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Visual Monitoring of Particulate Matter

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When particles are near or larger than the wavelength of the incident light, photons of all colors are scattered out of the beam path. --Figure 2.7, An Introduction to Visibility, National Parks Service 1999, ISSN 0737-5352-40

A visible emission is any visible airborne particles resulting from a chemical or physical process. Visible emissions are correlated with respirable particles, and can be measured by their effects on the opacity (or lack of transparency) of the air. Opacity can be monitored through visual assessment with only a stopwatch. Opacity is expressed as the percentage of light that is scattered or blocked by emissions. Examples of pollutants that change opacity are smoke stack emissions and dust.

Certifying community observers in EPA Method 9 for visual assessment of emissions can be written into a facility’s permits, read more in advocacy strategies.

Smoke School Certification

Citizens can get certified for this type of monitoring using EPA Method 9. This training is sometimes referred to as “smoke school,” Opacity Certification, Opacity Training, Visible Emissions Training, or VEO Training. In Smoke School, a smoke generator is used to create example plumes, and students learn to make visual estimates of relative opacity to within 5%.

Certification is valid in all states regardless of where it was obtained with the exception of Texas. There are several schools that will teach this method and certify those who complete the program. Certification can take a half day to two days, depending on who is administering the certification program. Method 22 can be conducted without certification, however familiarity with material in method 9 lectures and demonstrations is suggested.

To maintain your certification, you must pass the field certification test every 6 months. A list of other smoke schools can be found here, however Aeromaet is one of the cheaper options, takes less time and offers trainings in different locations around WI. To get certified, you must attend a public training or arrange for a site training. The total cost of training through Aeromet is $225 (this is for online training materials- $40, and Field Certification cost $185.) The lecture material needs to be completed every 3 years and recertification needs to be done every 6 months.

When you receive your EPA Method 9 certification, it also allows you to perform EPA Method 203a, 203b and 203c tests.

Method 9, Determining Opacity

According to the WI DEQ: If you see dust you can assume the visible emissions are above 10% opacity. Observers stand with their back to the sun and as close as possible to perpendicular to the emissions plume. The observers and take visual estimations of how obscured the view is through the plume in accordance with their training, rounding to the nearest 5%. The EPA states “opacity shall be determined as an average of 24 consecutive observations recorded at 15-second intervals,” adding up to a 6 minute observation period.

Method 203a, b & c

Please see the EPA technical documents here.

“Method 203A is virtually identical to EPA's Method 9… except for the data-reduction procedures, which provide for averaging times other than 6 minutes."

“Method 203B is virtually identical to EPA's Method 9... except for the data-reduction procedures, which have been modified to apply to time-exception regulations.” "A time-exception regulation means any regulation that allows predefined periods of opacity above the otherwise applicable opacity limit (e.g., allowing exceedances of 20 percent opacity for 3 minutes in 1 hour.)”

“Method 203C is virtually identical to EPA's Method 9…. except for 5-second reading intervals" instead of 145-second intervals.

Method 22- uncertified observation

Another method for measuring emissions visually is EPA Method 22. This method does not require certification. “This method determines the amount of time that visible emissions occur during the observation period (i.e., the accumulated emission time). This method does not require that the opacity of emissions be determined… At a minimum, the observer must be trained and knowledgeable regarding the effects of background contrast, ambient lighting, observer position relative to lighting, wind, and the presence of uncombined water (condensing water vapor) on the visibility of emissions.”

Method 22 states that this information is in Method 9 lecture courses, and suggests attendance at least one smoke school before performing a Method 22 test. While Method 22 is effective in reporting, it is only considered qualitative and not as powerful as method 9 which is considered quantitative.


The WI DEQ regulates opacity for Fugitive Dust under NR 425.04 and NR 415.076. Fugitive Dust regulations state materials handling operations “shall have fugitive emissions controlled to 20% opacity when wind speeds are less than 25 miles per hour except for 3 minutes in any hour when fugitive emissions may equal 50% opacity.” DEQ also regulates opacity under NR 415.076: PM emission limitations for crushed stone, sand and gravel plants. This regulation states “...operator shall control fugitive emissions from a road or other area used by haul trucks and from drilling so that visible emissions do not exceed 20% opacity at the source…” and that “emissions from activities not associated with processing equipment, including but not limited to roads, other areas used by haul trucks, storage piles and drilling, shall be controlled so that visible emissions do not exceed 5% opacity at the source.”

Here is an example form that can be used for submitting information to the WI DNR on opacity.