Ultrafine Particles in Ambient Air

What are ultrafine particles?

Ultrafine particles (UFPs) are airborne particles 100 nm in diameter or smaller¹ — a subclass of the fine particle fraction (below 1,000 nm) that includes the more commonly regulated PM2.5 and PM10. They originate from both natural processes and human activity, including combustion, vehicle exhaust, industrial emissions, and atmospheric nucleation events.² At this scale, UFPs behave differently from larger particles — in the atmosphere, in sampling systems, and inside the human body. Understanding that behavior is what makes their measurement both challenging and important.

Why measure ultrafine particles?

Standard air quality monitoring focuses on particle mass — the total mass of particles present in a given volume of air. PM2.5 and PM10 regulations are built on this approach. But UFPs are so small that even in very high numbers, they contribute almost nothing to measurable mass. Mass-based methods are effectively blind to their presence.

This matters for two reasons:

Atmospheric science. UFPs influence how aerosols form, grow, and interact with sunlight and clouds — processes that are central to understanding air quality and climate. Measuring UFP concentrations and size distributions gives researchers insight into particle formation dynamics and transport that mass measurements simply cannot provide.

Human health. When inhaled, UFPs penetrate deeper into the respiratory tract than larger particles and may enter the bloodstream. Their small size and large surface area relative to mass make them relevant to exposure assessment in ways that PM metrics don't capture. Learn more about UFP health effects.

How are ultrafine particles measured?

Because mass-based techniques cannot detect UFPs reliably, measurement approaches shift to counting particles by number rather than weighing them by mass.

Particle number concentration (PN) answers the question: how many particles are present per unit volume of air? This approach counts every particle regardless of size, making it sensitive to UFPs that would otherwise go undetected. PN measurements are particularly useful for tracking emission sources, exposure events, and formation dynamics.

Particle size distribution (PSD) goes further by pairing counting capability with sizing capability — producing a picture of how particles are distributed across a range of sizes at a given moment. This adds context that a single concentration number cannot: you can see whether a UFP population is dominated by freshly nucleated 10 nm particles or by particles that have grown through condensation and coagulation.

Whichever approach is used, a well-designed air sampling system is essential. Proper sampling ensures that the particles reaching the instrument are representative of what is actually present in the ambient air — accounting for losses in tubing, temperature effects, and other artefacts that can distort results.
 

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