Standard Test Methods for Condensable Particulate Matter (CPM): Procedures, Standards, and Applicati

Condensable particulate matter (CPM) is the fraction of stack emissions that exists in the vapor phase at stack conditions but condenses into liquid or solid particles after discharge into the atmosphere, and it is now a critical parameter in stationary source emission testing. Standard test methods such as US EPA Method 202, combined with filterable PM methods like EPA Method 201A and ISO 9096, enable industries and accredited third-party laboratories to quantify total particulate emissions accurately. This article provides a technical overview of CPM sampling trains, analytical procedures, quality control requirements, and typical industrial applications.

Technical Overview

Condensable particulate matter (CPM) is defined as material that is in the vapor phase (or passes through a heated filter as gas) at stack temperature but which condenses and/or reacts upon cooling and dilution in the ambient air to form liquid or solid particulate matter immediately after discharge from the stack. CPM is not retained on a heated filter during conventional particulate sampling, and therefore it is not measured by classical filterable PM test methods such as US EPA Method 5, Method 17, ISO 9096, ASTM D3685/D3685M, GB/T 16157, or the low-concentration gravimetric method HJ 836.

Total primary particulate emissions from a stationary source are the sum of two fractions:

• Filterable particulate matter (FPM): particles captured on a filter maintained at the stack temperature or at approximately 120 °C in an out-of-stack heated filter holder.
• Condensable particulate matter (CPM): material collected downstream of the heated filter in a condenser and impinger (or dry condenser) system, then recovered by solvent rinse, evaporation, and gravimetric analysis.

The principal reference method for CPM worldwide is US EPA Method 202, published in Appendix M of 40 CFR Part 51, with EPA Method 201A and Methods 5/17 commonly used for the filterable fraction. EPA's conditional method ALT-011 provides a dry condenser-coil alternative that reduces sulfate artifacts. In ultra-low emission (ULE) programs and secondary aerosol studies, CPM frequently accounts for a substantial share of total PM discharged after wet flue gas desulfurization, and its measurement is therefore essential for accurate emission inventories, permit compliance demonstrations, and control equipment optimization.

Testing Samples

CPM testing is performed on extracted stack gas samples obtained isokinetically from stationary source exhaust ducts. Typical sample matrices include the following.

Coal-Fired Boiler Flue Gas

Flue gas sampled downstream of air pollution control devices (selective catalytic reduction, electrostatic precipitator or baghouse, and wet flue gas desulfurization). These streams often contain sulfur trioxide, ammonium bisulfate from SCR ammonia slip, and fine sulfate aerosols that dominate the CPM fraction.

Steel and Sintering Process Off-Gas

Off-gas from sintering machine heads, blast furnaces, and other metallurgical processes, where alkali metal chlorides, sulfates, and heavy metal vapors contribute to condensable emissions.

Cement Kiln and Waste Incinerator Exhaust

Kiln tail gas and municipal solid waste incineration flue gas containing acid gases (HCl, SO2), volatile trace metals, and organic condensables that are converted to particulate form after release.

Gas-Fired Units and Chemical Process Vents

Exhaust from gas-fired boilers, gas turbines, and chemical process vents, where CPM is typically low in mass but still significant relative to very low filterable PM limits.

Testing Items

A complete CPM test campaign typically reports the following items:

• Total condensable particulate matter (CPM) mass concentration, in mg/dscm, and mass emission rate.
• Inorganic CPM fraction: the residue from the aqueous (water) condensate and rinses after evaporation and desiccation.
• Organic CPM fraction: the residue from the organic solvent (hexane or methylene chloride) extraction and rinses.
• Optional speciation of the inorganic fraction by ion chromatography (sulfate, nitrate, chloride, ammonium, calcium, magnesium) for source apportionment and control technology studies.
• Optional trace metal analysis of the condensate and filter catch by ICP-OES or ICP-MS.
• Paired filterable PM (FPM) results, allowing calculation of total PM = FPM + CPM, and combined PM10/PM2.5 totals when size-segregating cyclones are used.

Testing Methods

The universally accepted approach follows US EPA Method 202, either alone or coupled with filterable PM methods. Key variants are described below.

US EPA Method 202 (Wet Impinger Method)

The sampling train consists of a probe, a heated filter holder maintained at 120 °C ± 14 °C (out-of-stack configuration), a water-cooled or air-cooled condenser, a series of glass impingers immersed in an ice bath, and a silica gel drying impinger. The test is conducted isokinetically with sampling velocity maintained within 90–110% of stack velocity, and the minimum dry gas sample volume specified by the method is collected.

After sampling, the condenser, impingers, and connecting glassware are rinsed with water and with organic solvent. The combined aqueous fraction is purged with nitrogen (or clean air) at approximately 14 L/min for a minimum of one hour to strip dissolved sulfur dioxide, which would otherwise oxidize to sulfate and create a positive measurement artifact. The purged aqueous sample is evaporated in a preconditioned weighing dish, desiccated for 24 hours, and weighed to determine inorganic CPM. The organic solvent extracts are evaporated at room temperature, desiccated for 24 hours, and weighed to determine organic CPM. The final result is the blank-corrected sum of both fractions, expressed in mg/dscm.

