Determination of Condensable Particulate Matter (CPM) Emission Concentration

Condensable particulate matter (CPM) is the fraction of stack emissions that exists as vapor at flue gas temperature but condenses into liquid or solid particles immediately after release into the atmosphere, making it invisible to conventional filter-based particulate testing. Accurate determination of CPM emission concentration, typically performed by accredited third-party laboratories using EPA Method 202 dry impinger procedures, is essential for quantifying the true total particulate footprint of stationary sources. This article explains the technical principles, sampling train configuration, analytical workflow, and quality control requirements for CPM measurement in industrial flue gas.

Technical Overview

Condensable particulate matter (CPM) is defined as material that exists in the vapor phase at stack conditions but 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. Because CPM remains gaseous while passing through the heated filter of a conventional sampling train, it is not retained by standard gravimetric methods such as EPA Method 5, EN 13284-1, or GB/T 16157, which quantify only filterable particulate matter (FPM).

Total particulate matter (TPM) emitted by a stationary source is therefore the sum of two distinct fractions:

• Filterable particulate matter (FPM): particles captured on a heated filter during sampling, including coarse particles and filterable fine particles.
• Condensable particulate matter (CPM): vapors and condensable species that penetrate the filter and form particulate only after atmospheric dilution and cooling.
• Total particulate matter (TPM) = FPM + CPM, the scientifically complete indicator of particulate emissions.

Upon condensation, CPM forms predominantly sub-micrometer droplets and solids that contribute directly to ambient PM2.5 and serve as precursors for secondary aerosol formation. Field studies at ultra-low emission coal-fired power plants, steel plants, and cement kilns consistently show that CPM can account for 20 to 70 percent of total particulate emissions, and at very low FPM levels it frequently becomes the dominant particulate fraction. Determination of CPM emission concentration has consequently become a key element of ultra-low emission evaluation, environmental impact assessment, and fine particulate pollution control research.

Testing Samples

CPM testing applies to flue gas and exhaust streams from stationary combustion and process sources equipped with sampling ports and platforms that satisfy the siting requirements of GB/T 16157. Typical sample matrices include:

Coal-Fired and Gas-Fired Boiler Flue Gas

Flue gas sampled upstream or downstream of air pollution control trains, including saturated wet flue gas after wet flue gas desulfurization and wet electrostatic precipitation, where CPM levels commonly range from a few to several tens of milligrams per dry standard cubic meter.

Steel and Metallurgical Process Emissions

Exhaust from sintering and pelletizing machines, coking ovens, blast furnaces, electric arc furnaces, and rolling and finishing operations.

Cement Kilns and Building Material Furnaces

Flue gas from rotary cement kilns, lime kilns, drying systems, and grinding units equipped with baghouses or electrostatic precipitators.

Municipal and Hazardous Waste Incineration Flue Gas

Post-combustion and post-scrubbing gas streams containing complex inorganic and organic condensable species, including acid gases, heavy metal vapors, and semi-volatile organics.

Chemical, Petrochemical, and Pharmaceutical Process Vents

Reactors, dryers, absorbers, and thermal oxidizer stacks where semi-volatile organics and acid mist precursors are present.

Testing Items

A complete CPM characterization program typically reports the following parameters:

• Total particulate matter (TPM) emission concentration, mg/m3 at dry standard conditions.
• Filterable particulate matter (FPM) concentration, determined in parallel.
• Total condensable particulate matter (CPM) concentration, mg/dscm.
• Inorganic CPM fraction, quantified as the water-soluble residue mass.
• Organic CPM fraction, quantified as the solvent-soluble residue mass.
• Water-soluble ions in CPM, including SO42-, NO3-, Cl-, F-, NH4+, Na+, K+, Ca2+, and Mg2+, determined by ion chromatography.
• Metal elements in CPM, determined by ICP-OES or ICP-MS.
• Organic speciation by GC-MS and total organic carbon analysis for source characterization studies.

Testing Methods

Sampling Principle and Train Configuration

A representative flue gas sample is withdrawn isokinetically through a nozzle and heated probe to a filter holder maintained at 120 ± 14 °C, where filterable particulate matter is retained. The gas then flows through a cooled condenser and a series of dry impingers immersed in an ice bath, where water vapor, CPM, and soluble vapors are condensed and captured. Downstream silica gel protects a dry gas meter and pump, while calibrated temperature, pressure, and orifice measurements allow the sampled dry gas volume to be corrected to standard conditions.

