Due to their small size, ultrafine particles (UFPs) are considered particularly harmful to health. They can penetrate deep into the lungs, enter the bloodstream, and trigger inflammatory processes in organs. Unlike conventional particulate matter fractions (PM10 or PM2.5), they contribute only a small amount to the total mass of particles, but are present in very high numbers in the air—and have been largely unregulated until now. This is changing with the new EU Air Quality Directive.

Long-term study at an urban background site in Paris

For several years now, there have been more and more scientific studies on this topic. One example is the study being conducted in Paris between October 2019 and December 2022. The location – in Nelson Mandela Park near the Les Halles shopping center – was deliberately chosen because it reflects typical exposure conditions for the population without being directly adjacent to busy roads or individual sources of emissions. It thus already corresponds to the concept of a “supersite” as envisaged in the new EU directive.

Supersites are extensively equipped measuring stations that continuously record not only classic particle sizes such as PM10 and PM2.5, but also ultrafine particles and a variety of gaseous air pollutants.

In Paris, in addition to UFPs, nitrogen oxides (NO, NO2, NOx), ozone (O3), carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), and ammonia (NH3) were also measured. The measurement program was supplemented by parameters such as soot, organic aerosols, and the chemical composition of the particles. This broad-based measurement strategy enables a comprehensive assessment of air quality and a detailed analysis of the relationships between pollutants, emission sources, and meteorological conditions – in line with the expanded requirements of the new EU directive.

High-precision instruments for monitoring ultrafine particles

High-precision devices from Palas GmbH in Karlsruhe were used for the particle measurements. The ultrafine particles were measured with a U-SMPS (Mobility Particle Size Spectrometer), which consists of a bipolarly charged X-ray neutralizer (model 1297), an electrostatic classifier (DEMC 1000 and DEMC 2000) and an ENVI-CPC condensation particle counter (model 100). In addition, a Fidas® 200 optical aerosol spectrometer was used to measure PM10 and PM2.5 mass concentrations.

All devices were operated in accordance with the requirements of technical specifications CEN/TS 16976 (now EN 16976) and CEN/TS 17434, i.e., precisely the standards that are to be applied in accordance with the new directive. The systems used enabled reliable long-term measurement with high data availability and precise size resolution in the range from 8 to 400 nanometers.

Key findings: concentrations, correlations, and sources

The study found that the average UFP concentration over the entire measurement period was 8,100 ± 4,800 particles per cubic centimeter. Particularly striking: the WHO recommended guideline values for daily averages (10,000 #/cm³) and hourly averages (20,000 #/cm³) were exceeded on 353 days – about one third of the study period. Higher UFP values typically occurred during the morning and evening rush hours, indicating the importance of road traffic as the main source. This was confirmed by significant correlations with nitrogen dioxide (r = 0.58) and soot from fossil fuels (r = 0.51). Another peak at midday can be attributed to the formation of new particles from gaseous precursors through photochemical processes such as solar radiation or high temperatures. The particle size distribution showed that the majority of particles (around 60%) were in the so-called Aitken mode (20–100 nm) – typical of fresh emissions from combustion processes. A comparison with data from 16 European cities shows that Paris is in the upper range of UFP pollution, making it one of the most affected metropolitan areas.

This study impressively demonstrates that UFP measurements in urban areas are technically feasible, scientifically relevant, and necessary for health policy. The new EU directive now provides a regulatory framework that requires precisely such data sets. The measurement technology used by Palas already meets future requirements for accuracy, standard compliance, and data availability.

Study:

• Abbou, G., Ghersi, V., Gaie-Levrel, F., Kauffmann, A., Reynaud, M., Debert, C., Quénel, P., Baudic, A. (2024). Monitoring ultrafine particles in Paris: From total concentrations to size distribution measurements. Aerosol Air Qual. Res. 24, 240093. (https://doi.org/10.4209/aaqr.240093)