CEMS-CGA, RATA & Certification California | CEMS Testing Services

CEMS-CGA, RATA & Certification in California: 2026 Compliance Guide for Industrial Facilities

Is your Continuous Emissions Monitoring System actually producing data that regulators can rely on?

For many industrial facilities in California, having a CEMS (Continuous Emissions Monitoring System) installed is only the beginning. Depending on the applicable permit, air-district rules, and federal requirements, the system may need initial certification, quality assurance testing, Cylinder Gas Audits (CGA), Relative Accuracy Test Audits (RATA), ongoing QA/QC and periodic reporting.

For facilities regulated by the South Coast Air Quality Management District (South Coast AQMD), Rules 218, 218.2 and 218.3 establish important requirements for CEMS administration, certification, performance specifications and QA/QC. South Coast AQMD’s current rule book lists Rule 218 as amended March 5, 2021, and Rules 218.2 and 218.3 as amended September 2, 2022.

This makes professional CEMS Certification, CGA and RATA testing an important part of an industrial facility’s compliance strategy.

What Is CEMS?

CEMS stands for Continuous Emission Monitoring System.

A CEMS continuously measures one or more emission parameters from an industrial source.

Depending on the application, it can monitor:

• NOx
• SO₂
• CO
• CO₂
• Oxygen (O₂)
• Flow rate
• Moisture
• Other regulated pollutants

A CEMS can continuously generate emissions data that may be used for compliance demonstrations, reporting and regulatory decision-making.

South Coast AQMD explains that CEMS can be required to continuously demonstrate compliance with permitted concentration limits and, for applicable RECLAIM systems, automatically report mass emissions.

The key point:

A CEMS is not automatically a compliance instrument simply because it is installed.

It must meet the applicable certification and performance requirements.

What Are CEMS Certification, RATA and CGA?

These three terms are closely connected but have different purposes.

CEMS Certification

Demonstrates that the monitoring system meets applicable regulatory performance and quality requirements and can function as an official data-reporting instrument.

RATA — Relative Accuracy Test Audit

Compares CEMS measurements against an applicable reference method to determine whether the CEMS is sufficiently accurate.

CGA — Cylinder Gas Audit

Challenges the CEMS with certified calibration/audit gases to verify analyzer response and measurement performance.

Together, they help answer:

Can regulators trust the emissions data being produced by the CEMS?

Why Is CEMS Compliance Important in California?

Industrial facilities may have strict emission limits for pollutants such as NOx, SO₂ and other contaminants.

When CEMS data are used to demonstrate compliance, inaccurate monitoring can create a serious problem.

A facility may believe it is compliant while the monitoring system is:

• Drifting
• Out of calibration
• Producing invalid data
• Experiencing excessive downtime
• Outside required performance specifications
• Operating outside its approved span range
• Missing required QA/QC tests

South Coast AQMD Rule 218.3 provides requirements addressing CEMS performance, QA/QC, out-of-control periods and alternative data acquisition.

2026 California CEMS Regulatory Landscape

California’s CEMS requirements continue to evolve through local air-district rules, federal requirements and pollutant-specific monitoring programs.

For South Coast AQMD-regulated facilities, Rules 218, 218.2 and 218.3 remain central to CEMS requirements.

South Coast AQMD’s FY 2026–27 budget and objectives also continue to identify CEMS Certification as a compliance activity, demonstrating that CEMS review and certification remain an active regulatory function.

2026 takeaway:

CEMS should be treated as an ongoing compliance program—not a one-time installation project.

CEMS Certification: What Does the Process Involve?

For a new or modified CEMS, certification can involve multiple stages.

South Coast AQMD’s current CEMS application package describes a certification pathway that includes:

1. CEMS plan, where applicable
2. CEMS application
3. Quality Assurance Plan (QAP)
4. Initial District approval
5. Certification source-test protocol
6. District review/approval of the protocol
7. Certification source testing
8. Certification source-test report
9. District approval/final certification
10. Continuing QA/QC and periodic testing

Once certified, the CEMS remains subject to requirements involving maintenance, modifications, outages, records and periodic QA/QC testing such as RATAs.

