What is the difference between a RATA and a CGA?

A relative accuracy test audit (RATA) compares CEMS results with simultaneous reference method measurements taken by a stack testing team, so it checks the whole system, including sampling. A cylinder gas audit (CGA) injects certified audit gases into the analyzers at defined concentrations to check their response. Under Part 60 Appendix F, a CGA can satisfy the quarterly audit in three quarters, but a RATA is still required at least once every four calendar quarters.

Can a Part 75 CEMS also satisfy Part 60 requirements?

Sometimes. Certain New Source Performance Standards subparts, such as Subpart Da for electric utility steam generating units, allow monitors certified and quality-assured under Part 75 to be used for some Part 60 monitoring. Other subparts and state permits have their own requirements. Review the applicable subpart and your permit language, and confirm with your regulator before assuming one monitoring program covers both.

What is EPA Method 30B?

EPA Method 30B is a reference method for measuring total vapor-phase mercury in stack gas by drawing a known volume of gas through paired carbon sorbent traps, then analyzing the mercury captured on each trap. It is commonly used for relative accuracy testing of mercury CEMS and sorbent trap monitoring systems, and for mercury emissions testing. Trap quality and laboratory analysis both affect results.

How often is a RATA required under Part 75?

Under Part 75 Appendix B, RATAs are required semiannually, meaning once every two successive QA operating quarters. A monitor can qualify for an annual frequency by achieving better accuracy, such as a relative accuracy of 7.5% or less for SO2 and NOx monitors. A grace period of 720 consecutive unit operating hours applies when a RATA isn't completed in the quarter it's due.

CEMS Basics: EPA Part 60 vs. Part 75 Emissions Monitoring

A continuous emissions monitoring system (CEMS) measures pollutant concentrations, and often stack flow, continuously and records the data needed to demonstrate compliance with emission limits. Which quality assurance rules apply depends mainly on the regulatory program: 40 CFR Part 60 for New Source Performance Standards and the many rules and permits that reference it, and 40 CFR Part 75 for the Acid Rain Program and related trading programs. This article is a high-level orientation; your permit and the applicable subpart always govern.

What is a continuous emissions monitoring system?

A CEMS is the complete measurement chain from stack to report, not just an analyzer. A typical system includes:

Probe and sample handling: filters, heated umbilicals, conditioners or dilution probes that deliver a representative sample.
Gas analyzers: for pollutants such as SO2, NOx, CO, HCl or total hydrocarbons, plus a diluent (O2 or CO2).
Flow monitor: needed to convert concentration into mass emissions.
Opacity or particulate monitors where the rule requires them.
Calibration gas system: certified gases (EPA Protocol gases where required) for daily checks and audits.
DAHS: the software that validates data, calculates emissions in units of the standard, flags exceedances and produces reports.

What types of CEMS are there?

The main distinction is where the gas is measured and whether moisture is removed first.

Type	How it works	Strengths	Watch-outs
Cool-dry extractive	Heated line to a chiller or dryer; analyzers read on a dry basis	Familiar analyzers, easy maintenance access	Water-soluble gases (SO2, NO2, HCl) can be lost in condensate
Hot-wet extractive	Sample kept above dew point all the way to the analyzer	Suits HCl, NH3 and multi-gas FTIR	Heated lines and cells must stay hot
Dilution extractive	Probe dilutes sample with clean dry air, lowering dew point	Low sample flow; common at utilities	Dilution ratio stability; wet-basis readings
In-situ	Measures at or across the stack	No long sample transport	Access, alignment and calibration method

Some designs blur these categories. ENVEA’s MIR IS, for example, is a stack-mounted multi-gas analyzer with integrated permeation drying, which avoids long heated sample lines on wet or corrosive gas.

What does 40 CFR Part 60 require for CEMS?

Part 60 sets certification requirements through Performance Specifications in Appendix B and ongoing quality assurance through Appendix F. It applies to sources covered by New Source Performance Standards, such as utility and industrial boilers (Subparts Da and Db) and petroleum refineries (Subparts J and Ja), and its specifications are referenced by many NESHAP rules and state permits.

Commonly cited Performance Specifications include PS-1 (opacity), PS-2 (SO2 and NOx), PS-3 (O2 and CO2), the PS-4 series (CO), PS-6 (flow rate), PS-11 (particulate matter), PS-12A (mercury CEMS), PS-12B (sorbent trap monitoring), PS-15 (FTIR) and PS-18 (HCl).

For gas CEMS, Appendix F, Procedure 1 sets the core QA rhythm:

Daily calibration drift checks at two concentration levels. The CEMS is out of control if drift exceeds twice the applicable specification for five consecutive days, or four times the specification in any single check.
Quarterly audits. A cylinder gas audit (CGA) at 20–30% and 50–60% of span, or a relative accuracy audit (RAA), is allowed in three of four quarters.
A RATA at least once every four calendar quarters, with successive RATAs no closer than two months apart.

Separate procedures cover particulate CEMS (Procedure 2), opacity monitors (Procedure 3) and mercury monitoring systems (Procedure 5).

What does 40 CFR Part 75 require?

Part 75 establishes monitoring for units subject to the Acid Rain Program, generally fossil fuel-fired electric generating units, and is also relied on by the Cross-State Air Pollution Rule trading programs. It focuses on SO2 mass emissions, NOx emission rate, CO2 and heat input, which makes stack flow measurement central.

