My understanding is that most instruments for PM measurements don't actually measure weight but somehow "count" particles. If that is the case, then the density of the particles is assumed.
If we measure very dense particles or very heavy particles, these instruments might be less accurate.
Why don't we just report particles per cubic meter instead?
Dear Sébastien
This is a good question. Current air quality regulations focus on mass-based particulate matter (PM), and neglect ultrafine particle number concentrations.
The measurements are made in ug/m^3 because apparatus measured the mass of a sample (in ug) and divide by the volume of air.
However, there new instruments which can measure the concentration number but they are more complicated to operate and cost significantly more. Also there is a debate about if we should measure the number of particles per m^3.
Note that bioaerosols (pollen, fungi) are measured in particles per m^3. But not inorganic aerosols.
There are at least two reasons particulate matter to be measured in ug/m^3, and not particles per m^3:
- first, which is obvious, the unit m^-3 is ambiguous hence the mass density of the different aerosol particles is (very) different.
- second, some of the most widely used methods for measurement of the 'bulk' concentration (polluted air passed trough filter) obtain ug/m^3. Important remark is that actually this method is incorporated in the correct definition of PM10 and PM2.5, see Article2/points 18 and 19 of the attached file.
Thanks for your answers. Again, this is not my field, so am I to understand that there are some instruments that actually measure the mass of the particulates (that get caught in some medium)? I would assume that you weigh the medium before and after the test, and have a very accurate measure of air flow.
I was not aware of this. I always assumed measurements were done with things like the Grimm instrument.
The measurements of particulate matter can be classified as follows:
1. Mass-based, expressed in micrograms of particulate matter per cubic meter of air or per kg of air. The concentrations are expressed separately for PM 2.5 and PM10.0.
2. Number-based where we count the number of all particles per cubic meter of air (or per kilogram of air) regardless of their size.
3. Particle-size distribution based where we measure the aerosol spectrum (concentration of particles in a given interval of particle sizes)
The third group is probably the most important if we are seeking to gain a deep understanding of physical and chemical processes and verify our models based on the stochastic coagulation equation.
For further details please consult
https://tsi.com/getmedia/8814639a-b1b6-4cca-a27a-74f89f836990/Air-Quality-White-Paper_How-Air-Quality-is-Measured_2_US_5002324_RevA_We?ext=.pdf
Particle count does not take into account the fact that particles are of different sizes and different densities. The effects of particulate matter (e.g., health effects) are generally correlated with mass and not with count.
Pedro Lilienfeld is correct - we have many epidemiology studies that find significant health effects are linked to fine particle mass. These include studies that examine what happens when PM mass is reduced over large areas that show reductions in mortality and other effects, as well as new studies employing causal inference and other methods that address issues with confounding. Note measurements of mass, number, and surface area are all relevant, not to mention composition. It's possible ultra fine particles (high numbers) may have effects, and there is some evidence of that - It is important to note, however, that they tend to move into the accumulation mode (0.5um to 1 plus um) with time and distance away from sources, which reduces exposure distant from high ultra fine areas such as near roads. So while both kinds may have effects, you'd expect more population exposure to high mass, not high number. So far, the evidence has been increasing that we need to strengthen standards for fine particles and continue to assess the effects of ultra fine particles. I'm leaving out the difference in size and composition between fine and coarse inhalable particles (up to 10um).
Both number concentrations (typically particles per cm3) and mass concentrations (typically micrograms or miligrams per cubic meter, less frequently grains per cubic foot) can be directly measured, and are used in legislation (exhaust emissions from motor vehicles, non-road mobile machinery, large commercial jet engines, etc., mass concentrations also in ambient air quality).
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