CEMS and RATA Testing: What You Need to Know Under SCAQMD Rules 218.2 and 218.3

If your facility operates a Continuous Emissions Monitoring System, RATA testing is the single most consequential event in your compliance cycle. Pass, and your CEMS stays certified for another four quarters. Fail, and you're looking at lost certification, mandatory reversion to periodic source testing, and potential enforcement exposure.

SCAQMD replaced the old Rules 218 and 218.1 with new Rules 218.2 and 218.3. The changes tightened several standards and added requirements that directly affect how facilities operate and test their CEMS. This guide covers what changed, what it means for your testing program, and how to avoid the problems that cause RATA failures.

What Is a CEMS?

A Continuous Emissions Monitoring System measures pollutant concentrations in a stack or duct on a continuous, real-time basis. A typical CEMS installation includes gas analyzers for NOx and CO, an O2 or CO2 analyzer for diluent correction, a sample conditioning system (for extractive systems), a Data Acquisition System (DAS), and calibration gas delivery components.

The CEMS provides the emissions data that facilities report to SCAQMD — hourly averages, daily totals, and quarterly summaries. For former RECLAIM facilities now operating under command-and-control permits, that data is the primary compliance record. If the CEMS isn't certified, the data isn't valid.

What Is a RATA?

A Relative Accuracy Test Audit compares CEMS readings against simultaneous reference method measurements performed by a LAP-approved source testing company. The reference methods — typically Method 7E for NOx, Method 10 for CO, Method 3A for O2/CO2 — are the regulatory "truth."

During the test, the source testing crew collects a minimum of nine measurement runs while the CEMS records its own data over the same time intervals. The two datasets are compared using statistical calculations defined in 40 CFR Part 60, Appendix F.

The result is a relative accuracy percentage. If the RA falls within the allowable limit — typically 20% of the reference method mean, or within an applicable absolute threshold for low-concentration applications — the CEMS passes. If not, corrective action and retesting are required.

Rules 218.2 and 218.3: What Changed

Rules 218.2 (Continuous Emission Monitoring System — General Provisions) and 218.3 (Continuous Emission Monitoring System — Performance Specifications) replaced the older Rules 218 and 218.1. The new rules brought several significant changes.

NOx De Minimis Threshold Reduced

The NOx de minimis threshold dropped from 1.0 ppm to 0.5 ppm. This matters because the de minimis level determines which RATA acceptance criteria apply.

Units emitting NOx at or below 0.5 ppm are now evaluated against an absolute accuracy standard rather than the percentage-based relative accuracy. For facilities with ultra-low-NOx burners achieving sub-1-ppm levels, this change means a tighter compliance band. CEMS analyzers must be capable of accurate measurement at concentrations that would have been below the noise floor of older instruments.

New CO2 Standard

Rules 218.2/218.3 added a CO2 relative accuracy standard of 1.0%. Facilities using CO2 as a diluent in emission calculations need their CO2 CEMS to meet this standard during RATA testing. Previously, CO2 monitoring requirements were less explicit.

CO Standard for Low-Concentration Units

For units operating below 2.0 ppm CO, the rules established a new absolute accuracy standard. This addresses the same problem as the NOx de minimis change — percentage-based accuracy criteria don't work well at very low concentrations. A 20% relative accuracy at 1.0 ppm CO means the CEMS must read within 0.2 ppm of the reference method, which challenges both the CEMS analyzer and the reference method measurement.

96-Hour Non-Operating Allowance

The rules now include a 96-hour allowance for non-operating periods — time when the monitored unit is shut down and the CEMS is not recording valid data. This replaces less defined provisions in the old rules. Exceeding the 96-hour allowance without approved alternative monitoring can trigger data substitution requirements and potential compliance issues.

RATA Frequency and Scheduling

RATA testing is required every four calendar quarters. The quarterly clock starts from the most recent passing RATA. Most facilities align their RATA schedules to consistent annual windows, but the rules allow flexibility within the four-quarter cycle.

Strategic scheduling matters. Common considerations:

• Process stability — Conduct the RATA when the unit is operating at a representative, steady-state condition. Testing during startup, shutdown, or load swings introduces variability that can cause failures.
• Weather windows — Stack testing requires reasonable weather conditions. Wind, rain, and extreme temperatures affect both safety and measurement quality.
• Analyzer condition — Schedule the RATA after maintenance, not before. A RATA on an analyzer that's due for component replacement is a RATA at risk of failure.
• Turnaround avoidance — If a unit is coming down for a turnaround within the quarter, schedule the RATA before the shutdown rather than dealing with post-turnaround complications.

Missing the four-quarter deadline results in CEMS certification lapse. The facility must substitute data using default high values until a passing RATA is completed.

What Happens When a RATA Fails

A failed RATA has cascading consequences:

Immediate impact: The CEMS for that parameter on that unit loses certification as of the failed test date.