Part 75 is more prescriptive than Part 60. Under Appendix B:

Daily calibration error tests on gas and flow monitors.
Quarterly linearity checks on SO2, NOx, CO2 and O2 monitors, with a 168-operating-hour grace period.
Quarterly leak checks for differential pressure flow monitors and a quarterly flow-to-load ratio evaluation.
RATAs semiannually, reducible to annually when the monitor demonstrates better accuracy (for example, relative accuracy of 7.5% or less for SO2, NOx and flow), with a 720-operating-hour grace period.

Part 75 also prescribes substitute data for missing hours that grows more conservative as monitor availability drops, and it requires quarterly electronic data reports submitted through EPA’s ECMPS software. Appendices D and E provide optional fuel-flow and NOx estimation methods for certain gas- and oil-fired units.

Part 60 vs. Part 75 at a glance

	40 CFR Part 60	40 CFR Part 75
Primary program	New Source Performance Standards; referenced by many NESHAP rules and permits	Acid Rain Program; CSAPR trading programs
Typical sources	Boilers, refineries, kilns and other NSPS categories	Fossil fuel-fired electric generating units
Certification	Appendix B Performance Specifications	Appendix A specifications and test procedures
Routine QA	Daily drift; CGA or RAA quarterly; RATA every four quarters	Daily calibration error; quarterly linearity; semiannual or annual RATA
Missing data	Out-of-control data can’t be used for compliance	Prescribed substitute-data procedures
Reporting	Per §60.7(c), the subpart and the permit	Quarterly electronic reports via ECMPS

Some units fall under both. Certain NSPS subparts, including Subpart Da, allow Part 75-certified monitors to satisfy some Part 60 requirements, but that is not universal.

Where does mercury monitoring fit?

Mercury monitoring is usually driven by NESHAP rules rather than Part 60 or Part 75 alone. The Mercury and Air Toxics Standards for coal- and oil-fired power plants (40 CFR Part 63, Subpart UUUUU) and the Portland cement NESHAP (Subpart LLL) both allow compliance approaches that include:

Mercury CEMS, certified under PS-12A, which report vapor-phase mercury continuously.
Sorbent trap monitoring systems, certified under PS-12B, which collect mercury on paired carbon traps over a sampling period for laboratory analysis.

EPA Method 30B, a sorbent trap reference method, is commonly used for relative accuracy testing of both approaches, although other reference methods may also be allowed for mercury CEMS. MATS has been amended and reconsidered several times in recent years, so confirm which version of the requirements currently applies to your unit.

Equipment options in our line card include ENVEA’s SM-4 and SM-5 continuous mercury monitors, which ENVEA states comply with PS-12A, and Ohio Lumex’s complete sorbent trap program: Method 30B and PS-12B traps, the OLM30B sampler, the M324 trap analysis system, an A2LA-accredited ISO/IEC 17025 laboratory, and field services that include mercury monitor RATAs. Ohio Lumex also offers the 915J continuous mercury monitor for coal-fired boilers, cement kilns and incinerators.

What causes failed RATAs and QA tests?

Most failures trace back to the sample, not the analyzer. The usual suspects are:

Sample line or probe leaks that dilute the sample with ambient air.
Moisture and condensation that strip out SO2, NO2 or HCl before measurement.
NOx converter efficiency loss in analyzers that convert NO2 to NO.
Stratification at the sampling location, so a single point isn’t representative.
Plugged filters or probes that slow response or change flow.
Calibration gas problems, such as expired cylinders or incorrect certified values entered in the DAHS.
Flow monitor issues, including cyclonic flow and dirty or drifting sensors.

How should you choose CEMS analyzers?

Start with the permit, then match technology to the gas matrix. Questions to settle early:

Which pollutants, diluent and units of the standard does the rule require?
Is the gas wet, corrosive or high in particulate?
Do you need flow for mass emissions?
Which Performance Specifications must the system meet, and who will perform certification testing?

ENVEA’s MIR 9000 extractive analyzer measures up to ten gases with built-in paramagnetic O2 and is stated by ENVEA as compliant with 40 CFR 60 and 75. The STACKFLOW ultrasonic flow monitors are listed as PS-6 compliant, and ProScatter dust monitors are referenced for PS-11. Remember that a compliant analyzer is only the start: certification depends on site testing of the installed system.

Confirm requirements with your permit and regulator

Monitoring requirements come from the combination of federal rules, the applicable subpart, state implementation and your specific permit conditions. Rules also change. Before selecting equipment or changing a QA program, confirm requirements with your environmental staff, permit writer or the state agency.

Local CEMS and mercury monitoring support

Vaughn Associates’ Baton Rouge team supports emissions monitoring projects across Louisiana and the Gulf Coast, where refineries, chemical plants, power stations and pulp mills operate under both Part 60 and Part 75 programs. We represent ENVEA in our Louisiana and Gulf Coast territory and Ohio Lumex across our territories (North Carolina only in our North Carolina, Virginia, Maryland and D.C. territory).

Review our CEMS and mercury monitoring solutions, the ENVEA and Ohio Lumex line pages, or contact us to talk through an analyzer upgrade, a new CEMS or a sorbent trap program.