Public Lab Research note

EPA Method 9: Visual Determination of the Opacity of Emissions from Stationary Sources

by gretchengehrke | February 21, 2017 22:26 21 Feb 22:26 | #13943 | #13943

gretchengehrke was awarded the Basic Barnstar by stevie for their work in this research note.


This is a step by step guide to to assess the opacity of emissions, using EPA’s Method 9 “Visual Determination of the Opacity of Emissions from Stationary Sources.” Stationary emissions sources, such as industrial facilities with smokestacks, are subject to federal and state emissions opacity limits, meaning there are limits to the thickness of emissions they can release. Federal opacity limits are included in the Code of Federal Regulations, Title 40, Chapter 1 part 60, 61, and 62 (40 CFR Part 60), and state opacity limits can be found in that state’s administrative codes for air pollution or in their state implementation plan (SIP), available on the relevant EPA region’s website.

Limiting emissions opacity has been instrumental in reducing air pollution, and monitoring emissions opacity is important to keep industries accountable to complying with opacity rules. According to AeroMet, one of the leading opacity emissions observation training companies, opacity observations have brought about the most air pollution fines in the US (AeroMet online lecture chapter 1). Anyone can follow Method 9 to observe and report emissions opacity to their regional EPA or state environmental agency (e.g. your state DEQ/DNR). However, to prompt enforcement action such as leveraging fines, the observer must be officially trained in EPA Method 9 and have passed an opacity estimation field exam within 6 months.

If you are unsure of whether or not this method may be of use to you please see two additional research notes:
Thoughts on Method 9 and its Utility and


To conduct this method, you will definitely need:

  1. Your eyes. If you wear glasses, you must have worn the same prescription glasses during your certification testing. If you use binoculars, you must have used the same binoculars during your certification testing. If you wear sunglasses, you must have been wearing sunglasses during your certification testing (and it is not advised since it diminishes contrast between smoke and background colors).

  2. A watch where you can easily see 15-second increments.

  3. A Method 9 reporting sheet, which can be downloaded from most state DEQ offices. Here is a Method 9 reporting sheet from Georgia’s Department of Natural Resources:.

  4. In order to accurately estimate opacities, it is highly recommended that you take a field training class, available through many companies including:Aeromet,ETA,Compliance Assurance. If you are conducting this method for enforcement purposes, you must be certified to do so, which can be achieved through completion of a lecture course and field test, available through the companies listed above. Some states have funding available for citizens to offset certification costs -- please be in touch with your state environmental agency to find out if you could receive funds.

Other required information includes sun direction and direction of the plume, distance to the plume, temperature, and relative humidity. To provide this information, you will need:

  1. Either a compass or the ability to demonstrate direction based on the sun location, time of day, and your latitude.

  2. Either a handheld temperature and humidity probe, or a nearby weather station (which could also report wind direction if you do not have a compass).

  3. A method for estimating distance, which could include a rangefinder or a detailed map with a precise scale bar and identifiable viewing location.

  4. You may want to have a clinometer to demonstrate the viewing angle of the yourself relative to the plume, but that can be calculated from knowing the stack height and your distance to the stack too.


Please refer to the official Visible Emissions Field Manual for official guidance and operating procedures.

Find an appropriate viewing location

  1. Locate the plume.

  2. Find or estimate the stack height or height of the point of emission (e.g. height at which wind-whipped dust is released from a sand pile). If you are estimating the height of the point of emission, it may be worthwhile to take a photograph and label landmarks or other indicators of scale.

  3. Observe the direction of the plume.

  4. Make sure you are standing perpendicular to the direction of the plume. If the plume is vertical at its base (where you will estimate its opacity), then any viewpoint would be considered perpendicular to the plume.

  5. Observe the position of the sun.

  6. Position yourself such that the sun is in a 140-degree range behind you (i.e. it should not be directly in your peripheral vision, nor should you be looking toward the sun). It is best to avoid observation times when the sun is mostly overhead, so morning or late afternoon hours are best.

  7. From your current position, standing perpendicular to the plume and with the sun at your back, make sure you are at least “three stack heights” away from the point of emission. This means that if you are observing a plume from a 100 ft stack, you will need to be standing at least 300 ft away. Note that if you are estimating the height of the emissions point and may be standing closer than what you estimate three stack-heights to be, it may be worthwhile to use a clinometer to ensure your viewing angle of the plume is appropriate. You should be viewing the plume at 18-degrees or less. If you are observing the plume at more than 18-degrees, then you’ll have to adjust your opacity estimates using calculations found in the Method 9 field manual. For any distance or height that you estimate, be sure to acknowledge that by using a “~” in your observation form.