Condenser Coil Method (EPA ALT-011)

This conditional test method replaces the water-filled impingers of Method 202 with a chilled glass condenser coil. Water vapor and CPM condense on the coil surface with a short gas-liquid contact time, which substantially suppresses SO2 dissolution and the associated sulfate artifact. Recovery is simplified because the condensed film is rinsed directly from the coil. The gravimetric analytical sequence is otherwise identical to Method 202.

Combined Filterable and Condensable Trains (Method 201A + Method 202)

When total primary PM10 or PM2.5 is required, an in-stack assembly of a PM10/PM2.5 sizing cyclone and filter (US EPA Method 201A) is connected in series with the Method 202 condensation section. The cyclone/filter catch yields filterable PM by size fraction, and the impinger recovery yields CPM; their sum represents total primary particulate emissions of the selected cut size.

Related Filterable PM Baseline Methods

• US EPA Method 5 / Method 17: classical filterable PM determination with out-of-stack or in-stack heated filters.
• US EPA Method 201A: PM10 and PM2.5 filterable fractions at a constant sampling rate.
• ISO 9096 and ISO 12141: international gravimetric determination of particulate concentrations, including low-concentration sources.
• ASTM D6331: determination of mass concentration of particulate matter at low concentrations by gravimetric or beta attenuation techniques.
• GB/T 16157 and HJ 836: Chinese national and industry standards for filterable PM from stationary sources, routinely paired with Method 202-based CPM procedures in ultra-low emission assessments.

Quality Assurance and Quality Control

• Isokinetic sampling maintained within 90–110% for the full test run, with a minimum of three valid runs per test condition.
• Heated filter temperature controlled at 120 °C ± 14 °C to prevent premature condensation before the collection section.
• Mandatory nitrogen purge of the aqueous fraction (minimum one hour) to eliminate dissolved SO2 artifacts.
• Field train blanks and laboratory reagent blanks collected and blank-corrected in the final result.
• Calibrated analytical balance with 0.1 mg or better readability; residues desiccated for 24 hours before weighing.
• Gas metering system calibration, leak checks before and after each run, and full documentation per the applicable method and quality management requirements of accredited third-party laboratories.

Applications

Standardized CPM testing supports a wide range of regulatory, engineering, and research objectives:

• Compliance demonstration for total particulate emission limits, particularly where ultra-low emission standards (e.g., 10 mg/m³, 5 mg/m³, or lower) require quantification of both filterable and condensable fractions.
• Investigation and mitigation of visible stack phenomena such as blue plume or colored plume caused by sulfuric acid and ammonium sulfate mist.
• Assessment of secondary PM2.5 formation potential in emission inventories and air quality modeling.
• Performance evaluation and optimization of SCR systems, wet flue gas desulfurization, wet electrostatic precipitators, and additional polishing control devices.
• Permit applications, environmental impact assessments, and commissioning acceptance tests for new or modified stationary sources.
• Academic and industrial research on PM2.5 source profiles, ammonium bisulfate formation, and control technology development.

FAQ

What is the difference between filterable and condensable particulate matter?

Filterable PM is captured on a heated filter during sampling and represents particles already in solid or liquid form at stack temperature. Condensable PM passes through the heated filter in vapor form and only converts to particulate matter after cooling and dilution in the atmosphere; it is collected in the condenser/impinger section and quantified gravimetrically.

Why is the nitrogen purge required in EPA Method 202?

During sampling, sulfur dioxide dissolves into the impinger condensate. If not removed, dissolved SO2 slowly oxidizes to sulfate, which would be weighed as inorganic CPM and bias the result high. Purging with nitrogen for at least one hour strips the dissolved SO2 before evaporation, ensuring the result reflects true condensable particulate material.

How are CPM results reported?

Results are reported as blank-corrected mass concentration in milligrams per dry standard cubic meter (mg/dscm), referenced to the standard conditions specified in the method, together with the inorganic and organic sub-fractions. When combined with filterable PM data, the total particulate emission concentration is also reported.

How long does a CPM test take?

A single run typically requires one to three hours of sampling depending on stack concentration and required minimum gas volume, plus sample recovery, nitrogen purge, evaporation, and 24-hour desiccation. A complete test normally consists of three valid runs conducted at representative operating conditions, with analytical results available within several days.

Can CPM and filterable PM be measured in one sampling run?

Yes. A combined train, such as EPA Method 201A followed by the Method 202 backend, simultaneously collects size-segregated filterable PM and condensable PM in a single isokinetic sample, providing a complete mass balance of total primary particulate emissions.

What are the main sources of high CPM emissions?

Elevated CPM is commonly associated with sulfur trioxide and sulfuric acid mist from fuel sulfur combustion, ammonium bisulfate formed from SCR ammonia slip and SO3, aerosol carryover from wet flue gas desulfurization, alkali chlorides in metallurgical off-gas, and semi-volatile organic compounds. Control strategies include alkali/sorbent injection for SO3, ammonia slip optimization, wet electrostatic precipitators, and condensation polishing units.