How is Particulate Matter (PM) Measured?
Particulate Matter (PM) is a serious environmental pollutant linked to a variety of adverse health effects. Knowledge of PM is vital for communities that want to know about their air and take action to improve it.
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What is PM?
PM refers to a number of tiny particles that float around in the air and affect many things, including your health and the environment. Particulate matter isn't one substance, but is rather made up of a bunch of different substances. This means that there are also many different sources of PM.
Read more about what particulate matter is here.
How is PM Measured?
There are a number of ways to measure particulate matter; these methods vary widely in their cost, labor, and hardware requirements. For the purposes of this blog, these methods are going to be sorted into three categories: federal reference methods, federal equivalent methods, and low-cost sensors.
Federal Reference Methods
Federal Reference Methods, or FRMs, are the gold standard for measuring particulate matter and many other air pollutants. Given their high cost ($15,000 to $40,000), they are typically run and maintained by various governmental organizations and not individuals.
FRMs typically employ gravimetric methods to measure particulate matter. These involve intaking air to collect particles on filters, which are then weighed and analyzed in a lab (CARB). The actual mass concentration values are generated by "measuring a filter's weight before and after sample collection and then dividing the difference by the total volume of air sampled through the filter."
FRMs are the gold standard, but they are also the most expensive and require the most labor.
Federal Equivalent Methods
Federal Equivalent Methods, or FEMs, are similar to FRMs, but with a number of differences. To start, FEMs are not standardized. Instead, the term "FEM" refers to a method of collecting air pollutant data that "has been designated as an equivalent method" to FRMs (eCFR). This means that many different methods and pieces of hardware can be used in a Federal Equivalent Method.
One of the more commonly used FEMs is a Beta Attenuation Monitor or BAM for short. BAMs use "beta ray attenuation to measure the mass concentration of PM in ambient air" (WA Gov). This involves sending beta particles through a piece of filter tape that collects particulate matter in the air. The "difference in the degree of attenuation between the pre-and-post-sampled filter tape is directly proportional to the mass of PM in the sampled air."
While FEMs may be less expensive and less labor intensive than FRMs, they still aren't easily attainable by concerned individuals.
Low-Cost Sensors
There are many types of low-cost sensors that measure different air pollutants in different ways. The name, or adjective, "low-cost" is derived from the fact that they are cheaper than FRMs and FEMs. However, they can still run anywhere from the low hundreds to thousands. Given the diversity in data collection methods between low-cost sensors, there isn't just one way they collect data, even for a single pollutant like PM. However, some methods are more popular than others.
Our PurpleAir sensors (which are defined as low-cost) use laser counters to detect particles. Those measurements are then used to extrapolate PM values. This method of data collection is called optical particle counting; the devices themselves are sometimes called OPCs. According to the California Air Resources Board, "particles in the air have the ability to scatter light. Optical particle counters (OPC) measure the amount of light scattered by particles to estimate the number and size of particles present in the air." More information is available in our community article here.
(The above photos picture a PurpleAir Flex and PMS6003 laser counters respectively)
Which Method is Right For You?
FRMs and FEMs
For most individuals, purchasing and maintaining an FRM or FEM is out of the question. However, that doesn't mean you can't have FRM or FEM data. The EPA runs AirNow, which provides an interactive map that reports data from both FRM and FEM sources.
If you're looking for regulatory data, this is where you'll find it.
Low-Cost Sensors
Low-cost sensors are becoming increasingly popular for a couple of reasons. First, given their much-reduced price when compared to FRMs and FEMs, they are more reasonably attainable by individuals. Second, because of reason number one, there are more low-cost sensors deployed out in the wild than there are FRMs or FEMs. This means that you're much more likely to have a sensor near you that can provide hyper-local air quality readings.
If this sounds interesting to you, take a look at the PurpleAir Map. On it, you can view recent data from any public PurpleAir sensor. It's very possible that there might even be a sensor near you! If there isn't, or you want more localized data, consider getting a PurpleAir sensor yourself. This community article outlines the differences in our products and might help you determine which monitor is right for you.
Please note that there is no substitute for an FRM or FEM. If you're unsure about using a low-cost sensor, read the EPA's article on how they should be used to help support your decision.
| The EPA also offers the AirNow Fire and Smoke Map, which includes data from FRMs and FEMs, as well as PurpleAir sensors. This way, you can view regulatory and supplemental data at the same time. |
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