This is why certification should be planned early.

A facility should not wait until the CEMS installation is complete before considering the certification process.

What Is a RATA?

RATA = Relative Accuracy Test Audit.

A RATA evaluates how closely the CEMS measurement agrees with an applicable reference method.

In simple terms:

CEMS Result vs. Reference Method Result

The closer they agree, the more confidence regulators can have in the CEMS data.

South Coast AQMD defines relative accuracy in terms of the difference between CEMS-generated data and reference-method results, including the confidence coefficient.

A typical RATA may involve:

CEMS operating

↓

Reference-method sampling

↓

Simultaneous measurements

↓

Multiple test runs

↓

Statistical evaluation

↓

Relative accuracy calculation

↓

Pass / Fail determination

Why Is RATA Testing So Important?

Imagine your CEMS reports:

NOx = 45 ppm

But an approved reference method produces significantly different results.

Which number should regulators trust?

That’s exactly why RATA exists.

A successful RATA provides evidence that the CEMS is accurately measuring the emissions parameter within the applicable performance specification.

South Coast AQMD provides dedicated technical guidance for RATA-related issues, including RATA outliers and mass-emissions RATA for non-concurrent concentration and flow measurements.

What Is a CGA?

CGA = Cylinder Gas Audit.

During a CGA, the CEMS is challenged using gases with known certified concentrations.

Under federal EPA QA procedures, applicable CGA testing uses a zero-level gas and two upscale audit levels within specified ranges, with repeated injections designed to challenge the measurement system.

The basic concept is:

Known Gas → CEMS → Measured Value → Compare

If the CEMS response is outside the applicable tolerance, corrective action may be required.

RATA vs. CGA

Important:

Don’t assume that a CGA replaces a RATA in every regulatory program.

The applicable rule, permit and federal/state monitoring requirements determine what is required.

How Often Is RATA Required?

There is no single testing frequency for every California CEMS.

The interval depends on the applicable program.

For example, EPA’s Procedure 1 for applicable gas CEMS requires a RATA at least once every four calendar quarters, subject to the applicable provisions. It also provides for CGA-based quarterly QA in certain circumstances.

Other programs can have different requirements.

Your facility’s permit and applicable regulation control.

This is especially important for facilities operating under:

• Title V
• South Coast AQMD rules
• RECLAIM-related requirements
• Federal NSPS requirements
• Federal NESHAP/MACT requirements
• 40 CFR Part 60
• 40 CFR Part 75
• Pollutant-specific monitoring programs

What Happens When a CEMS Fails QA/QC?

This is where CEMS compliance becomes particularly important.

A failed QA/QC test can result in an out-of-control period under applicable requirements.

South Coast AQMD Rule 218.3 provides that CEMS data generated during an applicable out-of-control period can be considered invalid for emission quantification in a compliance demonstration.

The facility may then need to:

1. Identify the problem
2. Correct the CEMS
3. Perform the required QA/QC test
4. Establish when valid data resumes
5. Evaluate affected data
6. Use an approved alternative data-acquisition method where applicable
7. Complete required reporting

This can become expensive if the issue is discovered too late.

Common Reasons CEMS Fail RATA or QA/QC

1. Analyzer Calibration Problems

Incorrect calibration can create measurement bias.

2. Sampling-System Problems

Leaks, condensation, contamination or sample-line problems can affect readings.

3. Span Range Problems

The selected span must be appropriate for the applicable requirements.

4. Moisture Interference

Moisture can affect certain monitoring systems if not properly accounted for.

5. Stratification

Non-uniform pollutant concentrations across the stack can affect comparison between the CEMS and reference method.

6. Flow Issues

Poor flow measurement can affect mass-emission calculations.

7. Aging Components

Sensors, analyzers, pumps and other components can degrade over time.

