The Appropriate Measurement Metric for Ultrafine Particles: A Case for Number-Based Methods

As concerns about air quality and its impacts on health continue to rise, accurately measuring and understanding particulate matter (PM) in the atmosphere becomes increasingly critical. One of the most challenging aspects of air quality monitoring is the measurement of ultrafine particles (UFPs), which are particles smaller than 0.1 microns (PM0.1). These tiny particles pose significant health risks, yet traditional measurement techniques often overlook them. In this blog post, we will explore the differences between mass-based and number-based measurement methods, illustrating why number-based methods are essential for a comprehensive understanding of UFPs.

Mass-Based Measurement
Mass-based measurement techniques, such as gravimetric analysis, light scattering, beta attenuation, and tapered element oscillating microbalance (TEOM), have been the standard methods for monitoring particulate matter (PM2.5). These methods measure the mass of particles in a given volume of air, providing a straightforward metric for air quality.

However, mass-based measurements have significant limitations when it comes to UFPs:

• Sensitivity to Larger Particles: Mass-based methods are biased towards larger particles because they contribute more to the total mass. As a result, the presence of numerous smaller particles can be overlooked if their combined mass is negligible compared to larger particles (Zhang et al., 2010).
• Invisibility of UFPs: Despite being part of the PM2.5 category, ultrafine particles often remain undetected in mass-based measurements. This can lead to a misleading assessment of air quality and potential health risks (Le Berre et al., 2023).

Number-Based Measurement
Number-based measurement techniques, on the other hand, count the number of particles in a given volume of air, regardless of their size. This method provides a different perspective on air quality by highlighting the presence of numerous small particles that mass-based methods might miss.

Key advantages of number-based measurements include:

• Detection of UFPs: By counting particles, number-based methods ensure that ultrafine particles are detected and quantified, providing a more accurate picture of air quality (Le Berre et al., 2023).
• Health Risk Assessment: UFPs are particularly harmful to human health due to their ability to penetrate deep into the respiratory system and even enter the bloodstream. Number-based measurements are crucial for assessing the true exposure and risks associated with UFPs (Calderón-Garcidueñas and Ayala, 2022).

Key Findings from Recent Literature

A number of recent studies highlight the discrepancies between mass-based and number-based measurements for UFPs, emphasizing the importance of using number-based methods for accurate air quality assessment.

1. Low Emission Zone Monitoring:

Low emission zones (LEZs) are designated areas where access by certain polluting vehicles is restricted to improve air quality. Implemented in various cities worldwide, these zones aim to reduce harmful emissions from vehicles, particularly diesel engines, by accelerating the adoption of cleaner, low-emission vehicles. A study on the Leipzig low emission zone offers valuable insights into the importance of measuring ultrafine particles (UFPs) and highlights why classical metrics, such as PM10, often fall short in capturing the full extent of vehicular pollution's impact. (Löschau et al., 2017)

• Sensitivity of UFP Measurements
  Ultrafine particles (UFPs), particularly those with diameters between 30 to 200 nanometers, are highly sensitive indicators of pollution from motor vehicle emissions. The study in Leipzig demonstrated that measurements of these particles (PN30-200nm for number concentration and PM30-200nm for mass concentration) showed significant reductions from 2010 to 2016 after the introduction of the low emission zone in 2011 in contrast to classical PM methods. This sensitivity highlights how UFP metrics can detect changes in local emissions more effectively than broader metrics like PM10.
• Inadequacy of Classical Metrics
  PM10, which covers particles with diameters up to 10 micrometers, is a commonly used metric for air quality. However, it includes particles from a variety of sources, not all related to traffic emissions. The Leipzig study points out that PM10 is often too insensitive to specifically detect motor-related emissions because it encompasses larger particles from non-motor sources such as soil and road dust, which are less harmful than smaller, combustion-related particles.
  There are currently no legal limits for UFPs (like BC, PN30-200nm, and PM30-200nm) as there are for PM10, PM2.5, and NO2. However, UFPs provide more precise data on pollution from vehicle emissions, emphasizing their necessity for accurate environmental monitoring and public health assessments.
• Effectiveness of Environmental Zones
  The introduction of environmental zones in Leipzig led to a noticeable decrease in UFPs, specifically black carbon (BC) and particles in the PN30-200nm range. This decrease was more pronounced than the reduction in PM10 levels, illustrating the effectiveness of targeting UFPs to reduce harmful pollutants. The success of these environmental zones in reducing UFPs underscores their importance in improving air quality.
• Health Impact Correlation
  One of the most compelling findings of the Leipzig study is the correlation between UFP reduction and decreased health risks. While the reduction in PM10 might appear minor, the associated decrease in UFPs led to a substantial reduction in health risks. This demonstrates an over-proportional health benefit from focusing on UFPs. Reducing these particles can significantly lower the incidence of respiratory and cardiovascular diseases, which are closely linked to exposure to ultrafine particles.

