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Number- vs. Mass-Based UFP Measurements
Pros and Cons
There are a number of ways to measure fine and ultrafine particles, but does it really matter whether you use a mass-based or number-based measurement technique? We look at a few examples that investigate the nuance of using one method versus another.
Some common methods for measuring particulate matter (i.e. mass) of 2.5 microns in size or smaller (PM2.5) include those based on gravimetric, light scattering, or beta attenuation techniques; tapered element oscillating microbalance (TEOM) instruments are also common. However, these methods will often miss ultrafine particles (UFPs) which are 0.1 microns or smaller (PM0.1).
In mass-based measurements every nanogram of particulate matter counts the same as the next nanogram, regardless of the sizes of the particles from which they came.
In number-based measurements, each particle counts the same as the next particle, regardless of how large or small they are, regardless of how massive or (ultra)fine they are.
In figure 1, we have a hypothetical normalized mass size distribution that is centered at 190 nm.
If we assume that all the particles in the blue (mass-based) distribution in Figure 1 are spherical and have a density of 1.0 g/cc (often called ‘unit density’), we can make a number-based distribution. That number-based distribution is shown in the red dashed lines. The median of this number-based distribution is 100 nm, while the mass perspective produces a median of 190 nm. Recall that these two curves are describing the exact same population of particles, but just from two different perspectives.
Since the statistic of median differs between these two distributions in Figure 1, let’s dig a little deeper. In Figure 2, we can see that 50% of the mass contains 92 % of the particles. Or another way of looking at it, the upper 50 % of the mass is dominated by only 8 % of the particles.
This phenomenon – the total mass is dominated by a small percentage of the particles – happens routinely in ambient air measurements. This is the key reason that number-based measurements have value… while ultrafine particles are technically included within PM2.5 metrics, practically speaking, they’re nearly invisible. Let’s look at this phenomenon using some real ambient air measurements.
Real Ambient Air Measurements
Figure 3 shows another example where this effect can be observed directly by examining a particle size distribution that was collected from ambient air measurements at TSI’s headquarters. The red dashed line shows a relatively low count of aerosol with a broad size distribution centered at 100 nm. The calculated mass concentration was 12.3 µg/m3 with number concentration around 4,500 particles/cm3.
Fifteen minutes later, another scan was taken (shown in blue). The blue line indicates a spike in particles around 20-40 nm; as a consequence of this peak being present, the total number concentration climbs five-fold. However, the calculated particle mass concentration stays the same. Table 1 provides a comparison of the number-based and mass-based values for these two scenarios.
Two real ambient samples:
This example illustrates that by using only mass-based particle measurement systems, a significant amount of data is lost! From a mass-based perspective, these two measurements would be considered identical, even though the second measurement contains 5 times more particles.
Beyond that, the missed particles - especially the smaller ones - may well pose a not inconsiderable health risk.
Number-weighted data provides significant insight into the story of ultrafine particles in the atmosphere.
So, if we’re going to measure UFP’s in the atmosphere, what should we do from an instrumentation perspective?
We should certainly not use a measurement method that is only mass-based, nor should we use a technique that is significantly biased towards larger particles. Extinction-based techniques and light scattering are both heavily biased in this direction, so UFPs will practically be invisible to those instruments.
A number-based measurement technique reveals the presence of UFPs; simply by counting particles. Measuring number concentration elucidates information unknown to mass-based measurement systems.
Furthermore, the ability to measure particle number concentration and particle size distribution in the atmosphere provide additional insight that helps to complete the story both from an exposure perspective and also an atmospheric science perspective.
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