Ambient air particulate monitoring is of more interest than ever before. The reason is new public awareness of the health effects of small particles suspended in the air, in combination with increased vehicle fleets emitting such particles.
“PM” stands for particulate matter. The number (usually 10 or 2.5) indicates the size of the particulates monitored. PM10 refers to particles with diameter 10 µm or smaller, and PM2.5 to particles with diameter 2.5 µm or smaller.
The Measurement Process
Technically, a sampling PM2.5 monitor and a sampling PM10 monitor look and work by exactly the same principles. Ambient air is forced through a special inlet head (more on that below), and then through a fine particulate filter. Knowing the air flow, the weight of a clean filter, and the weight of the filter after being exposed to the air flow for a known time, you can calculate the weight per volume of the particles.
In its most primitive shape, the filters are actually weighted manually on a scale, but that is of course labour intensive and results in noticeably delays between sampling and having a measurement result. Many particulate monitors instead make use of a low-activity beta radiation source. Beta rays are attenuated as they pass through matter. By measuring the amount of passing rays before and after the sampling sequence, you can calculate the mass of the filter before and after sampling, and thereby the mass of the particles accumulated on the filter.
The Inlet Head
The inlet head is the device that differs between a sampling PM2.5 monitor and a sampling PM10 monitor. In both cases, it consists of a carefully designed and manufactured system of tubes and impact plates. When the air flows through the tubes and changes direction, centrifugal forces make the heavier particles hit and get stuck to the surface of the impact plates. As a result, only lighter and thereby (effectively) smaller particles can pass the head. By applying different dimensions to the tubes and plates, a “cut-off” diameter of the particles is set. Only the particulate fraction below the cut-off diameter continues with the air stream and accumulates on the subsequent filter.
“Cut-off” is actually a little misleading. There is no sharp edge where all particles above the limit is rejected and all below is accepted. The PM number tells the particulate diameter where 50% of the particles are rejected. However, the “sorting curve” of the standardised heads still mimics that of the human respiratory organs, and it gives a good estimation of what the population actually is exposed to.
Other Types of PM Monitors
There is also a group of PM monitors operating with a completely different technology. A laser beam is focused to a very small volume in the air flow. When a particle passes that volume, some or all of the light is reflected or refracted. This is detected by a sensor as a brief light pulse. The intensity of the detected light indicates the size of the particle. By measuring the air flow and accumulating number and sizes of particles over time, the PM10 and/or PM2.5 concentration can be determined. Some of these instruments use the same “cut-off” inlet heads as sampling monitors, some have “open” inlet heads and instead separate the PM10 and/or PM2.5 fractions solely by processing the signal from the detector.
Focus on PM2.5
So why are there two standards: both PM10 and PM2.5? Traditionally, PM10 was the standardised parameter to monitor. However, the smaller particles the deeper into the lungs they reach, and the more damage they cause. That, in combination with modern vehicle air pollution control technology causing relatively higher fractions of smaller than larger particles, has made focus move towards PM2.5 or even smaller particulate fractions. Nowadays, there are also instrument for measurements of e.g. PM1 and even PM0.1 concentrations. However, PM10 and PM2.5 monitors are still the most widely used instruments as of today.