2. Maritime Emissions Study:

A study by Le Berre et al. (2023) underscores the importance of ultrafine particles (UFPs) in urban air pollution, particularly from shipping emissions. UFPs, which constitute 80-90% of particle number concentrations, are often undetected by classical PM metrics like PM10 and PM2.5, which focus on particle mass rather than number. UFPs have a significant health impact due to their ability to carry harmful substances such as transition metals and PAHs, causing inflammation, oxidative stress, cardiovascular diseases, and respiratory issues. The study highlights that number-based measurements are crucial for accurately assessing the pollution and health risks posed by UFPs, as they provide a more detailed picture of particle distribution and concentration. This approach is essential for improving air quality monitoring and public health protection, addressing the limitations of mass-based methods which fail to capture the prevalence and impact of UFPs in urban environments. (Le Berre et al., 2023).

As we move forward in addressing air quality challenges, the importance of number-based measurements for UFPs cannot be overstated. Mass-based methods, while useful for larger particles, fall short in capturing the complete picture of air quality, particularly when it comes to ultrafine particles.

The AVL UltraFine Particle Monitor^TM, a condensation particle counter based instrument, represents a significant advancement in air quality monitoring technology, providing the necessary tools to accurately measure and understand UFPs. By adopting number-based measurement techniques, we can gain a deeper insight into the presence and behavior of ultrafine particles, ultimately leading to better public health outcomes and more informed environmental policies.

In the quest for cleaner air and better health, choosing the appropriate measurement metric for ultrafine particles (UFPs) is crucial. The "Umweltzone Leipzig" study highlights the critical need for integrating UFP measurements into standard air quality monitoring practices. Classical metrics like PM10 and mass-based methods, while traditional, often fail to capture the true extent of UFP pollution and the nuances of motor vehicle emissions and their health impacts.

By focusing on UFPs and embracing number-based techniques, cities can achieve more precise pollution control, better protect public health, and enhance our understanding of air quality. Our advanced instrument offers a comprehensive solution that ensures ultrafine particles are not overlooked, providing accurate, reliable, and detailed measurements of UFPs.

As we continue to combat air pollution, adopting these advanced metrics will be crucial in creating healthier urban environments. Invest in accurate air quality monitoring with the AVL UltraFine Particle Monitor^TM and join us in the fight against ultrafine particle pollution. Together, we can make a difference.

• Le Berre, L., D’Anna, B., 2023. Measurement and modeling of ship-related ultrafine particles and secondary organic aerosols in a Mediterranean port city. Toxics. Available at: https://www.mdpi.com
• Zhang, Q., Gangupomu, R.H., Ramirez, D., Zhu, Y., 2010. Measurement of ultrafine particles and other air pollutants emitted by cooking activities. International Journal of Environmental Research and Public Health. Available at: https://www.mdpi.com
• Löschau, G., Wiedensohler, A., Birmili, W., Rasch, F., Spindler, G., Müller, K., Hausmann, A., Wolf, U., Sommer, W., Anhalt, M., Dietz, V., Herrmann, H., Böhme, U., Kath, H.-G., Kühne, H., Umweltzone Leipzig – Abschlussbericht, Landesamt für Umwelt, Landwirtschaft und Geologie (2017), Schriftenreihe des LfULG, Heft XX/2017, Link: https://publikationen.sachsen.de/bdb/artikel/29757
• Calderón-Garcidueñas, L., Solt, A. C., Henríquez-Roldán, C., Torres-Jardón, R., Nuse, B., Herritt, L., Villarreal-Calderon, R., Osnaya, N., Stone, I., García, R., Brooks, D. M., González-Maciel, A., Reynoso-Robles, R., Pérez-Guillé, B., & Díaz, P. (2008). Long-term air pollution exposure is associated with neuroinflammation, an altered innate immune response, disruption of the blood-brain barrier, ultrafine particulate deposition, and accumulation of amyloid beta-42 and alpha-synuclein in children and young adults. Toxicologic Pathology, 36(2), 289-310.