Visible emissions opacity explained
EPA Method 9 is the standard visual test for the opacity of smoke and emissions from stationary sources. Here is how the method works, what observers do in the field, and how they get certified.
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Method 9 is a field method: a trained person, standing in the right place, reading a plume every 15 seconds. These are the parts every observer needs to know.
Method 9, Visual Determination of the Opacity of Emissions from Stationary Sources, is published by the U.S. EPA in 40 CFR Part 60, Appendix A. Permits and regulations use it to check opacity limits on stacks, vents and other sources.
Opacity is how much a plume blocks the view of the background behind it, from 0% (clear) to 100% (fully blocked). Observers read both black and white plumes and record opacity to the nearest 5%.
The observer records a reading every 15 seconds. Compliance is usually judged on the average of 24 consecutive readings, a six-minute average, compared with the opacity limit in the permit.
The observer stands with the sun behind them, within a 140-degree sector, with a clear view of the plume against a contrasting background. Readings are taken where the plume is densest and free of condensed water vapor.
Method 9 measures how opaque a plume is. Method 22 only records whether emissions are visible and for how long, and it does not require the six-month field certification. Many facilities need both.
Read about EPA Method 22Every Method 9 reading is recorded on a Visible Emissions Observation (VEO) form: the source, the observer position, sun and wind, the plume background and each 15-second reading. You can download a blank form from our resources page.
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The opacity of visible emissions from stationary sources, read visually by a certified observer in 5% increments.
Only an observer who has passed the Method 9 qualification test in the last six months. The test uses a smoke generator with 25 black and 25 white plumes.
Readings are taken every 15 seconds. The result is usually the average of 24 consecutive readings (six minutes), unless the regulation sets a different averaging period.
You can download the Visible Emissions Observation (VEO) form from our resources page.
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