8. Poor Maintenance

A CEMS requires continuous attention.

CEMS Certification Is Not “Set It and Forget It”

A common mistake is:

Install CEMS → Get certified → Forget about it.

That’s not how an effective compliance program works.

After certification, facilities must continue managing:

Calibration + QA/QC + RATA + CGA + Maintenance + Data Availability + Records + Reporting

South Coast AQMD’s certification materials specifically state that certified CEMS remain subject to scheduled and unscheduled maintenance, outages, modifications, records/reporting and ongoing QA/QC.

2026 Update: CEMS Technology Is Expanding

CEMS requirements are also evolving for specialized pollutants.

A notable example is ethylene oxide (EtO).

South Coast AQMD entered into a July 2025 Memorandum of Understanding with Parter to help develop performance specifications, verification test protocols and implementation guidance for an EtO CEMS certification program. The MOU runs through December 31, 2026 unless extended.

This demonstrates an important industry trend:

Continuous monitoring is expanding beyond traditional NOx/SO₂ applications.

Facilities with specialized toxic-air-contaminant requirements should monitor regulatory developments closely.

CEMS and Environmental Compliance Are Becoming More Data-Driven

Modern air-quality compliance increasingly depends on continuous data.

Instead of relying only on periodic stack tests, regulators can use monitoring systems to evaluate:

• Hourly emissions
• Daily emissions
• Operating conditions
• Exceedances
• Data availability
• Control-device performance
• Long-term trends

South Coast AQMD’s CEMS application materials state that applicable CEMS can continuously monitor compliance and automatically signal exceedances, breakdowns and out-of-control periods.

Accurate data is therefore a compliance asset.

Pros of CEMS, RATA, CGA & Certification

✅ 1. Continuous Compliance Visibility

CEMS provides ongoing emissions information rather than occasional snapshots.

✅ 2. Early Problem Detection

Abnormal emissions can potentially be identified faster.

✅ 3. Stronger Compliance Documentation

Certified monitoring data can support regulatory reporting where applicable.

✅ 4. Better Process Control

Operators can use emissions information to improve process operation.

✅ 5. Reduced Compliance Uncertainty

Routine QA/QC helps identify monitoring problems before they become major issues.

✅ 6. Better Long-Term Data

Historical CEMS information can help identify trends.

Challenges / Cons

❌ Higher Initial Cost

Installing a compliant CEMS can require significant capital investment.

❌ Ongoing Maintenance

Analyzers, sampling systems and data-acquisition equipment require regular attention.

❌ QA/QC Requirements

RATA, CGA and other tests require planning and qualified personnel.

❌ Data Availability Issues

CEMS downtime can create compliance complications.

❌ Technical Complexity

CEMS involve instrumentation, electronics, sampling systems, software and regulatory requirements.

❌ Regulatory Changes

Changes to rules or permits can require modifications to the monitoring program.

But the cost of maintaining a reliable CEMS is usually easier to manage than discovering a monitoring failure during a compliance deadline.

Which Industries May Need CEMS Services?

Potential applications include:

Power & Energy

• Power plants
• Cogeneration facilities
• Distributed generation

Manufacturing

• Chemical plants
• Metal processing
• Cement
• Glass
• Manufacturing facilities

Petroleum & Chemical

• Refineries
• Petrochemical plants
• Process facilities

Waste & Combustion

• Waste-to-energy
• Incineration
• Thermal processing

Industrial Boilers

• Large boilers
• Steam generators
• Process heaters

Other Regulated Sources

Facilities with permit or regulatory requirements for continuous monitoring.

How EES Helps With CEMS-CGA, RATA & Certification

Energy Environmental Solutions, Inc. (EES) can help facilities manage the technical and regulatory aspects of their CEMS programs.

1. CEMS Certification Support

EES can support facilities through the certification process, including planning, protocol development, testing and reporting.

2. RATA Testing

EES can coordinate and perform applicable Relative Accuracy Test Audits using the appropriate reference methods and procedures.

