Reference Method for the Determination of Fine Particulate Matter (PM.) in the Atmosphere

Introduction

Fine particulate matter with an aerodynamic diameter of 2.5m or less (PM.) is a key indicator of airquality and a major concern for public health. Accurate determination of PM. concentrations requires a robust, reproducible, and internationally recognised method. The most widely accepted reference method is described in the US Environmental Protection Agency (EPA) Compendium of Methods for the Determination of Criteria Air Pollutants (Method5.1) and the European Standard EN12341. Both protocols rely on sizeselective sampling with a highvolume air sampler and subsequent gravimetric measurement of the collected mass.

Principle of the Method

The method separates particles smaller than 2.5m from the air stream using a sizeselective inlet (usually a cyclone or impactor). The particles are drawn onto a preweighed, filterpaper substrate with a known flow rate. After a defined sampling period, the filter is reweighed under controlled temperature and humidity conditions. The mass difference, divided by the volume of sampled air, yields the PM. concentration (gm).

Equipment and Materials

• Highvolume sampler (flow rate 5001000Lmin)
• PM. sizeselective inlet cyclone or impactor calibrated for 2.5m cutoff
• Preconditioned filter media typically 47mm PTFE or quartz fibre, 0.2m pore size
• Analytical balance (precision 1g) with an environmental chamber (201C, 505% RH)
• Flow calibrator (thermal massflow or orifice plate) and a calibrated rotameter
• Sealing devices filter holders, stainlesssteel prefilters (optional)
• Documentation sampling log, chainofcustody forms

Sampling Procedure

1. Filter Preparation

1. Condition the filter in the balance chamber for at least 24h.
2. Record the initial mass (M) to the nearest 0.1g.
3. Place the filter in the samplers holder, ensuring a leakfree seal.

2. Flow Calibration

4. Calibrate the sampler flow using a certified flow calibrator. Adjust the fan speed until the target flow (e.g., 800Lmin) is achieved.
5. Document the calibrated flow rate and the temperature/pressure correction factor.

3. Sampling

6. Start the sampler and record the start time (t). Ensure continuous power and unobstructed inlet.
7. Sample for a minimum of 24h (or a defined period that yields a mass >5g to meet detection limits).
8. Stop the sampler and record the stop time (t). Calculate the sampling duration t = t t.
9. Measure the flow rate at the end of the run; if variation exceeds 5% adjust the final volume accordingly.

4. PostSampling Handling

10. Transfer the filter to the balance chamber without touching the filter surface.
11. Condition for another 24h and record the final mass (M).

Gravimetric Calculation

The concentration of PM. (C) is calculated by:

C = (M M)V

where:

• M = initial filter mass (g)
• M = final filter mass (g)
• V = total sampled air volume (m), derived from flow rate (Q, mmin) and sampling time (t, min): V = Qt

Example calculation:

Quality Assurance & Quality Control (QA/QC)

Field Blanks

Expose a preconditioned filter to the sampling environment for the same duration as a regular sample, but without air flow. Analyse it identically; the blank mass must be <1g to be acceptable.

Duplicate Samples

Collect at least 10% of the total samples as sidebyside duplicates. The relative difference should be <10%.

Calibration Checks

Perform flow calibrations before and after each sampling day using a traceable standard. Record correction factors.

Balance Verification

Use certified weight sets to verify balance linearity weekly. Balance drift >0.2g requires service.

Data Validation

• Reject any sample where M <5g (below detection limit).
• Check for outliers using the Grubbs test; values exceeding 2 from the mean are investigated.
• Apply a correction for temperature and humidity variations if the balance chamber deviates beyond the specified range.

References

1. U.S. EPA, Compendium of Methods for the Determination of Criteria Air Pollutants, 5th Edition, Method 5.1 (PM.FRM/FEM), 2020.
2. European Committee for Standardisation, EN12341:2021, Determination of the Mass Concentration of Particulate Matter (PM10 and PM2.5) in Ambient Air by Gravimetric Method.
3. World Health Organization, Air Quality Guidelines Global Update 2021, Chapter3, Particulate Matter.
4. Hinds, W.C., Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles, 2nded., Wiley, 1999.
5. ISO 215014:2008, Measurement of particulate air pollution Determination of particle size distribution Part4: Devices for the collection of PM2.5 and PM10 for subsequent gravimetric measurement.