Data implications: All CEMS data recorded after the certification lapse is invalid for compliance purposes. The facility must use substitute data — typically the maximum potential concentration or a default value specified in the rules — which almost always exceeds actual emissions. This inflates reported emissions and can push a facility over permit limits on paper, even if actual emissions haven't changed.

Corrective action: The facility must diagnose the cause of failure — analyzer drift, sample system issues, stratification in the duct, reference method problems — and correct it. A retest RATA must then be scheduled and passed.

Reversion to periodic testing: Until the CEMS is re-certified, the facility may be required to conduct periodic reference method source testing at increased frequency — often quarterly — to provide valid emissions data.

Enforcement exposure: A pattern of failed RATAs or extended certification lapses draws SCAQMD attention. Inspectors may issue Notices of Violation for operating with an uncertified CEMS or for data gaps in required reporting.

The cost of a single failed RATA — between retesting, substitute data penalties, and potential enforcement — can easily exceed $50,000. Preventing failures through proper CEMS maintenance and pre-RATA preparation is far less expensive.

Between-RATA Quality Assurance

Passing a RATA every four quarters isn't the only QA requirement. Between RATAs, facilities must perform:

Cylinder Gas Audits (CGAs)

CGAs are required quarterly (in quarters when a RATA is not performed). The CGA involves challenging each gas analyzer with certified reference gases at two concentration levels — typically a low-level and a mid-level or high-level gas. The analyzer response must fall within specified tolerances.

A failed CGA doesn't immediately invalidate CEMS data the way a failed RATA does, but it does require corrective action and may trigger an accelerated RATA requirement depending on the severity.

Daily Calibration Drift Checks

Most CEMS perform automated zero and span checks daily. The drift — the difference between the known calibration gas value and the analyzer's response — must stay within defined limits (typically 2.5% of span). Excessive drift indicates analyzer degradation and predicts future RATA problems.

DAS Verification

The Data Acquisition System must accurately record, reduce, and report CEMS data. DAS checks verify that the software correctly converts raw analyzer signals to calibrated concentrations, applies diluent corrections, calculates averages, and flags data quality codes.

Errors in the DAS can cause a facility to report incorrect emissions even when the analyzers are working properly. DAS verification catches these issues before they become reporting violations.

Common Causes of RATA Failures

Based on decades of performing RATAs at Southern California facilities, these are the issues we see most frequently:

Analyzer Drift and Degradation

Gas analyzers lose sensitivity over time. Optical components degrade, detector cells contaminate, pump diaphragms weaken. An analyzer showing marginal performance on daily calibration checks is at high risk of failing a RATA. Replace components before the RATA, not after.

Sample System Issues

For extractive CEMS, the sample conditioning system — filters, chillers, pumps, tubing — is often the weak link. Leaks, blockages, moisture carry-over, and response time delays all degrade accuracy. A complete sample system check and cleaning before each RATA is essential.

Stratification

Non-uniform gas distribution across the stack or duct cross-section causes disagreement between the CEMS sample point and the reference method traverse. This is a physical problem, not an instrument problem. Mixing devices, relocation of the CEMS probe, or adjustment of the reference method traverse can address it.

Reference Method Complications

Sometimes the RATA failure isn't a CEMS problem. Errors in reference method sampling — contaminated reagents, improper gas standards, equipment malfunctions, analyst technique issues — can produce reference data that doesn't represent actual stack conditions. Choosing a qualified, experienced LAP-approved testing company reduces this risk.

Who Needs CEMS and RATA Testing?

Any facility in the SCAQMD with installed CEMS must comply with Rules 218.2 and 218.3. The primary categories include:

• Refineries — Typically the largest number of monitored units per facility. Heaters, boilers, FCCU regenerators, SRU/TGU equipment.
• Power plants — Gas turbines, boilers, and combined-cycle units.
• Former RECLAIM facilities — Many transitioned from CEMS used for RECLAIM reporting to CEMS required under command-and-control permits.
• Large industrial combustion sources — Cement kilns, glass furnaces, large boilers at manufacturing facilities.
• Landfill gas-to-energy plants — Internal combustion engines and turbines firing landfill gas.

If your facility has CEMS installed — whether required by permit, rule, or federal regulation — Rules 218.2 and 218.3 apply to you.

Preparing for Your Next RATA

A successful RATA starts weeks before the test crew arrives. Here's a pre-RATA checklist:

1. Review analyzer performance data — Look at daily calibration drift trends for the past 90 days. Identify any analyzers showing upward drift trends.
2. Service the sample system — Replace filters, check tubing for leaks, verify chiller operation, confirm pump flow rates.
3. Verify calibration gases — Confirm that CEMS calibration gases are within certification date and match the concentration ranges expected during the RATA.
4. Confirm process conditions — Schedule the RATA during representative, stable operation. Coordinate with operations to avoid load changes during the test window.
5. Check test ports — Verify that source test ports meet Method 1 criteria, are accessible, and have safe working platforms.
6. Coordinate with your testing company — Share recent CEMS data, process operating parameters, and any known issues. The more information the test crew has beforehand, the smoother the test runs.

Frequently Asked Questions