3. CGA Testing

EES can perform applicable Cylinder Gas Audits using properly certified audit gases and the applicable QA/QC procedure.

4. CEMS QA/QC

EES can help facilities maintain an ongoing CEMS QA/QC program.

5. Test Protocol Development

Before testing, EES can prepare a protocol addressing:

• Applicable regulations
• CEMS configuration
• Reference methods
• Test conditions
• Sampling procedures
• Audit gases
• QA/QC
• Calculations
• Reporting

6. CEMS Performance Evaluation

EES can help evaluate:

• Analyzer performance
• Calibration
• Response
• Drift
• Sampling system
• Data acquisition
• Data availability
• QA/QC records

7. Troubleshooting Support

If a RATA or CGA fails, EES can help identify potential causes and coordinate corrective action and retesting.

8. Compliance Reporting

EES can assist with preparing the technical documentation required by the applicable regulatory program.

EES CEMS Compliance Process

STEP 1 — REVIEW

Permit + Applicable Rules + CEMS Configuration

↓

STEP 2 — PLAN

QA Plan + Test Protocol + Scheduling

↓

STEP 3 — PREPARE

Analyzer + Sampling System + Calibration + Audit Gases

↓

STEP 4 — TEST

RATA / CGA / Certification Testing

↓

STEP 5 — ANALYZE

Data Validation + Calculations + QA/QC

↓

STEP 6 — REPORT

Technical Report + Regulatory Documentation

↓

STEP 7 — MAINTAIN

Ongoing QA/QC + RATA + CGA + Monitoring

Why Choose EES?

CEMS compliance requires more than an instrument technician.

It requires an understanding of:

Instrumentation + Source Testing + Reference Methods + QA/QC + Air-District Requirements

EES can help bring these pieces together into one coordinated compliance program.

South Coast AQMD maintains a Laboratory Approval Program (LAP) under which laboratories and source-testing firms are evaluated method-by-method for personnel, equipment, documentation, QA and other criteria. Facilities should verify that the testing provider and applicable methods meet the requirements of their specific program.

2026 CEMS Compliance Checklist

Before your next CEMS audit, ask:

• Is my CEMS currently certified?
• Is the certification still valid?
• Is my QAP current?
• Is the RATA due?
• Is the CGA due?
• Are calibration gases properly certified?
• Is the analyzer within calibration?
• Are sampling lines leak-free?
• Is moisture correction working properly?
• Is the data acquisition system functioning?
• Are data-availability requirements being met?
• Have any CEMS modifications occurred?
• Does a modification require District approval?
• Are previous QA/QC records complete?
• Are there unresolved out-of-control periods?
• Is the applicable permit current?
• Have regulatory requirements changed?

Don’t Wait Until Your RATA Is Due

One of the most expensive mistakes is scheduling a RATA only days before the regulatory deadline.

A better strategy is:

Review → Prepare → Test → Correct → Retest → Report

This provides time to address unexpected problems.

If your CEMS fails on the first attempt and your deadline is tomorrow, your options become extremely limited.

Is Your CEMS Ready for Its Next Audit?

EES Can Help You Find Out.

If your facility has a CEMS and you need:

CEMS Certification
RATA Testing
CGA Testing
CEMS QA/QC
CEMS Performance Evaluation
CEMS Troubleshooting
CEMS Test Protocol Development
CEMS Compliance Reporting

EES can help you develop a testing and compliance strategy based on your applicable permit and regulatory requirements.

🚨 Don’t Let Invalid CEMS Data Become a Compliance Problem

Your CEMS is generating regulatory data every day.

Can you trust the numbers?

Let EES help you verify it.

Energy Environmental Solutions, Inc. (EES)
Phone: 714-630-5210 | 714-630-7844
Email: info@sourcetester.com
Website: sourcetester.org

CEMS Testing. RATA. CGA. Certification. Compliance.

One experienced team for your emissions-monitoring needs.