There are various methods to measure fine and ultrafine particles, but does it matter whether you use a mass-based or number-based measurement technique? Let's explore the nuances of each method.
Common methods for measuring particulate matter (PM) of 2.5 microns or smaller (PM2.5) include gravimetric, light scattering, and beta attenuation techniques. Tapered element oscillating microbalance (TEOM) instruments are also widely used. However, these methods often miss ultrafine particles (UFPs), which are 0.1 microns or smaller (PM0.1).
Differences Between Mass- and Number-Based Ambient Air Measurements
In mass-based measurements, every nanogram of particulate matter counts the same, regardless of particle size. In number-based measurements, each particle counts the same, regardless of its size or mass.
For example, consider a hypothetical normalized mass size distribution centered at 190 nm. If we assume all particles in this distribution are spherical with a density of 1.0 g/cc, we can create a number-based distribution. The median of this number-based distribution is 100 nm, while the mass-based perspective produces a median of 190 nm. These two curves describe the same particle population from different perspectives.
In Figure 2, we see that 50% of the mass contains 92% of the particles. Alternatively, the upper 50% of the mass is dominated by only 8% of the particles. This phenomenon, where total mass is dominated by a small percentage of particles, is common in ambient air measurements. This is why number-based measurements are valuable; while UFPs are included in PM2.5 metrics, they are nearly invisible in practice.
Real Ambient Air Measurements
Figure 3 shows a particle size distribution collected from ambient air measurements at TSI’s headquarters. The red dashed line indicates a low count of aerosol with a broad size distribution centered at 100 nm. The calculated mass concentration was 12.3 µg/m3, with a number concentration around 4,500 particles/cm3.
Fifteen minutes later, another scan (shown in blue) revealed a spike in particles around 20-40 nm, causing the total number concentration to increase five-fold. However, the calculated particle mass concentration remained the same. Table 1 compares the number-based and mass-based values for these two scenarios:
| Sample | PM0.7 (µm/m3) | Number conc. (#/cm3) |
|---|---|---|
| Red dashed line | 12.3 | 4.47 x 10^3 |
| Blue line | 12.3 | 2.21 x 10^4 |
This example illustrates that using only mass-based particle measurement systems results in significant data loss. From a mass-based perspective, these measurements are identical, even though the second contains five times more particles. The missed particles, especially the smaller ones, may pose considerable health risks.
Number- vs. Mass-Based Measurements: A Gateway to New Insights?
Number-weighted data provides significant insight into the story of ultrafine particles in the atmosphere. To measure UFPs effectively, we should avoid mass-based methods and techniques biased towards larger particles. Extinction-based techniques and light scattering are heavily biased in this direction, making UFPs practically invisible to these instruments.
A number-based measurement technique reveals the presence of UFPs by simply counting particles. Measuring number concentration provides information unknown to mass-based systems. Additionally, measuring particle number concentration and size distribution in the atmosphere offers insights that complete the story from both exposure and atmospheric science perspectives.
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