  8. Ensure that your vantage point has an unobstructed view of the plume you are assessing.

  9. Note that it can be difficult to have access to a viewing point that is (1) perpendicular to the direction of the plume, (2) with the sun at your back, (3) at least three stack-heights away, (4) with an unobstructed view of the plume, and (5) is legally and feasibly accessible. If your first attempt to find an appropriate viewing location that meets these 5 criteria fails, try again at different times of day and on days with different wind directions.

Observe the emissions

  1. On your Method 9 reporting sheet, sketch your observation diagram in the appropriate space, including the emissions point, sun location, viewing location, wind direction, and any identifiable landmarks.

  2. If possible, take a picture to document your position.

  3. Note the temperature, wind direction, and wind speed. You may be able to use local weather data. If there is a notable change in wind direction or speed during your observations and you are relying on a local weather station rather than a portable weather station, take note on your observation sheet of the times and estimated changes to wind.

  4. Take note of the background against which you are comparing the plume. Make mental notes about the texture of the trees or color of the sky. Your primary task is to estimate the extent to which your view to this background is obscured by the emissions you are monitoring, so be sure to take stock of the unobscured background nearby.

  5. Make note of where you will observe the plume. You’ll want to focus your gaze at the base of the plume, nearest the point of emissions (unless the plume contains steam, for which you will want to observe the plume before or after the condensed steam; see the VE Field Manual, page 12-13).

  6. Consult your watch such that you can note 15-second intervals.

  7. On the 15-second mark, observe the plume quickly. It is recommended that you spend no more than 3 seconds looking at the plume. Estimate the opacity of the plume in those 3 seconds.

  8. Record your opacity estimate in the appropriate space on your Method 9 observation sheet.

  9. Rest your eyes. It is very important to relax your gaze between observations so that your eyes do not fatigue.

  10. After 15 seconds have elapsed since you started your first observation, make your second observation. Follow steps 16-18.

  11. Continue making observations until you have made at least 24 measurements (6 minutes consecutively).

  12. If you have to miss a measurement on the 15-second interval for an unintended reason (e.g. something obscures your view of the plume, the wind changes direction and impedes your ability to view the plume from an appropriate angle, something gets in your eye, etc), simply mark the space with a horizontal line on your reporting form. Make sure you make 24 actual observations, not including missed intervals.

  13. Many regulations allow smokestack emissions to exceed 20% for 6 minutes per hour. Thus, if you are concerned that an industry is out of compliance with these stipulations, you may want to observe more than one six-minute set to demonstrate a pattern of opacity exceedance.

Calculate Results

Once you have recorded at least 24 opacity observations, you will want to find the highest 6-minute average. Note that some states and tribal governments require different averaging times. Most states use the federal standard of 6 minutes, but it may be worthwhile to check your state or tribe’s state implementation plan (SIP) or tribal implementation plan (TIP), or contact your state environmental agency to check.

  1. To start, add the estimated opacities of the first 24 measurements you made. If you had to miss a measurement interval unintentionally (see #21 above), make sure you add the first 24 actual measurements you made. Divide this sum by 24. Record this value as the average opacity for the six-minute section evaluated.

  2. To find the next rolling average value, start with your second measurement and sum 24 consecutive measurements from there (e.g. measurement numbers 2 through 25). Add these 24 values, divide by 24, and record this value.

  3. Find the next rolling average value by starting with your third measurement and summing 24 consecutive measurements from there, dividing by 24, and recording your value.

  4. Continue to find subsequent rolling average values until you have covered all measurements you made.

  5. Report the highest average value.

Report Results

The exact process for reporting your Visual Emissions Opacity monitoring results varies from state to state. It is highly recommended to contact your state environmental agency (e.g. your state DEQ or DNR) to discuss their preferred reporting process from community members, as information on their websites is often catered to industries who must conduct these tests to demonstrate compliance.

Generally, required documents that must be submitted to the appropriate division within your state environmental agency (usually termed the Division of Air Quality or something similar) include:

  1. the Method 9 reporting form

  2. documentation for your certification

  3. Possibly information regarding the company’s permit number

Be prepared to explain and defend your observations, particularly if you have estimated distances or heights of emissions outlets, missed observations, or observed steam plumes. Pictures can be very useful to demonstrate your location and add credence to your estimates.

Additional Resources

Please refer to the EPA’s Visual Emissions Field Manual for more information:

Further information about particulate matter and visible emissions on Public Lab’s website can be found at:

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Looks great!

i noticed two formatting issues. There seems to be a missing research note in the section:

If you are unsure of whether or not this method may be of use to you please see two additional research notes:
Thoughts on Method 9 and its Utility and

also, the numbers' sequence is being re-set by Markdown, so when you reference a number it isn't quite showing up.

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@stevie awards a barnstar to gretchengehrke for their awesome contribution!

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