Determination of Condensable Particulate Matter (Dry Impinger Method)

The reference procedure follows US EPA Method 202 (Appendix M to 40 CFR Part 51) and equivalent procedures such as T/CAEPI 12-2017, which adapt the dry impinger principle to Chinese sampling conventions based on GB/T 16157-1996. Essential procedural steps include:

• Leak testing and calibration verification of the complete sampling train before each test series.
• Isokinetic sampling at stable, representative process load with all control devices in normal operation.
• Maintaining the gas temperature exiting the condenser at or below 30 °C with the impinger train held in an ice-water bath.
• Purging the train with high-purity nitrogen after sampling, typically for approximately 15 minutes, to strip dissolved sulfur dioxide and suppress positive sulfate artifacts.
• Collection of a field train blank for background correction of each test campaign.

Sample Recovery and Fractionation

After sampling, all internal surfaces from the filter holder outlet through the final impinger are rinsed sequentially with ultrapure water to recover the condensable inorganic fraction, followed by a suitable organic solvent such as hexane or acetone as specified by the method to recover the condensable organic fraction. Both fractions are transferred to clean, pre-weighed vessels for gravimetric determination.

Laboratory Analysis and Calculation

The aqueous fraction is evaporated to dryness at approximately 105 °C, and the organic solvent fraction is allowed to evaporate under ambient conditions; both residues are desiccated and weighed to constant weight, with consecutive weighings differing by no more than 0.5 mg. The reported CPM mass is the sum of the inorganic and organic residues corrected by the field train blank, and the emission concentration is calculated as milligrams per dry standard cubic meter:

CPM concentration (mg/dscm) = (m inorganic + m organic − m field blank) / V std

Where required, speciation of the recovered fractions by ion chromatography, ICP-OES, ICP-MS, and GC-MS provides insight into CPM formation mechanisms and supports the selection of control strategies.

Quality Assurance and Quality Control

• Field train blanks collected for every test series, with blank correction applied within the limits specified by the method.
• Parallel duplicate sampling trains evaluated against relative deviation acceptance criteria.
• Calibration of nozzles, orifice meters, dry gas meters, thermocouples, and barometers traceable to national metrology standards.
• Constant-weight verification and controlled desiccator handling of all weighing vessels.
• Minimum dry gas sample volume requirements, typically no less than 1 cubic meter, to ensure a quantifiable residue mass.
• Full documentation of process load, fuel characteristics, and control device performance during each sampling run.

Applications

• Ultra-low emission performance assessment, verifying the real total particulate footprint of plants retrofitted with electrostatic precipitators, baghouses, wet flue gas desulfurization, and wet electrostatic precipitators.
• Environmental impact assessment and emission permit compliance demonstration for new and existing facilities.
• Engineering evaluation of CPM control technologies, including wet electrostatic precipitation, flue gas condensation and scrubbing, phase-change agglomeration, and low-temperature heat recovery systems.
• Scientific research into secondary PM2.5 formation, regional haze chemistry, and source apportionment.
• Corporate ESG disclosure, supply-chain environmental auditing, and sustainability verification.
• Technical support for emission standard development and regulatory research programs.

FAQ

What is the difference between filterable and condensable particulate matter?

Filterable particulate matter is captured on a heated filter during stack sampling and exists as solid or liquid particles at stack temperature. Condensable particulate matter penetrates the filter as vapor and transforms into liquid or solid particles only after the plume is diluted and cooled in the ambient atmosphere.

Why do conventional particulate methods not detect CPM?

Reference methods such as GB/T 16157-1996, HJ 836-2017, EPA Method 5, and EN 13284-1 define particulate matter solely as the material retained on a heated filter. Vaporous species pass through the filter unmeasured, so a separate condensing train is required to capture and quantify them.

Which standards govern CPM determination?

EPA Method 202 is the internationally recognized reference procedure, while T/CAEPI 12-2017 specifies the dry impinger method under Chinese sampling conventions. Filterable particulate matter is determined simultaneously per GB/T 16157-1996 or HJ 836-2017 so that total particulate matter can be reported as FPM plus CPM.

How is the sulfur dioxide interference artifact controlled?

Dissolved sulfur dioxide in the condensed water can oxidize to sulfate and bias results high. The impinger train is purged with high-purity nitrogen before sample recovery, and field train blanks quantify any residual background contribution.

What operating conditions are required during sampling?

Testing is performed at stable, representative process load with all pollution control devices in normal operation, using sampling ports and platforms that comply with GB/T 16157. A minimum of three valid sampling runs is normally executed for each test condition.

How is the result expressed?

CPM emission concentration is reported in milligrams per dry standard cubic meter, corrected for the field train blank, together with the parallel filterable particulate result and the calculated total particulate matter concentration.

Can CPM be monitored continuously?

No validated continuous emission monitoring system for CPM is currently regulated. Determination relies on periodic isokinetic stack sampling using impinger-based reference methods, which accredited third-party laboratories can integrate with routine compliance monitoring campaigns.