Method 9
Visual Determination of the Opacity of Emissions From Stationary Sources
40 CFR Part 60, Appendix A-4
From the method — regulatory text
Sections 1 and 2 of Method 9—Visual Determination of the Opacity of Emissions From Stationary Sources, as published at 40 CFR Part 60, Appendix A-4. Retrieved 10/07/2026. The full method is on the eCFR.
1. Principle and Applicability
1.1 Principle. The opacity of emissions from stationary sources is determined visually by a qualified observer.
1.2 Applicability. This method is applicable for the determination of the opacity of emissions from stationary sources pursuant to § 60.11(b) and for qualifying observers for visually determining opacity of emissions.
2. Procedures
The observer qualified in accordance with section 3 of this method shall use the following procedures for visually determining the opacity of emissions:
2.1 Position. The qualified observer shall stand at a distance sufficient to provide a clear view of the emissions with the sun oriented in the 140° sector to his back. Consistent with maintaining the above requirement, the observer shall, as much as possible, make his observations from a position such that his line of vision is approximately perpendicular to the plume direction, and when observing opacity of emissions from rectangular outlets (e.g., roof monitors, open baghouses, noncircular stacks), approximately perpendicular to the longer axis of the outlet. The observer's line of sight should not include more than one plume at a time when multiple stacks are involved, and in any case the observer should make his observations with his line of sight perpendicular to the longer axis of such a set of multiple stacks (e.g., stub stacks on baghouses).
2.2 Field Records. The observer shall record the name of the plant, emission location, type facility, observer's name and affiliation, a sketch of the observer's position relative to the source, and the date on a field data sheet (Figure 9-1). The time, estimated distance to the emission location, approximate wind direction, estimated wind speed, description of the sky condition (presence and color of clouds), and plume background are recorded on a field data sheet at the time opacity readings are initiated and completed.
2.3 Observations. Opacity observations shall be made at the point of greatest opacity in that portion of the plume where condensed water vapor is not present. The observer shall not look continuously at the plume, but instead shall observe the plume momentarily at 15-second intervals.
2.3.1 Attached Steam Plumes. When condensed water vapor is present within the plume as it emerges from the emission outlet, opacity observations shall be made beyond the point in the plume at which condensed water vapor is no longer visible. The observer shall record the approximate distance from the emission outlet to the point in the plume at which the observations are made.
2.3.2 Detached Steam Plume. When water vapor in the plume condenses and becomes visible at a distinct distance from the emission outlet, the opacity of emissions should be evaluated at the emission outlet prior to the condensation of water vapor and the formation of the steam plume.
2.4 Recording Observations. Opacity observations shall be recorded to the nearest 5 percent at 15-second intervals on an observational record sheet. (See Figure 9-2 for an example.) A minimum of 24 observations shall be recorded. Each momentary observation recorded shall be deemed to represent the average opacity of emissions for a 15-second period.
2.5 Data Reduction. Opacity shall be determined as an average of 24 consecutive observations recorded at 15-second intervals. Divide the observations recorded on the record sheet into sets of 24 consecutive observations. A set is composed of any 24 consecutive observations. Sets need not be consecutive in time and in no case shall two sets overlap. For each set of 24 observations, calculate the average by summing the opacity of the 24 observations and dividing this sum by 24. If an applicable standard specifies an averaging time requiring more than 24 observations, calculate the average for all observations made during the specified time period. Record the average opacity on a record sheet. (See Figure 9-1 for an example.)
EssentialRegs notes
Our plain-English reading of Method 9. Not the method text.
What it measures
The opacity of a visible emission plume — the degree to which it obscures the view behind it — expressed as a percentage, as judged by a trained and certified observer.
How it works
A certified observer stands at a position with the sun behind them and the plume viewed against a contrasting background, and records an opacity reading at fixed intervals over the observation period. Readings are averaged over the period the applicable rule specifies, and the average is compared with the opacity limit. The method sets the observer position, the reading interval, the averaging, and the training and certification the observer must hold.
Equipment
No instrument. A certified visible-emissions observer, recertified at the interval the method requires, with a data sheet recording conditions, position and readings.
When a rule cites it
Opacity limits on stacks, flares, fugitive dust sources and process units, and the compliance demonstrations behind them. It is also the method a rule usually names when an observer, rather than an instrument, is to judge visible emissions.
Notes for compliance staff
Method 9 is a human observation, so observer certification and the recorded conditions matter as much as the readings. Method 22 is the related method for whether visible emissions are present at all and for how long, without assigning an opacity percentage; rules that only need a yes-or-no on visible emissions cite Method 22 instead.
Cited by
Provisions in the corpus whose text cites Method 9. Subscribers land on the provision in the regulation reader; visitors see a preview of it (the GP05 sample opens in full).
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.