U.S. EPA Issues Memo on Adjusting Condensable PM Test Results

On November 21, 2024, the U.S. Environmental Protection Agency (EPA) issued a short Memorandum entitled: Condensable PM Adjustment for Ammonium Sulfate Formation in M202 Test Results. The actual correction method is provided by OTM-59, Method for the Correction of Residual Ammonia in Condensable Particulate Matter Samples Collected according to Method 202. The following sections provide background and explain why the memo was needed and what it means.

Background

Method 202 is used to quantify condensable particulate matter (CPM), that is, any material that passes through the heated filter and then condenses within the chilled impinger section or “back-half” of the sampling train. CPM results are then used with filterable PM emissions data to develop emissions rates for total particulate matter (TPM) as well as PM less than 10 microns in diameter (PM₁₀) and PM less than 2.5 microns in diameter (PM_2.5). Like all PM test methods, the actual measurement is gravimetric, meaning that the weight gain in the gas sample (i.e., “front-half” catch, including the filter catch and probe/nozzle rinsate and/or “back-half” impinger catch) is divided by the sample volume collected to derive the results.

The Issue

Ammonia and sulfur dioxide are typically gaseous at stack conditions; both pass through the heated filter, and both can be captured in the water in the back-half chilled impingers. Neither of these gases should be counted as particulate matter but through the miracle of chemistry, once they are dissolved into the impinger water, and a few electrons and protons do their thing, the liquid now contains dissolved ammonium sulfate. When the liquid is recovered, and the water is evaporated, ammonium sulfate, if present, will be part of the resulting solids, with the CPM results biased high due to the presence of ammonia and sulfur dioxide in the stack gas. Now that the annual PM_2.5 National Ambient Air Quality Standard (NAAQS) is very close to the background value in many areas of the country, any high bias in PM test methods is important to resolve and remove.

The Solution

To make the correction, the CPM residue (what’s left after the evaporation step) is first re-dissolved and then analyzed for ammonium and sulfate ions. The amount of ammonium sulfate in the residue is then determined using some quick chemistry and stoichiometry. The final correction is made by subtracting the mass of ammonium sulfate from the gravimetric CPM determination. Voilà, the CPM values are now corrected for ammonia and sulfur dioxide bias.

The Fine Print

U.S. EPA has included a couple of details and prerequisite conditions that must be met for use of the correction:

• The presence of ammonia and sulfur dioxide must be confirmed using independent measurements (Method 320 or other ammonia slip methodology).
• Ammonia must be measured concurrent with the M202 CPM measurement.
• If the presence of ammonia is associated with ammonia injection for NO_X control using selective catalytic reduction (SCR), the ammonia feed rate and SCR inlet and outlet NO_X concentrations must be provided.
• Clarifications regarding applicability:
  • The U.S. EPA memo says: This adjustment is not appropriate for determining compliance with emission limitations that were derived without applying this adjustment methodology. Where this adjustment is sought for a new source, the process … may be applied to an existing similar source, at the discretion of the permitting authority.
  • The new method says: The procedures included in this method and adjustments to CPM measurements described in this method are only intended for use in the context of New Source Review (NSR) permitting on a prospective basis, such as 1) to determine applicability or NSR permitting requirements, 2) support a permit application, or 3) derive emissions limitations in a permit.

The Bottom Line

U.S. EPA acknowledges Method 202 biases under conditions where ammonia and sulfur dioxide are present. In response, they have provided a methodology to correct this bias. This correction can only be used under certain conditions.   Because the final guidance is fairly recent, there has not been much discussion regarding its potential use beyond what U.S. EPA has identified. Given the prerequisites provided by EPA, our initial interpretation regarding use of the ammonium sulfate bias CPM correction is provided below:

	Good to Go	Unclear	Not Allowed
Compliance with Existing NSR PM10/PM2.5 Limit Derived from M202 Uncorrected Data			❌
Compliance with NSPS or NESHAP PM10/PM2.5 Standard			❌
Compliance with NSR Limit Derived from Corrected M202 Data	✔️
Compliance with a PM10/PM2.5 Limit Established using Dispersion Modeling
Compliance with a State Implementation Plan (SIP) PM10/PM2.5 Limit or Standard
Correcting M202 PM10/PM2.5 Data in a Visibility or NAAQS Modeling demonstration
Establishing an NSR PM10/PM2.5 Limit Derived from Corrected M202 Data	✔️

ALL4 can help you design and manage a stack test program, evaluate results, develop emissions factors and emissions inventories, and demonstrate compliance with air quality regulations and standards. If you have any questions or concerns about Method 202, or other aspects of PM measurement, feel free to contact Roy Rakiewicz (rrakiewacz@all4inc.com) or Gene Youngerman (eyoungerman@all4inc.com).

References and More Information

The memo summarized herein: Condensable PM Adjustment for Ammonium Sulfate Formation in M202 Test Results; FROM: Richard A. Wayland, Director, Air Quality Assessment Division; TO: Air Division Directors; 21 November 2024

OTM-59, Method for the Correction of Residual Ammonia in Condensable Particulate Matter Samples Collected according to Method 202; Undated