Tracking Particulate Matter: Federal Reference Method (FRM) or Federal Equivalent Method (FEM)?

With growing awareness of the potential impacts and effects of air pollution, federal agencies are strapping down and taking more diligent measures to track ambient particle concentrations. The US Environmental Protection Agency (EPA) is constantly redefining safe parameters of the National Ambient Air Quality Standards (NAAQS).

One of the most concerning airborne pollutants tracked is fine particulate matter, particularly Particulate Matter 2.5 (PM2.5).

As of today, there are two main methods of tracking PM2.5:

  1. Federal Reference Method (FRM) 
  2. Federal Equivalent Method (FEM).

    What is the Federal Reference Method (FRM)?

    The FRM is the primary acceptable sampling design used for collecting and testing concentrations of PM2.5. It's “an integrated, gravimetric method intended to provide a measurement of fine particle mass concentration over a 24h sampling interval”. Here, “gravimetric” means that the concentration of PM2.5 is determined by measuring the net mass gain in each sample to determine an accurate PM2.5 concentration level.

    This method uses a sample that collects atmospheric air through an inlet, which is designed to filter out insects and other larger ambient debris. Once the air has entered the sampling unit, the first chamber effectively removes any other debris bigger than 10 μm. This chamber is also referred to as the PM10 fractionator.

    The once-filtered air sample is then sent “downtube” towards a second chamber, also referred to as the WINS PM2.5 fractionator, which further refines the sample, removing any particles larger than 2.5 μm. The remaining particles are sent to a filter, which is weighed to determine the total PM2.5 gained per sample.

    The EPA has further requirements for methodology and sampling conditions in order to meet the requirements of the FRM.

    What is the Federal Equivalent Method (FEM)?

    The FEM is a system that ensures that alternative methods of sample collection and analysis are as consistent and accurate as possible with the FRM.

    In other words, the FEM is not the name of a sampler design but rather a system used to measure the accuracy of various sampling methods that deviate from the FRM method. It ensures that any variations in methods of collecting air samples for PM2.5 concentrations are accounted for and classified according to how closely they resemble the FRM.

    FEMs are distinguished into three classes: Class I, II, and III. This classification system determines how close the sampling method is to the FRM, with Class I being the most similar and Class III the least.

    Class I is, therefore, the most accurate of the three FEM methods, as it deviates the least from FRM. The FEM classification system will then determine the equivalency required for the sample to adhere to FRM standards.

    FEM Classification System:

    • Class I: Minimal deviations from the FRM.
    • Class II: More significant deviations, but still designed to collect 24h filter samples for gravimetric analysis.
    • Class III: Includes all other alternative methods designated as acceptable forms of sample collection deemed adequately comparable to the FRM.

    What’s the Difference between FRM and FEM?

    While the FRM is the accepted standard for analyzing PM2.5 concentration in air samples, the FEM is an equivalency system. In addition, the FRM is a method recognized by the EPA, while the FEM is a system used to determine whether alternative measurement techniques otherwise not accepted by the EPA meet the applicable requirements in the Code of Federal Regulations (CFR).

    Why are the FRM and FEM Relevant?

    Knowing the differences between the FRM and FEM helps to determine the legitimacy and accuracy of a given air sampling method that aims to evaluate ambient PM2.5 concentrations.

    If a sampling device or method does not meet the FRM standard, the FEM can be used to determine whether it meets the requirements set out by the EPA according to the CFR guidelines. It can then be classified into one of the three Classes mentioned to determine its efficacy and overall accuracy.

    Worried About Air Quality?

    Monitor the Particulate Matter levels around the world with our free, real-time PurpleAir Map. or join PurpleAir's mission to make air quality data accessible to everyone by investing in an air quality monitor for your home. 

    Together, we can be informed and make changes in our daily habits and the community to improve air quality.