich Airport shows how this can already be implemented in practice – and provides a concrete template for cities, airports, and environmental authorities across Europe.

Study and Figures:

© Zurich Airport

• Fleuti, Emanuel; Maraini, Silvio: Ultrafine Particles - Study on volatile and non-volatile particles at Zurich Airport (https://media.flughafen-zuerich.ch/-/jssmedia/airport/portal/dokumente/das-unternehmen/politics-and-responsibility/environmental-protection/technische-berichte/2022-01_zrh_nv-v-ufp-study.pdf?vs=1&rev=ea9ddf362e894291be5df69489b0eec1)