ISO 25597:2013 — Stationary Source Emissions — PM2.5 and PM10 Test Method Using Cyclone Samplers and Sample Dilution

1. Overview and Principle of ISO 25597:2013

ISO 25597:2013 specifies a test method for determining the mass concentration of PM2.5 and PM10 particulate matter in stationary source emissions using cyclone samplers combined with sample dilution. This standard addresses a critical gap in emission monitoring: while total particulate matter measurement methods have existed for decades, size-selective sampling for fine and coarse fractions in hot, moist stack gases requires specialized techniques to avoid sampling artifacts caused by condensation, chemical reaction, and particle agglomeration.

The principle relies on two complementary approaches. The basic cyclone sampling method uses a series of cyclones operating at their designed cut-point diameters to separate PM2.5 and PM10 fractions from the sample gas stream. The dilution sampling method conditions the hot stack gas by mixing it with filtered, dry dilution air before particle collection, thereby reducing the gas temperature and relative humidity to levels that prevent condensation and allow collection on filter media suitable for gravimetric analysis.

2. Equipment Requirements and Sampling System Design

2.1 Cyclone Sampling System

The cyclone sampling system requires a sampling probe, a series of cyclones with calibrated cut-points at 2.5 um and 10 um aerodynamic diameter, filter holders, a flow control system, and a vacuum source. The cyclones must be constructed from materials resistant to the chemical composition of the stack gas, typically Type 316 stainless steel or Hastelloy for corrosive environments. Each cyclone must be individually calibrated using monodisperse aerosol challenge particles to establish the actual cut-point at the intended sampling flow rate.

2.2 Dilution Sampling System

The dilution sampling system introduces particle-free, dry dilution air at a known ratio (typically 5:1 to 20:1) to condition the sample gas before particle collection. The dilution air must be filtered through HEPA and activated carbon stages to remove both particulate and gaseous interferents. The dilution ratio is maintained by mass flow controllers on both the raw sample and dilution air streams, with continuous monitoring of temperature and pressure at the sampling point.

3. Pre-Sampling and Gravimetric Procedures

3.1 Filter Conditioning and Weighing

The gravimetric analysis protocol specified in ISO 25597:2013 requires meticulous control of filter conditioning. Filters must be equilibrated at 20 C +/- 2 C and 50% +/- 5% relative humidity for a minimum of 24 hours before pre-sampling weighing. Post-sampling, filters are re-equilibrated under identical conditions for at least 24 hours before re-weighing. The analytical balance must have a readability of 0.01 mg or better, with calibration verified before each weighing session.

Electrostatic charge effects on the filter medium must be neutralized using a polonium-210 or similar ionizing source before each weighing. Control filters (field blanks) must be processed alongside sample filters at a frequency of at least 1 per 10 samples to assess handling and transport contamination.

3.2 Isokinetic Sampling Requirements

Isokinetic sampling, where the velocity of gas entering the sampling nozzle equals the free-stream velocity at the sampling point, is essential for representative particle collection. The standard requires that isokinetic conditions be maintained within +/-10% for particles larger than 2.5 um. For PM2.5, the isokinetic tolerance is relaxed to +/-20% due to the smaller particle size and lower inertia. The sampling nozzle diameter must be selected based on the stack gas velocity profile determined by pitot tube traverse measurements conducted prior to sampling.

4. Engineering Insights for Reliable PM Measurement

Field experience with ISO 25597:2013 has identified several practical challenges. First, the cyclone cut-point is sensitive to flow rate variations; a 5% deviation in flow can shift the D50 by 10-15%, causing misclassification of particles. Real-time mass flow correction to standard temperature and pressure (STP) conditions is essential. Second, filter loading must be controlled: excessive loading increases pressure drop and alters cyclone performance, while insufficient loading degrades gravimetric precision. The optimal filter loading is 2-5 mg for 47 mm filters.

Third, for high-moisture stacks (e.g., wet scrubbers, biomass combustion), the dilution ratio must be carefully selected to reduce the relative humidity below 60% at the filter face. Computational thermodynamic modeling of the dilution process using tools such as ASPEN Plus or Cantera can predict condensation risks before field deployment.

5. Frequently Asked Questions