Method 3A
Determination of Oxygen and Carbon Dioxide Concentrations in Emissions From Stationary Sources (Instrumental Analyzer Procedure)
40 CFR Part 60, Appendix A-2
From the method — regulatory text
Sections 1.0 and 2.0 of Method 3A—Determination of Oxygen and Carbon Dioxide Concentrations in Emissions From Stationary Sources (Instrumental Analyzer Procedure), as published at 40 CFR Part 60, Appendix A-2. Retrieved 10/07/2026. The full method is on the eCFR.
1.0 Scope and Application
What is Method 3A?
Method 3A is a procedure for measuring oxygen (O2) and carbon dioxide (CO2) in stationary source emissions using a continuous instrumental analyzer. Quality assurance and quality control requirements are included to assure that you, the tester, collect data of known quality. You must document your adherence to these specific requirements for equipment, supplies, sample collection and analysis, calculations, and data analysis.
This method does not completely describe all equipment, supplies, and sampling and analytical procedures you will need but refers to other methods for some of the details. Therefore, to obtain reliable results, you should also have a thorough knowledge of these additional test methods which are found in appendix A to this part:
(a) Method 1—Sample and Velocity Traverses for Stationary Sources.
(b) Method 3—Gas Analysis for the Determination of Molecular Weight.
(c) Method 4—Determination of Moisture Content in Stack Gases.
(d) Method 7E—Determination of Nitrogen Oxides Emissions from Stationary Sources (Instrumental Analyzer Procedure).
1.1 Analytes. What does this method determine? This method measures the concentration of oxygen and carbon dioxide.
| Analyte | CAS No. | Sensitivity |
|---|---|---|
| Oxygen (O2) | 7782-44-7 | Typically <2% of Calibration Span. |
| Carbon dioxide (CO2) | 124-38-9 | Typically <2% of Calibration Span. |
1.2 Applicability. When is this method required? The use of Method 3A may be required by specific New Source Performance Standards, Clean Air Marketing rules, State Implementation Plans and permits, where measurements of O2 and CO2 concentrations in stationary source emissions must be made, either to determine compliance with an applicable emission standard or to conduct performance testing of a continuous emission monitoring system (CEMS). Other regulations may also require the use of Method 3A.
1.3 Data Quality Objectives. How good must my collected data be? Refer to section 1.3 of Method 7E.
2.0 Summary of Method
In this method, you continuously or intermittently sample the effluent gas and convey the sample to an analyzer that measures the concentration of O2 or CO2. You must meet the performance requirements of this method to validate your data.
EssentialRegs notes
Our plain-English reading of Method 3A. Not the method text.
What it measures
The concentration of oxygen, carbon dioxide, or both, in a stack gas, read continuously by an instrumental analyzer.
How it works
Gas is drawn from the stack through a sample line and conditioning system to the analyzer, which reports a continuous reading. Before and after each run, the analyzer is checked with calibration gases of known concentration. The quality checks are calibration error, sampling system bias and drift between the start and end of a run. The method follows the same quality assurance approach as Method 7E.
Equipment
A heated or unheated sample line, a gas conditioner to remove moisture, and an oxygen or carbon dioxide analyzer, such as a paramagnetic oxygen analyzer or an infrared carbon dioxide analyzer, with a data recorder and certified calibration gases.
When a rule cites it
Performance tests that need oxygen or carbon dioxide concentrations, to correct a measured pollutant concentration to a reference oxygen level or to calculate an emission rate. It is used where an instrumental reading is preferred to an Orsat analysis or where the test already runs an instrumental analyzer system for other pollutants.
Notes for compliance staff
Method 3A reports oxygen and carbon dioxide concentrations. A test that also needs a velocity or flow rate still runs the other stack-sampling methods. Method 3B is the Orsat method for the same gases. The test report should include the calibration gas certificates and the check results for each run.
Cited by
Provisions in the corpus whose text cites Method 3A. Subscribers land on the provision in the regulation reader; visitors see a preview of it (the GP05 sample opens in full).
40 CFR Part 60 Subpart IIII
- Table 7 to Subpart IIII of Part 60 — Requirements for Performance Tests for Stationary CI ICE With a Displacement of ≥30 Liters per Cylinder
40 CFR Part 60 Subpart JJJJ
- Table 2 to Subpart JJJJ of Part 60 — Requirements for Performance Tests
40 CFR Part 60 Subpart OOOO
40 CFR Part 60 Subpart OOOOa
40 CFR Part 60 Subpart OOOOb
40 CFR Part 60 Subpart OOOOc
40 CFR Part 63 Subpart ZZZZ
- Table 4 to Subpart ZZZZ of Part 63 — Requirements for Performance Tests
The sections marked as regulatory text above are reproduced from the CFR. Everything under ‘EssentialRegs notes’ is our reference copy, not the method. The method as published in the CFR controls.